Literature Search for Reliability Data of Components in Electric Distribution Networks

Literature Search for Reliability Data of Components in Electric Distribution Networks

by

M.H.J. Bollen

EUT Report 93-E-276

ISBN 90-6144-276-1

August 1993

Eindhoven University of Technology Research Reports

ISSN 0167-9708

EINDHOVEN UNIVERSITY OF TECHNOLOGY

Faculty of Electrical Engineering Eindhoven, The Netherlands

LITERATURE SEARCH FOR RELIABILITY DATA

Coden:TEUEDE

OF COMPONENTS IN ELECTRIC DISTRIBUTION NETWORKS

by

M.H.J. Bollen

EUT Report 93-E-276 ISBN 90-6144-276-1

Eindhoven August 1993

CIP-DATA KONINKLlJKE BIBLIOTHEEK, DEN HAAG

Bollen, M.H.J.

Literature search for reliab.ility data of components in electric distribution networks I by M.H.J. Bollen. -Eindhoven: Eindhoven University of Technology, Faculty of Electrical Engineering. - Fig., tab. - (EUT report, ISSN 0167-9708 ; 93-E-276) With ref. ISBN 90-6144-276-1 NUGI832 Subject headings: power system reliability I distribution networks.

Abstract.

This report gives the result of a literature search for component lifetimes for use in reliability studies of distribution networks. Data are given for power transformers, circuit breakers and switches, protective equipment, fuses, voltage and current transformers, generators, "uninterruptable" power supplies, cables and acessories, busbars and large motors. For each chapter the available lifetime data is divided into recommended values, data from surveys, data used in reliability studies and ageing data. Also data for duration of the restore are given. Each chapter results in lifetime values that appear to be reasonable.

Keywords: power system reliability I distribution networks.

Bollen, M.H.J. LITERATURE SEARCH FOR RELIABILITY DATA OF COMPONENTS IN ELECTRIC DISTRIBUTION NETWORKS. Eindhoven: Faculty of Electrical Engineering, Eindhoven University of Technology, 1993. EUT Report 93-E-276

Address of the author.

M.H.J. Bollen University of Manchester Institute of Science and Technolgy Department of Electrical Engineering P.O. Box 88 Manchester M60 1 aD United Kingdom

iii

CONTENTS OF THIS REPORT

pag

1. Introduction 1 1.1. Aim and contents of this report 1 1.2. Terms used in this report 2 1.3. Conclusions 3

2. Power transformers 4 2.1. Recommended values 4 2.2. Data from surveys 6 2.3. Data used in reliability studies 11 2.4. Ageing data 16 2.5. Conclusions 19

3. Circuit breakers and switches 21 3.1. Recommended values 21 3.2. Data from surveys 22 3.3. Data used in reliability studies 31 3.4. Ageing data 41 3.5. Conclusions 44

4. Protective equipment; general and relays 47 4.1. Recommended values 47 4.2. Data from surveys 48 4.3. Data used in reliability studies 54 4.4. Ageing data 57 4.5. Conclusions 59

5. Fuses 60 5.1. Recommended values 60 5.2. Data from surveys 61 5.3. Data used in reliability studies 62 5.4. Ageing data 63 5.5. Conclusions 64

6. Voltage and current transformers 65 6.1. Recommended values 65 6.2. Data from surveys 66 6.3. Data used in reliability studies 67 6.4. Ageing data 68 6.5. Conclusions 69

7. Generators 70 7.1. Recommended values 70 7.2. Data from surveys 71 7.3. Data used in reliability studies 78 7.4. Ageing data 81 7.5. Conclusions 84

iv

pag

8. "uninterruptable" power supplies 86 8.1. Recommended values 86 8.2. Data from surveys 87 8.3. Data used in reliability studies 88 8.4. Ageing data 92 8.5. Conclusions 93

9. Cables and accessories 94 9.1. Recommended values 94 9.2. Data from surveys 96 9.3. Data used in reliability studies 108 9.4. Ageing data 114 9.5. Conclusions 116

10. Busbars. 120 10.1. Recommended values 120 10.2. Data from surveys 121 10.3. Data used in reliability studies 123 10.4. Ageing data 126 10.5. Conclusions 127

11. Large motors 129 11.1. Recommended values 129 11.2. Data from surveys 130 11.3. Data used in reliability studies 133 11 .4. Ageing data 134 11.5. Conclusions 135

12. References 136

v

1. INTRODUCTION

1.1. Aim and contents of this report

This report is the result of an extensive literature search after failure data of components in distribution systems. It started as part of a project aimed at the incorporation of reliability aspects in the design of industrial power systems. During the execution of this project it felt that there was a lack of data on failure rates (lifetimes) of industrial power system components. It was then decided to perfrom this literature search after component reliability data.

This report is divided in 10 more chapters, each dealing with one type of power system components. Every chapter is further divided into five sections: recommended values; data from surveys; data about ageing; conclusions. Within each of these sections the references are sorted to year of publication.

Not much information has been found on ageing of components. I therefore present the available information without drawing any conclusions. Also on repair and replace times no conclusions are given. These times are too much dependent on the specific situation to give recommended values. Furthermore, they can easily be assessed in an industrial environment, so there is less need for recommended values. I do however present the available data. These interested are able to draw there own conclusions.

-1- Introduction

1.2. Terms used in this report

In this report the ETTF (Expected Time To Failure) has been used to describe the failure behaviour of a component. For surveys it has been defined as

ETTF = #components * #years #failures

It is related to the failure rate A through

(1 )

(2)

An ETTF of 1000 years thus does not mean that this kind of component is expected to last a thousand years but that each year 1 %0 of the population will fail during the next few years (20 or 30 at most).

From a survey the ETTF can never be determined exactly. Where possible a confidence interval has been given in this report. The 95% confidence interval (designated as c.L) is approximated through the following expression (for a survey finding n failures in a population of N component-years):

. [N N] (3) c. ~. = n+2*/Ii' n-2*/Ii .

The repair proces is described through the term "repair time" which refers to the expected repair time which is again, in case of survey, an average repair time. No confidence interval is given for the repair time. The values should however be used with the same caution as the ETTF-values.

For some recommeded values the authors gave a range of values [e.g. IEEE, 1983; Kloeppel et al., 1990]. This is either a spread in the value for different locations and/or circumstances or a measure of the uncertainty in the recommended value. As it needs not to be the same as a confidence interval, the original term "range" has been used in this report.

Some authors distinguish between active and passive failures. The followingm definition for this is given by Billinton and Allan [1984]. A passive event is a component failure mode that does not cause operation of protection breakers and therefore does not have an impact on the remaining healthy components. Service is restored by repairing or replacing the failed component. Examples are open circuits and inadvertent opening of breakers. An active event is a component failure mode that causes the operation of the primary protection zone around the failed component and can therefore cause the removal of other healthy components and branches from service.

I often took over the terms as used by the authors of the papers I refere to. This causes a non-standard use of vocabulary. I hope the reader will except this. It is for instance not always clear what an author means with repair time. Is it the time needed to repair a component, or the time needed to restore the service. It the latter case switching time or replacement time seem more appropriate terms. In case the latter terms were used I took them over.

-2- Introduction

1.3. Conclusions

From the data found during this survey I would suggest the following ETTF values for components in distribution networks. For the reasoning behind these values, please refer to the corresponding chapters further on in this report.

MV /L V transformers MV/MV transformers HV /MV transformers MV and LV circuit breakers disconnect switches electromagnetic relays electronic relays (single function) electronic relay systems fuses voltage and current transformers standby generators

probability of fail-to-start continuous generators UPS invertor

rectifier underground cables (1000 meters) cable terminations cable joints busbars (one section) large motors

: 500 - 1000 years : 75 - 100 years : 40 - 70 years : 1000 - 5000 years : 250 - 1000 years : 250 - 1000 years : 100 - 200 years : 10 - 30 years : 1000 - 5000 years : 2000 - 3000 years : 5 - 20 days : 0.5 - 2 % : 1 - 3 years : 0.5 - 2 years : 10 - 30 years : 40 - 75 years : 1000 - 3000 years : 500 - 2000 years : 500 - 2000 years : 15 - 30 years

-3- Introduction

2. POWER TRANSFORMERS

2.1. Recommended values

2.1.1. Green and Bourne [1972) give average component failure-rates for electrical components. For power transformers they give:

less than 15 kV 15 kV to 33 kV 33 kV to 132 kV 132 kV to 400 kV

: ETTF = 190 years : ETTF = 57 years : ETTF = 29 years : ETTF = 16 years

2.1.2. IEEE standard 500 [lEEE,1983) gives reliability data for components of nuclearpower stations. The data has been derived by a Delphi-method combined with the results from several surveys and data bases. For station service transformers the following data are, recommended (for catastrophic failures only)

liquid filled single phase transformers 0-600 Volt: ETTF = 650 years; range = < 200, 1300 > 601 V -15 kV: ETTF = 600 years; range = <250,1500> 15 - 40 kV: ETTF = 225 years; range = < 100,600 > restore time = 1260 hours; range = < 12.8, 3740>

liquid filled three phase transformers. 0-600 Volt: ETTF = 700 years; range = < 350, 2200 > 601 V- 15 kV: ETTF = 875 years; range = < 350, 1850 > 15 - 40 kV: ETTF = 300 years; range = < 140, 1200 > restore time = 1300 hours; range = < 12.8, 3700>; based on [lEEE,1980)

Dry type single phase transformers 0-600 Volt: ETTF = 700 years; range = < 200, 1400 > 601 V-15 kV: ETTF = 350 years; range = <160,800> 15 - 40 kV: ETTF = 130 years; range = < 50, 500 > restore time = 28.0 hours; range = <0.5, 720>; based on [lEEE,1980)

Dry type three phase transformers 0-600 Volt: ETTF = 500 years; range = < 150, 1000 > 601 V-15 kV: ETTF = 340 years; range = <140,750> 15 - 40 kV: ETTF = 105 years; range = <80,290> restore time = 28.0 hours; range = < 0.5, 720>; based on [lEEE,1980)

For substation transformers the following data are recommended (catastrophic failures only)

Liquid filled single phase transformers 2-30 kV: ETTF = 340 years; range = < 200, 700 > 31-72 kV: ETTF = 300 years; range = <150,500> 73-145 kV: ETTF = 240 years; range = <130,500>

-4- power transformers

146-242 kV: ETTF = 65 years; range = <40,120> 243-346 kV: ETTF = 185 years; range = <110,1250> 347-550 kV: ETTF = 190 years; range = <125,1300> over 550 kV: ETTF = 120 years; range = <60,285>

liquid filled, three phase transformers 2-30 kV: ETTF = 250 years; range = <150,400> 31-72 kV: ETTF = 175 years; range = <120,300> 73-145 kV: ETTF = 120 years; range = <90,250> 146-242 kV: ETTF = 70 years; range = <45, 165> 243-346 kV: ETTF = 110 years; range = <65,250> 347-550 kV: ETTF = 90 years; range = <55,200> over 550 kV: ETTF = 80 years; range = <50,250>

2.1.3. Kloeppel et al. [1990) present recommended values of component data for reliability studies. They were based on data from several industries in Eastern Germany as well as form the public supply. For transformers the following data are recommended:

MV/LV

30kV/MV

ETTF = 100 years; range < 65, 170 > repair time = 18 hours; range = < 6, 30 >

ETTF = 50 year; range < 30, 80 > repair time = 48 hours; range = < 8, 120 >

110 kV/MV ETTF = 30 years; range = <20,45> repair time = 120 hours; range = < 8, 240 >

220 kV /MV ETTF = 20 years; range = < 16, 25 > repair time = 180 hours; range = < 12, 360>.

2.1.4. The IEEE gold book [lEEE,1990) gives recommended values for the components of industrial and commercial power systems. For power transformers it recommends:

liquid filled transformers: ETTF = 161 year

rated power between 300 and 10,000 kVA: ETTF = 169 year

rated powers above 10,000 kVA: ETTF = 65 year.

These values are based on the results of the 1979 IEEE survey [Aquilino, 1980). This survey is further discussed in Section 2.2.5.


2.2. Data from surveys

2.2.1. Dickinson (1962) reports on an AlEE survey undertaken in 1959. Responses were recieved from 33 compagnies, covering 85 plants. For power transformers the results are:

above 15 kV below 15 kV

above 500 kVA below 500 kVA

: ETTF = 50 years; c.i. = <25,300>

: ETTF = 130 years; c.i. = <65,260> : ETTF = 110 years; c.i. = <90,140>

The repair time (average down time) reported was:

above 15 kV below 15 kV

above 500 kVA below 500 kVA

: 2000 hours;

: 250 - 300 hours : 8 - 15 hours

2.2.2. Todd [1964) gives outage rates for transmission and distribution facilities derived from 5 - 11 years experience at Indianapolis Power & Light Company.

138/34.5 kV transformers: ETTF = 70 years; outage duration exceeding 4 hours in all cases; time of occurrence between 7 AM and 6 PM in all cases.

34.5/4.1 kV transformers: ETTF = 180 years; outage duration less than 2 hours

2 - 4 hours exceeding 4 hours

time of occurence 7 AM to 6 PM 6 PM to 10 PM

:20% :30% : 50% :80% : 20%

2.2.3. Connor and Parkins [1966) report about a 14-year survey of faults in networks with nominal voltages between 2 and 33 kV. The results for transformers (excluding lightning) are shown in the table below. Table 1 gives failure rates per 100 transformers per year.

system vol tag. 2 • 5 kV 6.6 kV 11 kV 11 kV 22 kV 33 kV method of earthing sol id/res sol id/res arc sup cot l sol id/res sol id/res sol id/res bushing fai lure 0.03 0.04 0.04 0.04 0.06 0.06 wincHna fai lure 0.06 0.17 0.22 0.23 0.22 0.25 oil QUal;tv . . 0.01 . . .

overload 0.07 0.03 0.08 0.07 0.09 . taD-chsnqe mechanism 0.01 0.01 0.01 0.02 0.04 0.44 miscellaneous 0.05 0.06 0.04 0.06 0.07 0.65 total 0.25 0.27 0.38 0.39 0.49 1.41

Table 1: failure rate per 100 transformers per year according to a 14-year survey reported by Connor and Parkins {1966]


The number of transformers connected to the system under study was 14,561 in 1952 and 37,464 in 1965. The following ETTF-values can be derived from the table above:

2-5 kV : ETTF = 400 years; 6.6 kV : ETTF = 370 years; 11 kV arc sup coil : ETTF = 260 years; 11 kV solid/res : ETTF = 260 years; 22 kV : ETTF = 200 years; 33 kV : ETTF = 70 years;

all voltages : ETTF = 260 years; c.i. = < 240,270 >.

2.2.4. An IEEE sponsored survey of electrical equipment reliability in industrial plants was completed during 1972 IIEEE,19741. This survey included a total of 1982 equipment failures that were reported by 30 compagnies covering 68 plants in nine industries in the United States and Canada. The results for power transformers are given in Table 2.

average actual estimated restore down time (hours) time (hours)

ETTF (years) c. i . industry plant repair replace

LfCl\Jid filled

All vol tages 240 [190 325] 529 219 378 73.4

600 • 15 000 V

All sizes 340 [250 500] 174 49 382 74.3

300 • 750 kVA 270 [170 750] 61 10.7 49 3.7

751 • 2 499 kVA 400 [260 870] 217 64 297 39.7

2 500 kVA and above 310 [200 700] 216 60 618 150

Above 15 000 V 77 [55 130] 1076 1260 367 71.5 Dry type

o • 15 000 V 275 [190 500] 153 28 67 39.9

Rectifier above 600 Vol t 34 [23,60] 380 80 300 20.0

Table 2: results of IEEE survey completed during 1972.

The last four columns give: the industry average anf the plant average of the actual downtime per failure and the average estimated time to fix the failure during 24 hours work day in case of repair of the failed component and in case of replace with a spare.

2.2.5. The IEEE performed another survey after failures of power transformers in industrial plants and commercial buildings in 1979 [Aquilino, 1980, 19831. Table 3 gives the results for liquid filled power transformers and rectifier transformers.

DOwer transfonmers rectifier transformers ETTF c. i. restore time (hours) ETTF c. i. restore time (hours)

(years) repair replace (year-s) repair replace All sizes 160 [130 215] 365.1 85.1 50 [30 140] 2316 41.4

300 • 10 000 kVA 170 [130 230] 297.4 79.3 65 [30 300] 1664 38.7

Above 10 000 kVA 65 [35,400] 1178.5 192 30 [15,111> 2707.2 60

Table 3: Results of IEEE survey of 1979.


The table gives ETTF and confidence interval in years, as well as average estimated time to fix the failure during 25 hour work day in case of repair of the failed component and in case of replace with a spare.

For power transformers the number of failures was high enough (about 100) to justify a further subdivision of the failures.

A subdivision to failure initiating cause yields: transient overvoltage disturbance :16.4% overheating : 2.7% insulation breakdown : 48.1 % mechanical failure : 12.7% shorting by external object : 3.6% malfunction of protection : 4.5% improper operating procedure : 3.6% loose connection or termination : 7.3% others : 0.9%

A subdivision to failure contributing cause yields: abnormal temperature exposure of aggressive chemicals or moisture normal ageing severe weather conditions lack or malfunction of protective device loss or degradation of cooling medium improper operating procedure or testing error inadequate maintenance others

: 5.5% : 14.4% : 13.3% : 4.4% : 10.0% : 10.0% : 3.3% : 7.8% : 31.1%

A subdivision to suspected failure responsibility yields: manufacturer defective component or improper assembly improper application inadequate installation and testing prior to start-up inadequate maintenance inadequate operation outside agency others

A subdivision to type of failure yields: winding : 53% others : 47%

A subdivision to failure characteristic yields: automatic removal by protective device : 75 % partial failure reducing capacity : 5% manual removal : 20%

-8-

: 33.3% : 3.1% : 6.3% : 26.0% : 4.2% : 9.4% : 17.7%

power transformers

2.2.6. Cigr6 held an international survey on failures in large power transformers in service in 1978 [CIGRE,1983]. Data was received from 13 countries (Australia, Austria, Belgium, Canada, Czechoslovakia, France, Finland, Italy, japan, Switzerland, United Kingdom, USA and USSR). The analysis took in more than 1000 failures that occurred between 1968 and 1978, relating to a total population of more than 47,000 unit-years. The results are summarized in table 4.

ETTF (veersl c.f.(veersl

Power station transformers 60 - 100 kV 65 [40 190]

100 - 300 kV 45 [35 60]

300 - 700 kV 32 [22 54]

without on-load tab-changers 44 [326n

with on-load tab-changers 42 [33 sn Substation transformers

60 - 100 kV 48 [42 56]

100 - 300 kV 45 [40 50]

300 - 700 kV 46 [30 110]

Table 4: Results of 1978 Cigre survey.

2.2.7. One ofthe Dutch regional electricity authorities, N. V. PNEM, performed a survey after the reliability of power system components in their 10 kV network, covering the period 1980 - 1986 [van Amelsfoort et.al, 1986]. For 10 kV/380 V transformers the survey resulted in:

ETTF = 650 yr; c.i. = [500,900].

2.2.8. Waumans [1986] presents the result of failure registration in a number of medium and low-voltage networks in the Netherlands from 1979 to 1984. For transformers (apparently MV/LVI he gives:

ETTF = 500 years

2.2.9. A survey performed by the Dutch utilities [VDEN,1987] results in the following values for 10kV/400 Volt transformers:

ETTF = 950 yr; c.i. = [700,1400]

2.2.10. Pijls [1988] analysed failures in the electricity system supplying a number of chemical plants, during 8 years. The rated voltage of the system under study was 10 kV. For transformers 10 kV /6 kV and 10 kV /2 kV he found:

ETTF = 280 years; c.i. = < 90 years, 00 >

2.2.11. Franke [1990] observed during 12 years, failures in a chemical plant in Eastern Germany. Early failures in the wear-in phase (1.5 ... 2 years) have been removed from the data. For MV/LV transformers up to 1.6 MVA he reports:

ETTF = 85 years; c.i. = < 55,200>


Included in this value are: intervention by the protection due to internal fault; alarm signal by Buchholz relay; over temperature.

2.2.12. Bovy et al. [1991] analysed the outage occurances in some 4000 km of underground 10 kV network during 6 years. For transformer stations they found:

ETTF: 625 years; repair time: 12 hours; ETTF for phase-to-ground-faults: 7000 years; probability of multiple fault due to phase-to-earth fault: 19%

2.2.13. Bruggeman et al. [1991] present some statistics about various types of transformer faults from British utilities. They divide the faults by the place of occurence. Their results for ground-mounted, cable-connected distribution transformers are given Table 5.

cOO1)Onent where the faut t occured failure rate per 1000 unit.vr ETTF (vr)

cable tenminations 0.69 1 450 other external connections 0.13 7700 windings and comections 0.52 1 900 tanks radiators etc. 0.15 6,700 (mostly corrOSion) taa-changers: mechanical 0.06 17 000 tap-changers: eLectrical 0.008 12 500 other sites mostly accessories 0.08 12 500

'Ilknown 0.14 7 000

all c ts 1.778 550

Table 5: Results of survey by Bruggeman et al.[1991J.

2.2.14. Verplanke [1991] studied failures that occured in the power system of a chemical industry in The Netherlands between 1970 and 1991. For 50 kV/11 kV transformers he reports

ETTF = 20 years; sample too small for c.i.


2.3. Data used in reliability studies

2.3.1. Dickinson [1957] gives for transformers in an industrial power system a typical value of

ETTF = 300 years.

He states:" whether available, actual service experience should preferably be used. Actual failure rates wil vary widely depending on quality of equipment, installation, maintenance and on service conditions." The typical repair time (electrical downtime) after failure of a power transformer is

repair time = 360 - 720 hours.

2.3.2. Dickinson [1960] uses the following values for power distribution systems for refinery process units. These are based on several published papers and on service reports obtained from refineries. For transformers he uses:

ETTF = 350 years; repair time = 70 - 500 hours.

2.3.3. Gangel and Ringlee [1968] calculate service interruption rates and restoration times for a distribution circuit. They use the following data for distribution transformers:

ETTF = 200 - 1000 years; repair time = 1 - 5 hours.

2.3,4. Guertin and Lamarre [1975] performed a reilability analysis of a high-voltage substation. For HV/MV transformers they use:

ETTF = 10 years; repair time = 1000 hours;

maintenance is performed every year during 6 hours . •

2.3.5. Bocker and Kaufman [1977] use the following values for a public distribution grid:

110/10 kV transformer: ETTF = 50 years; repair time = 90 days. 10 kV/400 V transformer: ETTF = 500 years; repair time = 44 hours.

2.3.6. Lonsdale and Hitchen [1977) use the following value for transformers 33/11 kV and 33/6.6 kV in a public distribution network in the Northwest of England. Data were based on an examination of system performance over recent years.

ETTF = 170 years.

2.3.7. McNab [1977] uses the following values for 33/11 kV transformers in a public distribution network in the South of Scotland:

ETTF = 40 years; repair time = 14 hours.

2.3.8. Snaith [1977] uses the following values for 3.3 kV/415 V transformers in the electricity supply for a nuclear power station:


ETTF = 170 years; repair time = 24 hours. maintenance interval = 1 year;

2.3.9. Allan, Dialynas and Homer (1979) use the following values for 33/11 kV transformers in a reliability study of a distribution system:

ETTF = 500 years; repair time = 343 hours; time to maintenance = 4 years; maintenance time = 8 hours.

2.3.10. Allan et al. [1980] use, in a reliability study of electrical auxiliary systems of a power station, the transformer reliability data shown in Table 6.

ETTF (ye.rs) repair ti. ti. to dur.tfon of active passive maintenance 118 f ntenance

400/23.5 kY 17 vr 18 vr 144 hr. 1 vr 168 hr. 132/11 kY 37 vr 42 vr 196 hr. 1 vr 168 hro 23.5/11 kY 70 vr 170 vr 196 hra 1 vr 168 hrs

11/3.3 kY 70 vr 170 vr 120 hrs 1 vr T2 hrs

3.3 kY/415 Y 70 yr 170 yr 120 hrs 1 yr T2 hrs

Table 6: values used by Allan et al. [1980J.

2.3.11. Nelson and Johnson [1982) compare three distribution voltages for the power supply to a gas centrifuge uranium enrichment plant. They use the following reliability data (based on 1974 IEEE survey and other available data) for power transformers:

345/13.8 kV : ETTF = 280 years; 345/24.9 kV : ETTF = 250 years; 345/34.5 kV : ETTF = 230 years.

2.3.12. Koval [1983) uses for a 50 MVA transformer in an industrial power system: ETTF = 85 years; replacement time = 168 hours; restoration time = 12.0 hours.

2.3.13. Williams and Mudge [1983) use values drawn from distribution system statistics. For failures of a 33/11 kV transformer they use:


2.3.14. Sillinton and Goel [1986) use the following data for 25 kVA, 14.4 kV/240 V pad-mounted transformers in the reliability analysis of an existing 14.4 kV distribution network in Saskatchewan, Canada:

ETTF = 200 years; replacement time = 48 hours.

2.3.15. Dialynas and Allan [1986] use for 33/11 kV transformers, when including local generating facilities in the reliability evaluation of power distribution systems.

ETTF = 500 years; repair time = 343 hours; time to maintenance = 4 years; duration of maintenance = 8 hours.


2.3.16. Anderson et al. [1987) use the following values for HV/MV transformers in their study after the reliability of HV substations:

mechanical failures ETTF = 130 years; repair time = 12 hours

electrical isolation failures ETTF = 750 years; repair time = 120 hours

maintenance is performed every 330 days during 12 hours

2.3.17. Billinton (1987) uses for 14.4 kV/LV transformers in the Anaheim distribution system of the Saskatchewan Power Corporation the following data:

ETTF = 200 years; replacement time = 48 hours;

2.3.18. Dialynas and Allan [1987] describe a reliability model for a power distribution network with local generation. For transformers 33 kV/11 kV they use:

ETTF = 500 years; repair time = 342 hours

2.3.19. Goldberg et al. (1987) use the following values for distribution transformers:

ETTF = 500 years; repair time = 5.5 hours.

2.3.20. Kevers (1987) uses for 150 kV/ 10 kV transformers values of


The values are based on failure statistics in high-voltage networks.

2.3.21. Dialynas (1988) presents a reliability analyses of a Greek 150/20 kV transmission and distribution system. He uses the following values for failures in power transformers:

permanent failures temporary failures

: ETTF = 7.6 years. : ETIF = 21 years.

The transformers present in the system under study are 150/20 kV transformers.

2.3.22. Dialynas and Papadopoulos (1989) have determined the reliability of a. "typical" 20 kV rural distribution system in Greece. For a 150kv/20kV transformer they use:

ETTF = 25 years.

2.3.23. Fransen [1989] uses, in a reliability study of an industrial supply, for shortcircuits in transformers a value of

ETTF = 88 years; repair time = 11.5 hours


This value is based on failure statistics during 7.5 years in the power system of a large chemical plant in The Netherlands.

The transformers under study were mainly 30 kV/10 kVand 10 kV/ 2 kV.

2.3.24. Roos [1989] compares the supply reliability for different configurations of medium-voltage power systems. For independent failure of HV /L V transformers he uses the following values:


2.3.25. Whiting [1989] determines the reliability of power-supplies to broadcast transmitting stations. Component data are taken from various sources. For 11 kV/415 V transformers he uses:

ETTF = 50 years; expected repair time = 120 hours.

2.3.26. Horton et al. [1989a] determine the reliability of service of an underground 21 kV feeder of Pacific Gas and Electric. They have used the following data for distribution transformers:


2.3.27. Horton et al. [1989b] use their Distribution Reliability Assessment Model [DREAM] to compute reliability indices for distribution feeders. They use the following data for power transformers:

ETTF = 500 years; repair time = 330 minutes

2.3.28. Duke et al. [1989] assess the reliability of an industrial distribution system. For 66/11 kV transformers they use:


2.3.29. Allan and Inga-Rojas [1990] describe a method for distribution system reliability. In an example they use for transformers:

ETTF = 9 years

2.3.30. Sallam et al. [1990] calculate, as an example, reliability indices for the MVnetwork of Port-Fouad, Egypt. They have used the following data for power transformers:

HV/MV transformers: ETTF = 330 years; repair time = 130 hours

MV/LV transformers: ETTF = 250 years; repair time = 1.85 hours.

2.3.31. Mohan Rao and Sekhar [1990] compare the reliability of a number of distribution systems. They use the following values for 13.8 kV/480 Volt transformers:



2.3.32. Allan at al. [1991] present a reliability test system for distribution networks. The ETTF values used for power transformers are summarized in Table 7.

voltaaes cermanent fai lures active failures temoorarv failures 138/33 kV 100 vr 100 vr 20 vr 33/11 kV 70 vr 70 vr 20 vr 11 kV/415 V 70 vr 70 vr

Table 7: ETTF values used by Allan et al.[1991J

2.3.33. Dialynas and Koskolos [1991] determine the reliability performance of industrial power systems. They study an existing industrial system in Greece. For transformer failures they use the following values:

permanent failures temporary failures

: ETTF = 7.6 years ; repair time = 89.0 hours. : ETTF = 21.2 years ; repair time = 1.5 hours.

Maintenance is performed once every 2 years during 19 hours.

The transformers present in their study are 20 kVI 6.6 kV 10 MVA, 20 kV/ 380 V 1.6 MVA, and 6.6 kV/380 V 1.25 MVA.

2.3.34. Prescott et al. [1991] use the following values for the failure of a distribution transformer:


2.3.35. Volkmann et al. [1991] use a value of ETTF = 500 years for distribution transformers. The value has been determined from failure reports by Pacific Gas and Electric.

2.3.36. Kj"lIe and Sand [1991] use the following values for transformers in distribution systems in Norway:

22 kV rural network: ETTF = 100 years; repair time = 3.0 hours;

11 kV urban network: ETTF = 600 years; repair time = 7.8 hours.

2.3.37. Warren [1992] examines different UPS system configurations to formulate the most reliable system. The failure rates used are based on field statistics and MIL-HDBK-217-E. For MV-LV transformers he uses:

ETTF = 300 years.


2,4. Ageing data

2.4.1. Parascos and Arceri (1976) present bathtub curves for underground network transformers. Only "internal winding failures" have been taken into account. The resultys are shown in Figure 1. The numbers with the curves show the production years. Newer types show less wear-out but (relatively) more wear-in.

Figure 2 shows the results for "heavy tank corrosion requiring removal". As expected. only a wear-out phase is visible.

w ... " '" w

'" ~ ~ ., ~

~

" z z

0.3

0.2

" ae 0.1

\

\ , \ \

\ \ \

, I

/ 1939-1952

1953·1962

2 , 6 B 10 12 14 16

AGE OF EQUIPMENT (YEARS)

Figure 1: Failure rate of underground network transformers. internal winding failure. According to Parascos and Arceri [1976J.

w ...

0.40

~ 0.30 w

'" ~ ~

;; ~ 0.20

'" ~ z z " .. 0.10

5 10 1S 20 25 30 35 40

AGE OF EQUIPMENT (YEARS)

Figure 2: Failure rate for underground network transformers; heavy corrosion of tank material. According to Parascos and Arceri {1976J.


2.4.2. The IEEE performed a survey after failures of power transformers in industrial plants and commercial buildings in 1979 [Aquilino, 1980, 1983). Table 8 gives some results on ageing of liquid filled power transformers with rated power between 300 and 10,000 kVA. Age is defined in this survey as the age at the end of the reporting period.

Age ETTF (years) conf. interval 1 ~ 10 years 138 [90 3251

11 - 25 years 188 [135 3001

above 25 years 165 [140,2801

Table 8: Influence of age.

2.4.3. Cigre held an international survey on failures in large power transformers in service in 1978 [CIGRE, 1983). Data was received from 13 countries (Australia, Austria, Belgium, Canada, Czechoslovakia, France, Finland, Italy, japan, Switzerland, United Kingdom, USA and USSR). The analysis took in more than 1000 failures that occurred between 1968 and 1978, relating to a total population of more than 47,000 unit-years. Some results for ageing are summarized in Table 9.

60 - 100 kV 100 - 700 kV age (yrl ETTF (yrl c. i. ETTF (yrl c. i .

o - 5 36 [28 481 57 [48 701

5 - 10 49 [38 681 49 [41 601

10 - 20 58 [48 741 43 [37511

Table 9: influence of age.

2.4.4. Bar et al. [1990) use a Delphi-method to determine the position of the knee in the bath-tube curve, i.e. the place where ageing starts to become important.

For "oil-filled" MVILV transformers they find in case of good circumstances : 42 years; average circumstances : 35 years; bad circumstances : 25 years.

For cast-resin isolated transformers they find in case of good circumstances : 38 years; average circumstances : 32 years; bad circumstances : 21 years.

2.4.5. Ducket and McDonough [1990) have performed a study after the ageing of transformers. During 5 years they have observed the failure behaviour of a population with a known age structure. The data has been provided by California, Power and Light. Over 8600 events of failure, which resulted in pole type transformers being removed from the system and scrapped, occured during the years 1984 through 1988. These units were purchased from a single supplier between the years 1947 and 1988. The information available is: the number of transformers purchased for each year between 1947 and 1988; the number of failures for each year between 1984 and 1988. From this information an estimation is made of the failure rate as a function of time. The result is shown in Figure 3.


" 1000r---------------------------------,

100

10

0.1L---~-----L----~--~~--~----~

o 5 10 15 20 25 30

Yee,.

Figure 3: Failure rate of transformers as a function of their age, according to Bucket and McDonough [1990}.

For the first 5 years the failure rates are fairly accurate and can be assumed constant. This leads to a value of:

ETTF = 93 years; c.L = <85,103>

I slightly changed the data of Ducket and McDonough [1990]. The number of failures for the first year of operation has been doubled. For the first year they give a failure rate of 0.423 % whereas the failure rate for the years 2 through 6 is around 1 %. As transformers are put into operation the whole year through, but the counting of failures stops at the end of the year, the first year of operation is on the average just half a year.

2.4.6. Horton et al. [1990] provide estimates of failure rates of underground distribution system components. The estimates are based on information collected from a number of utility sources throughout the United States of America. For each component and each year they used the number of units installed or removed and the number of failures which occured. This data has been fitted to a Weibull distribution. From over 88,000 single-phase pad-mounted transformers installed by the NELPA (Northwest Electric Light & Power Association) utilities between 1968 and 1988 they find a constant failure rate:

ETTF = 350 years; c.L = [340,370].


2.5. Conclusions

Table 10 and Table 11 summarize the data on transformer life time given in this chapter. Table 10 gives recommended data from surveys, where Table 11 gives data used in reliability studies. A distinction has been made between MV/LV transformers (e.g. 10kV/400V). MV/MV transformers (e.g. 33/11 kV) and HV/MV transformers (e.g. 150/10kV).

Section Reference MV/LV MY/MY HV/MV unknown 2.1.1. Green and Bourne 1972 190 29·57 16 2.1.2. IEEE 1983 340-875 2.1.3. Kloeppel et al. 1990 100 50 20-30 2.2.1- Dickinson 1962 110-130 50 2.2.2. Todd 1964 180 70 2.2.3. Connor and Parkins 1966 200-400 70 2.2.4. IEEE 1974 270-400 77

2.2.5. AQUilino 1980. 1983 65-170 2.2.6. CIGRE 1983 32-65 2.2_7. van Amelsfoort et at. 1986 650 2.2.8_ Walln8ns 1986 500 2_2.9. VOEN 1987 950 2.2_10_ Pi j Is 1988 280 2.2.11. Franke 1990 85 2_2.12. BoVY et al. 1991 625 2.2.13. Bruggeman et at. 1991 550 2.2.14_ Verplanke 1991 20

Tab/e 10: Summary of transformer life times: recommended values and data from surveys_

Most surveys result in an ETTF value in the order of 500 years for MV/LV transformers. Deviations from this value are given by: Green and 80urbe [1972] (data from the 60's 7); Kloeppel et al. [1990] (data from Eastern Germany 7); Dickinson [1960] (data from the 50's 7); and by Franke [1990] (data from Eastern Germany 7). Values around 500 years are also used in most of the reliability studies. Values below 100 years can clearly be described as non-realistic. From the surveys a clear increase in reliability is visible in the course of time: from about 100 years in the 50's to over 500 years for the latest surveys. Improved manufacturing and maintenance clearly are fruitfull.

MV/MV transformers are less reliable than MV/LV transformers. Lifetimes of 50 to 75 years seem to follow from the surveys. Higher values are given by Todd [1964] and Pijls [1988]. A lower value is given by Verplanke [1991]. The latter two (both data from chemical plants in The Netherlands) use a small population, which could explain their deviating values. It might also be due to the specific operating condition. Some reliability studies use a value of 500 years. Apparantly the value for MV/LV transformers has been used. Most other studies use values consistent with the results from surveys.

HV/MV transformers are even less reliable than MV/MV transformers, with lifetimes between 25 and 30 years. Some reliability studies however use lifetimes up to 330 years. These values are certainly not realistic.

MV/MV as well as HV/MV transformers become somewhat better during the years, due


to improved manufacturing and maintenance. This improvement in time can also be concluded from Figure 1.

From the data presented in this chapter I would suggest the values given in Table 12.

Section Reference MY/LV MY/MY HV/MY u:>known

2.3.1. Dickinson 1957 300 2.3.2. Dickinson 1960 350 2.3.3. Gangel and Rinal .. 1968 200·1000 2.3.4. Guertin and Lamarre 1975 10 2.3.5. Bocker and Kaufman 19n 500 50 2.3.6. Lonsdale and Hitchen 19n 170 2.3.7. McNab 19n 40 2.3.8. Snaith 19n 170 2.3.9. Allan Dialvnas Homer 1979 500 2.3.1D. Allan et at. 1980 70 70 17·37 2.3.11. Nelson and Johnson 1982 230·280 2.3.12. Koval 1983 85 2.3.13. WiLLians and Mudge 1983 80 2.3.14. Billinton and Goel 1986 200 2.3. IS. Oistynes and Allan 1986 500 2.3.16. Anderson et at. 1987 lID 2.3.17. Billinton 1987 2DD 2.3.18. Dlstynes and Allan 1987 500 2.3.19. Goldbero et al. 1987 500 2.3.20. Kevers 1987 IS 2.3.21. Dialynas 1988 7.6 2.3.22. Dialynas and Papadopoulos 1989 25 2.3.23. Fransen 1989 88 2.3.24. RODS 1989 650 2.3.25. \/hitin •. 1989 50 2.3.26. Horton et at .. 1989a 500 2.3.27. Horton et at. 1989b 500 2.3.28. Duke et at .. 1989 40 2.3.29. Allan and Inga-Ro"ss 1990 9 2.3.30. Sallam et at .. 199D 250 33D 2.3.31. Mohan Roa and Sekher 199D 300 2.3.32. Allan et at. 1991 70 70 100 2.3.33. Dialynas and !Coskalos 1991 7.6 7.6 2.3.34. Prescott et at. 1991 42 2.3.35. volkmam et at. 1991 500 2.3.36. Kj.lle and Sand 1991 100·600

2.3.37. Warren 1992 300

Table 11: Summary of transformer life times: data used in reliability studies.

existing transformers new transformers MV/LV transformers 400 . 60D years 500 .. 1000 vears

MY/MY transformers 50 .. 75 years 75 . 100 years HY/MV transformers 30 . 50 years 4D • 70 vears

Table 12: Suggested values for power transformers.

·20- power transformers

3. CIRCUIT BREAKERS AND SWITCHES


3.1.1. Green and Bourne [1972] give average component failure rates for electrical components. For circuit-breakers they give:

general (less than 33 kV): 415 V to 11 kV: 33 kV: 132 kV: 275 kV: 400 kV:

ETTF = 57 years ETTF = 76 years ETTF = 38 years ETTF = 29 years ETTF = 16 years ETTF = 11 years

3.1.2. The IEEE gold book [IEEE. 1990] recommends the ETTF-values given in Table 13. all values are in years:

o - 600 V All All 'vee. 0-600A 600 A + 600 V +

Fixed circuit breaker 192 240 285 105 57

Metal-clad circuit breaker 330 370 430 330 280

motor starter 72 65

enclosed disconnect switch 165

Table 13: recommended values (ETTF in years) according to the IEEE gold book.

3.1.3. Kloeppel et al. [1990] present recommended values of component data for reliability studies. They were based on data from several industries in Eastern Germany as well as from the public supply. For circuit-breakers the following values are recommended:

below 1 kV: ETTF = 650 years; range = <200. 1000> repair time = 4 hours; range = <3.8>

6. 10 kV: ETTF = 135 years; range = <85.150> 20 kV: ETTF = 75 years; range = < ...• 115>

repair time = 12 hours; range = <12.27> 30 kV: ETTF = 26 years; range = <12.27> 110 kV open: ETTF = 48 years; range = <37.63>

repair time = 15 hours; range = < 12.24> 110kV closed: ETTF = 1000 years

repair time = 60 hours; range = <36. 72>

For disconnect switches the following values are recommended:

below 1 kV:

10-30 kV:

100 kV:

ETTF = 300 years; repair time = 3 hours. ETTF = 400 years; range = <200.600> repair time = 3 hours. ETTF = 110 years; range = <85. 140> repair time = 12 hours.

-21- circuit breakers and switches

3.2. pata from surveys

3.2.1. Dickinson (1962) reports on an AlEE Survey undertaken in 1959. Responses were received from 33 compagnies, covering 85 plants. For circuit breakers the results are:

Fixed Circuit breakers above 15 kV: 2.4 - 15 kV:

ETTF = 70 years, sample too small ETTF = 85 years, c.L < 60, 150 yr>

Draw-out metal-clad circuit breakers above 600 V: ETTF = 96 years, c.L = <70, 140> 600 V and below: ETTF = 61 years, c.L <53,73>

Circuit breakers used as motor starters. above 600 V: ETTF = 23 years; c.L = < 19, 30>

Contactor-type motor starters above 600 V: ETTF = 50 years; c.i. = <41,66> 600 V and below: ETTF = 57 years; c.L = <54,60>

The reported repair time is:

Circuit breakers used as motor starters: above 600 V: average 14-27 h: min. 2 h; max: 240 h

Contactor-type motor starters above 600 V: average 15-17 h; min. 0 h; max: 32 h 600 V and below: average 5-8 h; min. 1 h; max: 125 h

For disconnect switches the results are:

Open switches: Enclosed switches:

ETTF = 102 yr, c.i. = <80,140 yr> ETTF = 230 yr, c.L = < 170, 365 yr>

3.2.2. Connor and Parkins (1966) report about a 14-year survey of failures in networks with nominal voltages between 2 and 33 kV. The results for switchgear are given below. The number of switchgear units in service has increased from 12,000 in 1951 to 23,000 in 1965. The authors note that the fault incidence of switchgear of all voltage has remained practically constant over the period under consideration. The manufacture of smaller units, to reduce costs, appears to be cancelled out by improving materials and manufacturing techniques.

Circuit breaker failures ETTF = 6500 years; Tripping or closing mechanism ETTF = 4000 years; A.R. tripping or closing mechanism ETTF = 12,000 years; Current transformers ETTF = 2700 years; Voltage transformers ETTF = 17,000 years; Other failures of outdoor switchgear ETTF = 3300 years; Other failures of indoor switchgear ETTF = 3400 years; Small wiring and auxiliary switches ETTF = 12,500 years;


Failure of metalclad fuse switch Failure of metalclad

oil-immersed isolator or switch Failure of air-break isolator Miscellaneous Total

ETTF = 1400 years;

ETTF = 7100 years; ETTF = 10,000 years; ETTF = 1100 years; ETTF = 290 years.

3.2.3. Selseth [19721 presents results from a survey of outage records at the Norwegian State Power System, over the period 1967-1970. The survey covered a population of 2,479 circuit-breaker years. The following results are derived for circuit breakers:

ETTF = 25 years; c.i. = <20,30>

A subdivision of failures yields: mechanical parts : 30%; electrical parts : 24%; unknown parts : 12%; others : 34%.

3.2.4. An IEEE survey of electrical equipment reliability in industrial plants was completed in 1972 IIEEE,19741. This survey included a total of 1982 equipment failures that were reported by 30 compagnies covering 68 plants in nine industries in the United States and Canada. The results for circuit breakers are given in Table 14.

average actual estimated restore down time (hours) time (hours)

ETTF (vears) c.L industry Dlant reoair restore

Fixed tYDe All voltages 194 [140 310] 5.8 4.0 31.7 4.5 Below 600 Volt 225 [160 375] 4.7 4.0 6.0 2.0 Above 600 Volt 57 [30 500] 10.6 3.8 44.5 12.0

Metalclad drawout All voltages 330 [260 425] 129 7.6 54.2 3.9 Below 600 Volts 370 [280 560] 147 4.0 47.2 2.9 Above 600 Vo l ts 280 [210 430] 109 168 62.4 5.2

Disconnect switches Enclosed 160 [130 220] 3.6 2.8 50.1 13.7 Open 350 [170 ...

Table 14: Results from 1972 IEEE survey {IEEE, 19741.

The last four columns give the industry average and the plant average of the actual down time per failure and the average estimated time to fix the failure during 24 hour work day in case of repair of the failed component and in case of replace with a spare.

A subdivision to failure characteristic yields for circuit breakers: fail-to-close : 5% failed while opening : 9% mal-trip : 44% damaged while successfully opening: 7% damaged while closing : 2% failed while operating : 32%


A subdivision to damaged part yields: insulation : 21 % mechanical : 18% auxiliary or protective device : 34% other : 27%

3.2.5. Lonsdale and Hitchen [1977] give the following data based on an examination of system performence over recent years:

Switchgear 33kV, 11 kV, 6.6 kV: ETIF = 500 years.

3.2.6. Mazza and Michacca [1981] as well as Heising [1986] presents the results of a worldwide survey after failure of circuit breakers in the voltage range above 63 kV. The survey was performed by working group 13.06 of CIGRE between 1974 and 1977. Their results are summarized in Table 15 and Table 16. A distinction is made between major failures and minor failures. A major failure occurs when the breaker can no longer perform all of its fundamental functions, or when intervention within 30 minutes is necessary. All other failures are referred to as minor failures. The results for the major failures are shown in Table 15; those for the minor failures in Table 16.

reoair time

vol tOQe ETTF c.i. average modi ...

63·100kY 250 yr <200 300. 29 hrs 5 hr. 100·200kY 60 yr <55 70> 95 hrs 12 hrs

200-300kY 40 yr <35 45. 60 hr. 11 hr. 300-500kY 22 yr <19 25,. B4 hr. l' hra >500kY 10 yr <8 12' 142 hr. 27 hr.

Table 15: Results for major failures, according to Cigre survey [Heising, 1986J_

The data below holds for the voltage range from 63 to 100 kV. The data for other voltage classes is too far away from our area of interest.

Does not open on command ENTF = 60,000; c.L <35,000, 130,000>

Does not break the current ENTF = 400,000

Does not close on command ENTF = 18,000; c.L < 13,000, 26,000 >

Failures without a command to open or close ETIF = 450 yr; c.i. = <350,600 yr>

(ENTF = Expected Number of operations To Failure)

The repair time has been defined as the time required to get to site, analyse the failure, obtain spare parts, repair and return the circuit-breaker to service, excluding deliberate delays.

-24- circuit breakers and switches •

repair time in hours voltaae ETTF (vears) c.; . average medh.rn

63-100kV 60 [55 6n 16_5 5_0

100-200kV 24 [22 26] 19_9 5_0

200-300kV 16 [14 In 27_7 6_0

300-500kV 6 [5 n 73_1 8_0

>500kV 20 [15 40] 58_2 9_0

Table 16_- Results for minor failures, according to Cigre survey [Heising, 19861_

A subdivision to origin of the failure yields for major failures: mechanical : 70 % electrical (main circuit) : 10 % electrical (auxiliary and control circuit) : 20 %

A subdivision to cause of the failure yields: design or manufacture : 45 % incorrect erection : 10 % incorrect maintenance : 8 %

Condition of the circuit-breaker when failure was discovered:

major failures in service during maintenance

minor failures in service during maintenance

: 90 % : 5 %

: 70 % : 25 %

Characteristics of the major failure: Does not close on command : 33.7 % Does not open on command : 14.1 % Closes without command : 1.7 % Opens without command : 5.2 % Does not make the current : 1.6 % Does not break the current : 1.9 % Fails to carry current : 2.5 % Internal breakdown : 5.3 % External breakdown : 4.0 %

For the major failure "internal or external breakdown" the following values are derived from the survey results:

63-100 kV : ETTF = 4000 years ; c.i. = <2000,11000> 100-200 kV : ETTF = 550 years ; c.i. = <400,750> 200-300 kV : ETTF = 600 years : c.i. = <400,1100> 300-500 kV : ETTF = 400 years ; c.i. = <250,800> >500 kV : ETTF = 60 years ; c.i. = <40,110>


3.2.7. According to Heising (1983) the expected life time of vacuum power circuit breakers is:

ETTF = 1.400 years.

This value has been based on field experience between November 1971 and January 1982.

3.2.8. Waumans (1986) presents the result of failure registration in a number of medium and low-voltage networks in the Netherlands, from 1979 to 1984. For MV circuitbreakers he gives:

ETTF = 500 years

For disconnect switches he finds:

ETTF = 1000 years.

3.2.9. One of the Dutch regional electricity authorities, N.V. PNEM, performed a survey after the reliability of power system components in their 10 kV network, covering the period 1980 - 1986 [van Amelsfoort et aI., 1986). For circuit breakers the survey resulted in:

ETTF = 350 yr c.i. = [280,440)

The survey resulted for disconnect switches in:

ETTF = 6000 yr c.i. = [4000,11000).

3.2.10. Stanek and Venkata (1988) compare failure rates of equipment in four different coal mines. For "molded-case circuit breakers 400-A frame size" they report:

mine 1: ETTF = 4.3 years; mine 2: ETTF = 65 days;

repair time = 1.25 h; mine 4: ETTF = 0.5 years;

repair time = 2.0 h.

The authors conclude that these failure rates are one to three orders of manitude higher than those of similar devices in other industries.

3.2.11. Norris (1989) reports on an IEEE survey conducted in 1985 on the reliability of circuit breakers in industrial and commercial installation. Due to the low number of responses, 13 plant location, detailled analysis was useless. The results are summarized below:

0-600 Volt air-magnetic: ETTF = 130 year, c.L = <90,200> ETTFO = 13 year ETTNFO = 44 days


repair time (average failure duration) = 2.8 hours

601 - 15,000 Volt air-magnetic ETIF = 170 year ;sample too small ETIFO = 740 days ETINFO = 10 days repair time = 2.25 hours

34.5 - 138 kV bulk oil ETIF = 21 year, c.L = < 13,64> ETIFO = 680 days ETINFO = 16 days repair time = 41.11 hours

345 kV air blast and SFs ETIF = 11 year, c.i. = <8,19> ETIFO = 215 year ETINFO = 11 days repair time = 171.45 hours

ETIFO = Expected Time To Fault Operation ETINFO = Expected Time To Non-Fault Operation

For increasing rated voltages the time-to-failure of the circuit breakers decreased considerably where as the repair time increases. The non-availability increases from 0.006 % for low-voltage breakers to 4 % for 345 kV breakers.

For air-magnetic breakers the ETIF is about equal for low-voltages and medium-voltages despite the far greater amount of operations for medium-voltage breakers.

3.2.12. Radwan and EI-Marsafawy [1990] present failure data for MV circuit-breakers collected from 1980 to 1984 in the Upper Egypt Zone of the Egyptian Unified Power System network. The total number of circuit breakers considered was 881 (107 indoor and 774 outdoor); leading to a population of 3054 unit-years.

66 kV circuit breakers; all types ETIF = 12 years; c.i. = <9,20> repair time = 3.6 hours

66 kV oil-filled circuit breakers ETIF = 35 years; c.L = < 18, ... )

66 kV air-blast circuit breakers ETIF = 5 years; c.L = <3,8>

33 kV circuit breakers; all types ETIF = 23 years; c.L = < 18,35 > repair time = 3.8 hours

33 kV oil-filled circuit breakers ETIF = 21 years;c.L = <15,35>

33 kV air-blast circuit breakers ETIF = 25 years; c.L = < 15, 70>

11 kV circuit breakers; all types


ETTF = 49 years; c.L = <37.70> repair time = 2.1 hours

6.6 kV circuit breakers; all types ETTF = 100 years (only two failures reported) repair time = 85 hours.

A subdivision to origin of failure yields:

66 kV circuit breakers electrical insulation to ground: 63 % electrical controls: 13 % seals gaskets: 10 %

33 kV circuit breakers seals gas: 50 % operating mechanism: 35 %

11 kV circuit breakers air system: 28 % operating mechanism: 28 % isolating contacts: 14 %

A subdivision to cause of failures yields:

66 kV circuit breakers pollution: 60 % (repair time = 1 hour) wear/ageing: 20 % (repair time = 13 hours)

33 kV circuit breakers wear/ageing: 60 % (repair time = 2 hours) maintenance: 5 % (repair time = 14 hours) others: 35 % (repair time = 20 minutes)

11 kV circuit breakers wear/ageing: 50 % (repair time = 50 minutes) installation: 20 % (repair time = 1 hours)

3.2.13. Franke [1990] observed. during 12 years. failures in a chemical plant in Eastern Germany. Early failures in the wear-in phase (1.5 ... 2 years) have been removed form the data.

6 kV circuit-breakers ETTF = 170 years; c.L = < 100. 250>

Included in this value are - short-circuit in the breaker - failure in the control part - incorrect intervention

3.2.14. Ishibashi et al. [1990] looked for vacuum failures of vacuum interrupters between 1965 and 1987. All known failures occurred in the first 8 years of production

1965-1973: 23 failures; 1.5 x 106 unit-years ETTF = 50.000 years


1974-1987: no failures; 8 x 106 unit-years ETTF > 1.000.000 years

3.2.15. Filter and Jones [1990] performed laboratory tests on loadbreak elbow connector opening. Their test program was designed to address the issue of switching speeds and their influence on switching reliability.

Average speed = 0.38 m/s; stationary piston sample reliability = 86.7 %; c.i. < 71,97 >

Average speed = 0.38 m/s; floating piston sample reliability = 100 %; c.i. < 92,1 00 >

Average speed = 0.61 m/s sample reliability = 100 %; c.i. <98,100>

Average speed = 1.17 m/s; sample reliability = 100 %; c.i. <98,100>

Average speed = 1.70 m/s; sample reliability = 99 %; c.i. <97,100>

3.2.16. Working Group 13-06 of CIGRE performed a follow up of their 1977 survey (discussed in Section 3.2.6) during 1988-1989 [Cigre, 1989]. Some of their results are given in Table 17.

regair time vol taae ETTF c. i. 8veraae mediLrn

63·100kV 400 vr <600 1500> 45 hr. 24 hr. 100·200kV 140 vr <100 150> 49 hr. 8 hrs 200·300kV 90 vr <75 125,. 70 hI's 10 hr. 300·500kV 60 vr <50 15> 199 hr. 10 hr. >500kV 55 vr <40 100> 464 hr. 120 hr.

Table 17: Results of 1988-89 Cigre survey {Cigre, 19911.

3.2.17. Bovy et al. [1991] analysed outage occurances in the 10 kV cable networks of a Dutch utility, during 6 years. For circuit breakers they find:

ETTF = 5,000 years; repair time = 3.2 hours ETTF for phase-to-earth faults = 10,000 years; probability of multiple fault following phase-to-earth fault: 41 %

3.2.18. Volkmann et al. [1991] give the following values for failures in underground distribution systems for the source of their data they refer to two internal reports by Pacific gas and Electric.

Non-load break elbow Molded splice Switch

: ETTF = 5,200 year : ETTF = 5,200 year : ETTF = 250 year

For overhead distributions they give, Switch (urban feeder) : ETTF = 1,300 year Switch (rural feeder) : ETTF = 800 year


These two values were computed from historical outage records for a group of 85 rural and 95 urban feeders. The feeders were selected from areas known to provide accurate and compehensive reports of distribution circuit failures. The data span a period of approximately 5 years.

The average repair times. from historical outage data. are:

Molded splice (rural) (urbani

Elbow (rural and urbani

overhead switch (rural) (urban)

underground switch (rural)

recloser (rural) (urban)

(urban)

: 303 minutes : 205 minutes : 210 minutes

: 95 minutes : 116 minutes : 79 minutes : 226 minutes : 200 minutes : 74 minutes



3.3.1. Dickinson [1957) gives a typical value of

ETTF = 100 years

for circuit breakers. Typical values of the repair time are between 4 and 8 hours.

3.3.2. Dickinson [1960) uses the following values for power distribution systems for refinery process units. These are based on several published papers and on service records obtained from refineries.

Circuit breakers, fixed : ETTF = 200 years ; repair time = 40 hours.

Circuit breakers, drawout : ETTF = 250 years ; repair time = 4 hours.

Switches, integral with equipment : ETTF = 1000 years; repair time = 40 hours.

Switches, single throw : ETTF = 500 years ; repair time = 40 hours.

3.3.3. Capra et al. [1969) use the following data for a reliability analysis of 12 kV underground distribution systems:

Switches, 600A

Switches, 200A

: ETTF = 500 - 2000 years;

: ETTF = 500 - 2000 years;

3-phase interrupters, 600A : ETTF = 100 - 200 years;

3-phase interrupters, 200A : ETTF = 100 - 200 years;

1-phase interrupters, 100A : ETTF = 200 - 500 years.

3.3.4. Grover and Billinton [1974) use the following data for a study after substation reliability:

High-voltage breakers short-circuits incorrect trip stuck-breaker probability

Low-voltage breaker short-circuits incorrect trip stuck-breaker probability

disconnect switches short-circuit mal-trips

: ETTF = 30 year : ETTF = 5 year : 0.5 %

: ETTF = 100 year : ETTF = 100 year : 6 %

: ETTF = 50 years : ETTF = 5 years


The average outage duration is 2.09 hours. the switching time 3 hours. Maintenance is performed every 4 years during 4 hours.

3.3.5. Guertin and Lamarre (1975) performed a reliability analysis of a high-voltage substation. For high-voltage circuit-breakers they use

ETTF (fault) = 30 years; repair time = 70 hours; maintenance every 4 years during 20 hours;

ETTF (mal-trip) = 5 years: repair time = 2.0 hours

stuck-breaker probability = 1 % repair time = 3.0 hours

For low-voltage circuit-breakers they use ETTF (fault) = 100 years; repair time = 50 hours; maintenance every 10 years during 5 hours;

ETTF (mal-trip) = 30 years repair time = 2.0 hours

stuck-breaker probability = 4 % repair time = 3.0 hours

For load-break switches they use: ETTF (fault) = 50 years repair time = 10 hours maintenance every 5 years during 2 hours; ETTF (mal-trip) = 50 years; repair time = 2 hours stuck-breaker probability = 3 % repair time = 2 hours

3.3.6. Bocker and Kaufmann [1977) use the following values for a public distribution network:

Switchgear bay 10 kV with circuit breaker: ETTF = 140 years; repair time = 9 hours.

Switchgear bay 10 kV with load-break switch: ETTF = 300 years; repair time = 9 hours.

3.3.7. Allan et al. [1977) use the following data for 11 kV circuit breakers in the electrical auxiliary systems of power stations:

Active failures : ETTF = 67 years; Passive failures : ETTF = 200 years;


repair time stuck probability time to maintenance duration of maintenance

= 20 hours; = 0.1 %; = 1 year; = 50 hours.

3.3.8. McNab [1977) uses the following values for a public distribution network in the South of Scotland:

33 kV circuit breakers: ETTF = 110 years; repair time = 3.6 hours.

11 kV circuit breakers: ETTF = 72 years; repair time = 2.8 hours.

3.3.9. Snaith [1977) uses the following values for the electricity supply of a nuclear power station:

3.3 kV or 415 Volt circuit breaker ETTF = 200 years time to maintenance = 0.5 years repair time = 2 hours

Contactor starter ETTF = 500 years time to maintenance = 0.2 years repair time = 2 hours

3.3.10. Chang [1977) uses the following data for load break connectors to evaluate distribution system design:

ETTF = 300 years; replace time = 4 1/2 hours.

3.3.11. Allan et al. [1979) use the following values in a reliability study of a distribution system:

33 kV circuit breaker ETTF = 200 years repair time = 75 hours time to maintenance = 4 years maintenance time = 8 hours

11 kV circuit breaker ETTF = 500 years repair time = 18 hours time to maintenance = 4 years maintenance time = 8 hours

11 kV and 33 kV isolators ETTF = 1000 years repair time = 75 hours time to maintenance = 4 years maintenance time = 8 hours


3.3.12. Allan et al. (1980) use, in a reliability study of electrical auxiliary systems of a power station, the circuit breaker reliability data shown in Table 18. They further use:

Stuck probability of normally closed breakers : 0.1 % Stuck probability of normally open breakers : 1.3 %

~ ETTF repair

active passive ti.

(years) (years) (hours)

400 kV 6 20 54 132 kV 40 125 45 11 kV 200 140 48 3.3 kV 200 140 36 415 V 200 140 36

time to I duration of maintenance

(years) (hours)

1 168 1 168 1 24 1 8 1 24

Table 18: Circuit breaker data used by AI/an et al. [1980}.

3.3.13. Adams and Jasmon (1981) use the following data for circuit breakers in a distribution system:

Active failures : ETTF Passive failures : ETTF Repair time Stuck probability Time to maintenance Duration of maintenance

= 10 years; = 7 years;

= 20 hours; = 0.5 %; = 2 Yz years; = 12 hours.

3.3.14. Nelson and Johnson (1982) compare three distribution voltages for the power supply to a gas centrifuge uranium enrichment plant. They use the following reliability data (based on 1974 IEEE survey and other available data) for circuit breakers:

13.8 kV 24.9 kV 34.5 kV

: ETTF = 278 years; : ETTF = 265 years; : ETTF = 253 years.

For switches they use:

13.8 kV 24.9 kV 34.5 kV

: ETTF = 670 years; : ETTF = 450 years; : ETTF = 330 years.

3.3.15. Ruoff and van Meteren (1983) use a value of

ETTF = 380 yr

for a circuit breaker being normally closed and

ETTF = 750 yr

for a circuit breaker being normally open. This applies to short circuits in circuit breakers in distribution systems.


3.3.16. Williams and Mudge (1983) use a value of

ETTF = 500 year

with a repair time of 75 hours for 33 kV circuit breakers and a value of

ETTF = 800 year

with a repair time of 5 hours, for 11 kV circuit breakers. The values have been drawn from distribution system statistics.

3.3.17. Koval (1983) uses for circuit breakers in an industrial power system:

Opens without command: ETTF = 36 years; replacement time = 16 hours; restoration time = 20 hours.

Short circuit: ETTF = 140 years; replacement time = 48 hours; restoration time = 2.5 hours.

For switches he uses: ETTF = 300 years; replacement time = 8 hours; restoration time = 2.7 hours.

3.3.18. Declerq et al. (1985) compare two methods for HV/MV substation reliability. They use the following data for MV circuit breakers:


3.3.19. Dialynas and Allan (1986) use the same values as Allan et al. (1979) when including local generation facilities in the reliability evaluation of power distribution systems.

3.3.20. Anderson et al. [1987) use the following data for circuit-breakers in their study after the reliability of HV substations.

mechanical faults : ETTF = 250 years; : repair time = 12 hours;

electrical isolation failures : ETTF = 230 years; : repair time = 12 hours;

maintenance is performed every 240 days during 8 hours;

They use the following data for disconnectors:

mechanical faults electrical isolation failure

: do not occur : ETTF = 500 years


: repair time = 6 hours maintenance is performed every year during 2 hours.

3.3.21. Dialynas and Allan [1987] describe a reliability model for a power distribution network with local generation. For 33 kV circuit-breakers they use

ETTF = 200 years; repair time = 75 hours;

For 11 kV circuit-breakers they use ETTF = 500 years; repair time = 18 hours;

For isolators they use: ETTF = 1000 years repair time 75 hours

3.3.22. Goldberg et al. [1987] use the following values:

Switches ETTF: 250 years repair time = 1.5 hours

Splices ETTF = 1700 years (old splices) ETTF = 17000 years (new splices) repair time = 1.5 hours

Elbow ETTF = 1700 years (old elbows) ETTF = 17000 years (new elbows) repair time = 3.5 hours

3.3.23. Allan (1988) uses in a reliability study a value of ETTF = 100 years for a short circuit in a circuit breaker.

3.3.24. Dialynas [1988] uses a value of ETTF = 13 years for 150 kV circuit breakers and a value of ETTF = 9.6 years for 20 kV circuit breakers.

He uses a value of ETTF = 75 years for 150 kV isolators.

3.3.25. Dialynas and Papadopoulos [1989] use the following values:

ETTF = 300 years for a 20 kV circuit breaker.

ETTF = 300 year for a 20 kV recloser.

ETTF = 250 years for a 20 kV sectionaliser.

ETTF = 250 years for a 20 kV isolator.


3.3.26. Fransen [1989] uses a value of

ETTF = 90 years

for short circuits in circuit breakers. The value is based on failure statistics during 7% years in the power system of a large chemical plant. The repair time used is 8 hours. The voltage levels under study are mainly 30 kV. 10 kV and 2 kV.

3.3.27. Whiting [1989] determines the reliability of power supplies to broadcast transmitting stations. Component data are taken from various sources.

11 kV circuit breakers: ETTF = 85 years repair time = 48 hours

415 V circuit breakers: ETTF = 200 years repair time = 24 hours

3.3.28. Horton et al. [1989a] determine the reliability of service of an underground 21 kV feeder of Pacific Gas and Electric. They use the following component data:

Switches ETTF: 250 years repair time = 1.5 hours

Splices ETTF = 1700 years (old splices) ETTF = 17000 years (new splices) repair time = 1.5 hours

Elbow ETTF = 1700 years (old elbows) ETTF = 17000 years (new elbows) repair time = 3.5 hours

3.3.29. Horton et al. [1989b] use their Distribution Reliability Assessment model (DREAM) to compute reliability indices for distribution feeders. They use the following data:

Splice ETTF = 5300 years repair time = 90 minutes

Elbow ETTF = 5300 years repair time = 210 minutes

Switch ETTF = 250 years repair time = 90 minutes


3.3.30. Duke et al. [1989] assess the reliability of an industrial distribution system. For circuit breakers they use the following data:

66 kV circuit breakers ETTF = 300 years; repair time = 133 hours.

11 kV circuit breakers ETTF = 350 years; repair time = 51 hours.

3.3.31. Allan and Inga-Rojas [1990] describe a method for distribution system reliability. In an example they use for circuit breakers:

Open breakers probability failing to close: 1 % ETTF = 50 years

Closed breakers ETTF = 25 years

3.3.32. Sallam et al. [1990] calculated, as an example, reliability indices for the MVnetwork of Port-Fouad, Egypt. They use the following data:

13.8 kV circuit breakers: ETTF = 280 years repair time = 3 hours

Manual switches ETTF = 10,000 years repair time = 2.0 hours

Disconnect switches ETTF = 170 years repair time = 3.6 hours

3.3.33. Mohan Rao and Sekhar [1990] compare the reliability of a number of distribution systems. They use the following data:

13.8 kV disconnect switch (enclosed) ETTF = 160 years repair time = 3.5 hours

13.8 kV metalclad circuit breaker ETTF = 280 years repair time = 83 hours

480 Volt metalclad circuit breaker ETTF = 370 years repair time = 4 hours


3.3.34. Allan at al (1991) present a reliability test system for distribution networks. For this network they use for circuit breakers the ETTF data in Table 19.

permanent active terJ1lOrary failures failures fai lures

138 kV 175 vrs 285 vrs 20 vrs 33 kV 500 vrs 670 vrs 50 vrs 11 kV 170 vrs 250 vrs 17 vr.

Table 19. Circuit breaker data used by Allan et al. [1991 J.

3.3.35. Dialynas and Koskolos (1991) use a value of

ETTF = 9.61 yr

for permanent failures of circuit breakers. The repair time is 280 hours. Maintenance is performed every two years during three hours.

the stuck breaker probability used is 0.1 %.

The system under study contains circuit breakers of 20 kV, 6.6 kV and 380 Volt.

For a 150 kV breaker they use ETTF = 12.9 yr, with a repair time of 21.0 hours.

They use a value of ETTF = 75 yr for isolators (6.6 kVand 150 kV).

The repair time used is 50.0 hours.

Maintenance is performed once every two years during 3.0 hours.

3.3.36. Dortolina et al. (1991) use in a substation reliability evaluation study for circuit breaker failures:

ETTF = 66 years for active failures; ETTF = 33 years for passive failures.

The repair time used is, in both cases, equal 24 hours. Maintenance is performed once every three years during 24 hours.

They use a value of ETTF = 500 years for switches, with a repair time of 12 hours.

3.3.37. Kj"lIe and Sand (1991) give the following values for circuit breakers in distribution networks in Norway:

22 kV rural network ETTF = 59 years; repair time = 6.0 hours.

11 kV urban network ETTF = 77 years; repair time = 5.4 hours.


For disconnectors they give the following values:

22 kV rural network ETTF = 71 years; repair time = 1.5 hours.

11 kV urban network ETTF = 1400 years; repair time = 0.8 hours

3.3.38. Warren [1992] examines different UPS system configurations to formulate the most reliable system. The failure rates used are based on field statistics and MIL-HDBK-217E. For low-voltage circuit breakers the following value is used:

ETTF = 1650 years.


3.4. Ageing data

3.4.1. Wiseman [1976] presents "the actual bath tub curve for power circuit breakers". It is reproduced in Figure 4. The curve is based on more than 600,000 breaker service years.

3.4.2. Mazza and Michacca [1981] present the results of an enquiry on circuit-breaker failures and defects in service, observed during the period 1974-1977. The enquiry was performed by Cigr6 Working Group 13.06. The survey concerns 77,892 breaker-years in service, obtained from 102 utilities in 22 countries.

0 ...

~ hilUf. R8'8 r."..SHd in pIorcent per ,..e •

•

0 ~ /

~ /

, 3 • • • 7 I II 10

AGE IN YEARS

Figure 4: Circuit breaker failure rate as a function ofage, according to Wiseman [19761.

A distinction is made between major failures and minor failures. A major failure occurs when the breaker can no longer perform all of its fundamental functions, or when intervention within 30 minutes is necessary. All other failures are referred to as minor failures. This survey is discussed in detail in Section 3.2.6.

Figure 5 shows the failure rate as a function of age. For minor failures, it shows a steady decrease. For major failurs no trent is present .

c • , -, u

• 10

g 08 -~. . -~ •

u _

~ . , ~ --i E.o 6 u c - . = -- -u • ~ ~

~

• 0

~: .04 _ u -_ z • < ~ -- -~ ! < ~ ~ 0

~ ~ .02 ~ 0 ~ _ x

- ~ < < ~ -

l-

-

-

-

Q • • • • •

\

• \ , , , , • )$

\ \

I

2

P-, , , • , , , , , , • , • , • , • , • , Minor failures (.r) • Q , "15 Oefaillances .ineures • , , , , , , ,

• , , , ,

I '~ , , , ,

Major failures (MF) p../ OHaillanees ujeures

, , " --,

- " . " , r-- ;"', // "', ,

/ '0.. " .... ...0...

• , ' ..... --<l"' .... -' '..., l/ .~ '<\

, I ~ '1

4 6 8 10 12 YEARS

Figure 5: Relation between the failure rate and the age of circuit breakers according to Mazza and Michaca [19811.


3.4.3. Kawamura et al. (1990) observed the fail behaviour of 77 kV air-blast type circuit breakers used by The Kansas Electric Power Co., Kansai, Japan. The majority of the breakers was manufactured between 1965 and 1970.

The causes of fault were divided in three categories. initial failure (poor manufacturing, poor construction, etc.)

- random failure (natural disaster, contact of birds or animals, mistake - wear-out failure (natural deterioration, poor maintenance, etc.)

Figure 6 shows the failure rate as a function of age. The upper figure for all failures, the three lower ones after a subdivision for failure cause. The failure rate for all failures shown no clear trent, whereas the initial failures show a decreasing failure rate and the wear-out failures an increasing one.

Rat, of fults du 10 .11 ~.UH TOlal numHf of tinll;1 bu-.ken: 9S(J

(17kV Air blllli typt')

Uud ,II'

(I) [nitial r.il.,1 ,-. Poor IUIIII.ehrilil.

Balhillb ear"

.: W,ar-ollt r.illr~

_ I )1I;liol (.ilure \

: ~:Rr40m railll~ •.......

(D) Random f.ilnt ... Nalull diusler. (I) W,.r-od r.ihlrr ... Nahul dtteriontioA.

0.015

0.01

Contaet of birds. .. im,b, .tt.

M iSlah. ete

10 IS 20 25

Poor mllintmann!

Figure 6: Bathtub for circuit breakers according to Kawamura et at. {1990}.

3.4.4. Bar et at (1990) use a Delphi-method to determine the position of the knee in the bath-tube curve, i.e. the place where the wear-out phase starts to become important. For low-voltage disconnect switches they find in case of

good circumstances 32 years; average circumstances 26 years; bad circumstances 17 years.

For medium-voltage minimum-oil circuit breakers in an MVtLV substation they find in case of

good circumstances average circumstances bad circumstances

26 years; 20 years; 13 years.


For oil filled circuit breakers the values are in case of good circumstances 27 years; average circumstances 21 years; bad circumstances 12 years.

For synthetic isolated circuit breakers they find in case of good circumstances 23 years; average circumstances 18 years; bad circumstances 10 years.

For ·open· circuit breakers they find good circumstances 30 years; average circumstances 23 years; bad circumstances 18 years.

For medium-voltage minimum oil circuit breakers in an MV substation they find in case of

good circumstances average circumstances

and for air-blast breakers, good circumstances average circumstances

14 years 13 years

11 years 11 years

3.4.5. Horton et al. [1990] provide estimates of the failure rates of underground distribution system components. The estimates are based on information collected from a number of utility sources throughout the United States of America. For each component and each year they used the number of units installed or removed and the number of failures which have occured. This data is fitted to a Weibull distribution.

From 364,000 load break elbows installed by NELPA (Northwest Electric Light and Power Association) utilities from 1968 to 1988 they find the following expression for the failure rate:

A(t) = 0.09x10·3t.

In terms of Weibull distributions this would imply: shape factor = 2; characteristic time-to-failure = 150 years.

As only 20 years of observation are available, the latter time has no relation to an ETTF value found under the assumption of an exponential distribution (2400 years in this case).

3.4.6. Kanai et al. [1991] determine parameters for the lifetime distribution of vacuum interrupters from field data. They find a Weibull distribution with a characteristic number of operations of 271,600 and a shape factor of 2.041. With the time as parameter they find a characteristic lifetime of 3201 months (267 years) with a shape factor of 1.235.


3.5. Conclusions

The values for circuit breakers presented in this chapter are difficult to compare. Circuit breaker failures include, among others, incorrect opening (either due to a failure of the protection or due to a failure of the breaker itself) and short circuits in or near the circuit breaker. Table 20 and Table 21 summarize the values for "short circuit in or near a circuit breaker". Table 20 gives recommended values and data from surveys. Table 21 gives data used in reliability studies.

In Table 20 it has been assumed that "failure in electrical part" and "failure of electrical insulation to ground" are synonym for "short circuit". A comparison with the other values in the table shows that this assumption is probably not correct. From Table 20 the conclusion could be that the ETTF is in the range from 1000 to 5000 years, for LV and MV circuit breakers. For higher voltages circuit breakers become less reliable. From follow-up surveys by CIGRE and IEEE it follows that circuit breakers are more reliable nowadays than in the past.

In Table 21 it has been assumed that "active failures" and "electrical insulation failures" are synonym for "short circuit". It is remarkable that the data used in reliability studies are systematically too low. No clear explanation for this has been found.

Table 22 and Table 23 summarize the ETTF values for disconnect switches. The results from surveys differ over a wide range: from 100 to 6000 years. This is partly due to the increase in reliability due to improved manufacturing and maintenance methods. But, apparently, different types of disconnect switches have fairly different life times. This is also made clear by the values presented by Volkmann et al. (1991). The same spread in values is visible in Table 23, showing the data used in reliability studies.

From the values presented here, an ETTF value in the range 250 - 1000 years seems reasonable.

Section Reference ETTF Remarks 3.2.3 Selseth 19n 100 failure in electrical part 3.2.4 IEEE 1974 925 fixed type

1570 draw-out 3.2.6 Mazza and M;chacca 1981 4000 63-100 kV

550 100·200 kV 600 200-300 kV 400 300-500 kV 60 >500 kV

3.2.12 Radwam and El-Marsafawy, 1990 19 66 k.V· electrical insulation to grCM.l"ld

3.2.17 Bovy et al. 1991 5000 10 kV

Table 20. Summary of circuit breaker life times; short circuits in circuit breakers; recommended values and data from surveys.


Section Reference ETTF Remarks 3.3.4 Grover and Billinton 1974 30 hinh-voLtafte

100 low-voltage 3.3.5 Guertin and Lamarre 1975 30 hiah·voltsae

100 low-vol taae 3.3.7 Allan et al. 19n 67 11 kv' active failures 3.3.12 Allan et at. 1980 6 400 kV' active failures

40 132 kV' active failures 200 11 kV' 3.3 kV 415 V· active failures

3.3.13 Adams and Jasmon 1981 10 active fai lures 3.3.15 Ruoff and van Meteren 1983 380 normallYooen

750 normaLlv closed 3.3.17 Koval 1983 140 3.3.20 Anderson et al. 1987 230 electrical insulation failures 3.3.23 ALLan 1988 100 3.3.26 Fransen 1989 90 3.3.34 Allan et at. 1991 285 139 kV' active failures

670 33 kV' active failures 250 11 kV' active failures

3.3.36 Dortolina et al. 1991 66 active fai lures

Table 21. Summary of circuit breaker life times; short circuits in circuit breakers; data used in reliability studies.

Section Reference ETTF Remarks 3.1.2 IEEE 1990 165 enclosed 3.1.3 Klo...,.1 et al. 1990 300 < lkY

200-600 10-30 kV 85-140 100 kV

3.2.1 Dickinson 1962 102 aDen

230 enclosed 3.2.2 Connor and Parkins 1966 7100 oB - ilrmersed

10 000 air-break 3.2.4 IEEE 1974 350 looen

160 enclosed 3.2.8 WallnBns 1986 1000 3.2.9 van Amelsfoort et at. 1986 6000 3.2.18 Volkmam et at. 1991 250 switch (underaround)

5200 elbow (underaround) 5200 sol ice (underaround) 1300 overhead switch' urban feeder 800 overhead switch' rural feeder

Table 22. Summary of disconnect switch life times; recommended values and data from surveys.


Section Reference ETTF Remarks 3.3.2 Dickinson 1960 1000 integral with equi-"t

500 single throw

3.3.3 CaDre et al. 1969 500·2000 3.3.4 Grover and Billinton 1974 50 short c i rcui t

5 mal·triD 3.3.11 Allan et at. 1979 1000 3.3.14 Nelson and Johnson 1982 670 13.8 kV

450 24.9 kV 330 34.5 kV

3.3.17 Koval 1983 300 3.3.20 Anderson et al. 1987 500 electrical insulatfon faflure 3.3.21 Dialvnas and Allan 1987 1000 3.3.22 Goldberg et at. 1987 250 switches

1700 old splices/ elbows

17000 new sDl;ces/elbows 3.3.24 Dialynas 1988 75 150 kV 3.3.25 Dialynas and Paoadoooulos 1989 250 20 kV 3.3.28 Horton et at. 1989a 250 3.3.29 Horton et al. 1989b 250 3.3.32 Sallam et at. 1990 170 3.3.33 Mohan Roa and Sekhar 1990 160 enclosed 3.3.35 Dialvnas and Koskolos 1991 75 3.3.36 Dortolina et al. 1991 500 3.3.37 Kjolle and Sand 1991 71 22 kV rural network

1400 11 kV urban network

Table 23. Summary of disconnect switch life times; data used in reliability studies.


4. PROTECTIVE EQUIPMENT; GENERAL AND RELAYS


4.1.1. IEEE standard 500 [IEEE, 1983] gives reliability data for components of nuclearpower stations. The data has been derived by a Delphi-method combined with the results from several surveys and data bases. For protective relays the following values are recommended (catastrophic failures only):

All types: Spurious operation Fails to open Fails to close

: ETTF = 2850 years; : ENTF = 2 x 106; : ENTF = 300 x 103;

Overcurrent alarm switches: ETTF = 200 years repair time = 0.5 hours.

Thermal overload relay ETTF = 250 years; range = < 65, 1000>

Overload, time delay relay ETTF = 75 years

Under-voltage, time delay ETTF = 90 years repair time = 0.6 hours

Under-voltage, instant ETTF = 50 year repair time = 0.6 hours

Over-voltage ETTF = 200 years

range = range = range =

(ENTF = Expected Number of operation To Failure)

-47-

<1200,11,000> < 1 x 1 06, 7 x 106 > < 1 50 x 103, 1 x 106 >

Protective equipment.


4.2.1. Dickinson [1962] presents the results of an AlEE survey in 1959. For "Protective relays, including switchgear, starter overhead etc." the results are:

ETTF = 170 yr; c.i. = <160,180 yr>

The repair time reported is 5 hours.

4.2.2. Connor and Parkins [1966] report about a 14-year survey on faults in networks with nominal voltages between 2 and 33 kV. The results for protective gear are given below. There are approximately 50,000 protective gear in service. This figure includes approximately 15,000 installations of current-release-operated equipment with time-limit fuses.

The following subdivision according to cause is given for faults in protective gear: relays and components: ETTF = 970 years; 9.1 % incorrect settings: ETTF = 930 years; 9.5% failure of trip supply: ETTF = 4800 years; 1.9% AC trip circuit and t.1. fuses: ETTF = 350 years; 25.7% wiring defects: ETTF = 2300 years; 3.8% pilot cables: ETTF = 1700 years; 5.2% incorrect connections: ETTF = 4800 years; 1.9% incorrect circuit diagram: ETTF = 20,000 years; 0.5% interference with secondary wiring: ETTF = 3700 years; 2.4% testing errors: ETTF = 2700 years; 3.3% vibration or mechanical shock: ETTF = 450 years; 20.0% incorrect characteristic: ETTF = 1600 years; 5.7% unkown at time of original report: 11 .0%

Based on 50,000 protective gear-units and a 14-year covering period, the following values are found for "all causes":

ETTF = 89 years; c.i. = <87,91 >

The number of system faults caused initially by relays and protection is extremely small, and on the average only about two cases occur per year (on a population of 50,000 that implies: ETTF = 25,000 years). Cases of protection failing to operate correctly when faults on the system occur are about 200 per year (this would imply ETTF = 250 years, although this value is of no meaning as the number of faults occuring in the system is not known). The average percentage of correct operations of relays for the past 14 years is approximately 94%, and has tended to improve slightly in recent years.

The percentage of correct operations of H.V. fuses is now approaching 90%.

4.2.3. The IEEE sponsored 1972-survey [lEEE,1974] (discussed in Section 3.2.4) resulted for protective relays in:

ETTF = 5000 years, c.i. = [2500,30000]

-48- Protective equipment.

The survey gives for the failure characteristic "mal-trip of circuit breaker": fixed type : ETTF = 440 years; draw-out : ETTF = 750 years.

For "failure of circuit breaker due to auxiliary or protective device" the survey results in: fixed type : ETTF = 570 years; draw-out : ETTF = 970 years.

4.2.4. Patterson and Teague [1974] present field experience with solid-state protective relays over the period 1965-1973. Their results, as shown in Figure 7, are based on a total of more than 10,000 equipment-years of service. The failure rate decreases from 0.25 per year to 0.03 per year, according to the authors, due to manufacturer's design and quality control improvements and due to the user's increased familiarity with the installation, setting testing and maintenance of the solid-state equipment.

,.

- - - - - - ., - -CAlf NO""" Yf ......

IUCUUCAl COI""'fCTlONS

1165 ,_ 1M' ,... It" 1110 1"1 "" "" C ... LtNO ..... vr ..... !

Figure 7. Field experience (left) and types of field problems (right) with solid-state protective relays, according to Patterson and Taegue [19741.

4.2.5.The CIGRE survey [Mazza and Michaca, 1981] described in Section 3.2.6 gives the following values for protection related failures.

Does not open on command: 63-100 kV : ETTF = 2400 years 100-200 kV : ETTF = 525 years 200-300 kV : ETTF = 220 years 300-500 kV : ETTF = 125 years > 500 kV : ETTF = 40 years

Opens without command: 63-100 kV : ETTF = 5600 years 100-200 kV : ETTF = 1400 years 200-300 kV : ETTF = 550 years 300-500 kV : ETTF = 520 years >500 kV : ETTF = 60 years

; c.i. , c.i. , c.i.

= = =

<1500,5000> <400,700> < 170,300>

; c. i. = < 100,175 > ; c.i. = <30,60>

; c.i. = < 3000,35000 > ; c.i. = <1000,2700> ; c.L = <400,1000> ; c.i. = <350,1250> ; c.L = <40,110>

4.2.6. Yoguchi et al. [1984] investigated 1037 failures, occured during April 1977 and March 1980, on typical relay equipment for transmission line, busbar, transformer and distribution line protection in Japan. A total of 14,755 pieces of equipment have been observed.


They give the following data for protective relay equipment:

phase comparison relay equipment ETTF = 7.5 years c.L = < 6.5, 8.5 >

directional comparison relay equipment ETTF = 10.4 years c.i. = <9, 13>

differential relay equipment for transformers ETTF = 70 years c.L = <60,90>

differential relay equipment for busses ETTF = 20 years c.L = < 15, 25 >

The transmission line equipment (phase and directional comparison) has duplicated main protection and complicated backup protection. This explains, according to the authors, the low value of ETTF.

Data obtained for the elements of the equipment is given below.

Main relay (static type) phase-comparison relay undervoltage relay overcurrent relay distance relay voltage balance relay bus protective relay logic element Time Auxiliary relay Control power source unit

: ETTF = 265 years : ETTF = 890 years : ETTF = 1 500 years : ETTF = 480 years : ETTF = 470 years : ETTF = 750 years : ETTF = 75,000 : ETTF = 3,400 years : ETTF = 23,000 years : ETTF = 230 years

For the main relay the percentages of unnecesarry and missing operating modes are 42% and 58%, respectively.

The failures described previously were detected by the manual periodic test, automatic monitoring, routine inspection and power system faults.

4.2.7. Franke [1987) gives data for 6 types of secundary equipment, obtained form 30 years of service experience in the power system of a chemical plant in the German Democratic RepubliC. There seem to be however some contradictions in his data. I will give here only his value for the probability of failure per interruption (mittere Fehlverhaltens quote):

RD 7/ RD 110 "Complicated relay system" ROU/ROS differential protection RSZ 3f/RSf independent overcurrent time RSZ 3t dependent overcurrent time high-voltage automatic transfer system low-voltage automatic transfer system

: 0.02 ± 0.01 % : 0.4 ± 0.2 % : 0.0061 ± 0.0003 % : 0.01 ± 0.005 % :0.3 % : 0.03 %


4.2.8. Chan [1988) compares the performance of different protection arrangements. for which he uses two relays.

Relay A probability of fail-to-operate: 10 % probability of mal-operation: 5%

Relay B probability of fail-to-operate: 6 % probability of mal-operation: 7 %

He uses two indices to quantify relay performance: DSFPI (Discriminative System Fault Performance Index) NSFPI (Non-system Fault Performance Index)

DSFPI = 1 _ Number of faults incorrectly cleared total number of system faults

NSFPI = 1 _ non-system fault circuit-breaker operations number of circuit-breakers installed

He gives values for these indices for the EHV. transmission and sub-transmission systems of Hong Kong. (based of observations from 1981 to 1987)

DSFPI = 95.1 % ; c.L = < 94.96 >

From the NSFPI an ETTF for mal-trip can be determined:

ETTF = .--;;;1=", 1-NSFPI

This leads to the following ETTF values:

1981 ETTF = 16 years 1982 ETTF = 15 years 1983 ETTF = 18 years 1984 ETTF = 20 years 1985 ETTF = 42 years 1986 ETTF = 33 years 1987 ETTF = 37 years

He further uses two indices for relay performance:

the index of correct performance:

k = correct operations C correct operations + mal operations


the index of fail-to-operate:

kf

= fail-to-operate correct operations + fail to-operate

The results for three types of distance relays are given in Table 24.

Relay 1 (43 units) Relay 2 (56 units) Relay 3 (115 units)

k, Ko k, Ko k, Ko 1980/82 0.2 X 99.3 X 1.3 X 91.2 X 0 90.5 X 1983/85 1.2 X 98.7 X 1.6 X 90.9 X 3.9 X 97.5 X 1985/87 1.4 X 99.0 X 2.5 X 98.3 X 2.6 " 90.5 X

Table 24: Relay data according to Chan [1988J.

4.2.9. Heising and Patterson (1989) give typical failures rates for protective relays.

Electromagnetic, time overcurrent Electronic single function ( 1970's design) Electronic relay system

: ETTF = 1000 yr. : ETTF = 300 yr : ETTF = 20 yr

The values are obtained from published and unpublished industry and general Electric sources.

Figure 8 gives the failure rate of electronic relays and electromagnetic relays as a function of complexity. For very complex systems an electronic relay will become more reliable than an electromechanic one.

The authors also present some data on the effectiveness of continuous monitoring; over the period from 1977 to 1984 a total of 48 out of 55 failures which could result in incorrect operation were detected by continuous monitoring.

4.2.10. The Canadian Electric Association [19901 gives detailled failure data for high-voltage

100 r---------------

Number 01 Functions in a Protection System

Figure 8. Relay failure rate versus functional complexity, according to Heising and Patterson [1989J.

equipment. For each component they give a sub-component "control and protection equipment". The results for this SUb-component are given in Table 25. Rated voltages are from about 100 kV to 700 kV.


ETTF c.i. Cables 8.5~r [6 12]

Lines 19.8 yr [18 22]

Transformer banks 34.8 yr [31 39]

Circuit breakers 63 yr [57 70]

Synchronous compensator 7.4 yr [5 12]

Static compensator 88 da"" [75 1 05 days]

Shunt reactor bank 27 yr [20 40]

Shunt caDScitor bank 62 yr [40 110]

Series capacitor bank 600 days [450 900 days]

Table 25: Failure of control and protection equipment, according to the Canadian Electrical Association 119901.

4.2.11. Vlutters [1991 J gives failure rates for earth leakage relays, derived from testing of over 44,000 installation in Bavaria, Germany. A subdivision to the place of the installation yields:

Workshops : ENTF = 80 ; c.L = <55,160> Offices, meeting rooms : ENTF = 75 ; c.i. = <50,130> Assembly halls : ENTF = 60 ; c.i. = <40,115> Houses : ENTF = 35 ; c.i. = <30,45> appartment buildings : ENTF = 28 ; c.i. = <24,32> Cellars, heating rooms, etc. : ENTF = 20 ; c.i. = <14,32> Outdoor equipment : ENTF = 13 ; c.i. = <8,50>


4.3. Data used in reliabilitv studies

4.3.1. Snaith (1977) uses the following values for the electricity supply of a nuclear power station.

Time delay relay; probability of failure on demand: 1 %

under-voltage relay; unrevealed fault: ETTF = 100 years; maintenance interval = 0.5 years.

4.3.2. Allan and Adraktas (1982) use the following failure rates for a reliability study. The values are based on information given by [Green and Bourne, 1972) and [IEEE, 1974)

Fault detector Relay Trip signal device Breaker

: 0.04 failures per year : 0.005 failures per year : 0.03 failures per year : 0.008 failures per year

(ETTF = 250 yr); (ETTF = 2000 yr); (ETTF = 300 yr); (ETTF = 1250 yr).

All these refer to "dormant fail-to-trip" situations.

The "fault detector" includes appropriate current transformers, voltage transformers and comparators. The "relay" contains operating and restraint coils. The "trip signal device" contains the trip signal device and associated power supply. The "breaker" is the actual fault breaking component.

4.3.3. Ruoff and van Meeteren (1983) use a value of ETTF = 10 years for incorrect tripping of a circuit breaker. The probability of fail-to-open they use is 1 %.

4.3.4. Bunch, Stalder and Tengdin (1983) use past performance from a number of electric utilities to establish reliability targets for automated distribution system equipment which are deemed acceptable. They arrive at the following targets for the protection:

false trip detected by self test false trip not detected by self test failure to trip detected by self test failure to trip not detected by self test

: ETTF = 20 years; : ETTF = 40 years; : ETTF = 10 years; : ETTF = 40 years.

4.3.5. Anderson et al. (1987) describe a study after the reliability of HV substations in which they include the secondary equipment. For failures of the secundary equipment they distinguish between (dormant) fail-to-trip and mal-trip. The second one is incorporated in their study by increasing the failure rate of the circuit breaker, the first one by introducing a failure probability at operation for the circuit breaker. They have used the following data:

failure of circuit-breaker due to mal-trip failure probability at operation

: ETTF = 800 years : 0.5 - 2.0 %.


4.3.6. Allan [1988) uses in a reliability study for an incorrect trip of a circuit breaker (thus for an incorrect trip of a relay) the value ETTF = 100 years. The value for the propability of fail-to-open used is 6 %.

4.3.7. Fransen [1989) uses a value of ETTF = 140 years of incorrect interventions by a relay or by an operator. The value is based on 7% years of failure data in the supply to a large chemical plant. The repair time used is 4 hours.

4.3.8. Mohan Rao and sekhar [1990) compare the reliability of a number of distribution systems.

Incorrect trip due to failure of a protective relay: ETTF = 1700 years; repair time = 5 hours

4.3.9. Dortolina et al. (1991) determined the reliability of three different designs for a 400 kV switching substation in Venezuela. They explicity include the behaviour of the protective relaying system by extending the circuit-breaker representation. Their circuit-breaker model is a logical series connection of a processing and decison subsystem (PDs), a switching subsystem (55) and a auxiliary subsystem (AS). The latter is considered totally reliable. The PDs has an ETTF of 20,000 years for active failures; an ETTF of 39 years for passive failures and a repair time of 12.0 hours. The 55 has an ETTF of 65 years for active failures; an ETTF of 200 years for passive failures and a repair time of 24.0 hours. The probability of a breaker being stuck is 0.5 %.

4.3.10. Koglin et al. [1991) describe a reliability study of an EHV system. For the protection they use the following data:

missing operation of protection for EHV line: expected value of conditional probability for one end of the line: 0.46 %.

unnecessary operation of protection for switching bay: expected value of conditional probability; 0.03 %.

4.3.11. Kula et al. [1991] include the reliability of the protection in the design of highvoltage substations. They use the following data:

probability of failure of relay : 1 % probability of failure of communication between relay and circuit breaker: 2% probability of fail-to-operate of circuit breaker : 2% probability of failure of the circuit-breaker failure protection : 2%

4.3.12. Lau et al. [1991) give failure data for components of modern computer-based protective equipment. These data are based on a review of experience with similar components in other applications and engineering judgment of a few experts in the field. They give the following ETTF values:

logic processor : 6.7 years; communication processor : 9 years;


input interface input card input protection output interface output card monitor Ckt

outputs (32) overall card

watch-dog-timer power supply

: 14 years; : 7 years; : 50 years; : 14 years; : 12 years; : 20 years; : 8 years; : 12 years; : 10 years.

All types of failure take 8 hours to repair. Periodic maintenance is carried out once in two years to check out the failures that cannot be detected otherwise.

4.3.13. Kialle and Sand [1991] give the following values for control equipment in distribution networks in Norway:

22 kV rural network ETTF = 20 years; repair time = 0.8 hours;



4.4. Ageing data

4.4.1 . Gusciora [1988) treats bathtub curves, Weibull distibutions and relay reliability. for a number of relays and related apparatrus he determined the parameters of the weibull distribution for the lifetime.

For ·snap-action switches before process fixes· he finds the superposition of two Weibull distributions:

char. life = 7 million operations; shape factor = 0.9 char. life = 20 million operations; shape factor = 4.5

For ·snap-action switches after process fixes· he finds the superposition of two other Weibull distributions:

char. life = 15 million operations; shape factor = 3.5 char. life = 15 million operations; shape factor = 5.5

Infant mortality of reed relays that had been declared ·marginally defective": char. life = 18 billion operations; shape factor = 0.2

General-purpose cia per style heavy-duty relay type 1 char life = 410,000 operations; shape factor = 3.8

General-purpose cia per style heavy-duty relay type 3 char. life = 265,000; shape factor = 7.8

4.4.2. Bar et al. [1990) use a Delphi method to determine the position of the knee in the bath-tub curve, i.e. the age where the wear-out phase starts to become important.

For an overcurrent-time relay in a MV-substation they find, in case of good circumstances : 15 years; average circumstances : 13 years.

The same values are given for polygon-shaped relays and for differential relays.

For signalling equipment they find, in case of good circumstances : 18 years; average circumstances : 17 years.

For the protective relay in an MV/LV substation they find: Buchholz relay

good circumstances average circumstances bad circumstances

overcurrent-time relay good circumstances average circumstances bad circumstances

: 21 years; : 20 years; : 20 years;

: 14 years; : 9 years; : 6 years;


differential relay good circumstances average circumstances bad circumstances

: 15 years; : 13 years; : 10 years.

For the on-off coil in an MV-substation they find, in case of good circumstances : 22 years; average circumstances : 21 years.

For the auxiliary contacts they find, in case of good circumstances : 17 years; average circumstances : 16 years;

For the remote control they find, in case of good circumstances : 20 years; average circumstances : 18 years.

4.4.3. Ugokwe [1992] presents the results of a reliability case study on the ·offline 1774 programmable logic controller (PLC)". From a total of 20 failures of this type of PLC they conclude that the failure rate consists of three Weibull-distributions, with the following shape factors:

p = 0.457, 0 - 20 days; p = 1.842, 7 - 25 days; p = 2.185,60 - 150 days.


4.5. Conclusions

Failures of the protection can be divided into "fail-to-trip" and "incorrect trip·. The latter can be further divided into ·spontaneous fail-to-trip" and "fail-to-trip due to fault in another zone". Unfortunately, many studies do not make this distinction. This makes it difficult to compare the values.

Table 26 and Table 27 summarize the values for relay life time given in this chapter. The values show a large spread. Several reasons for this can be given: difference in definition of failure of the relay; different life times for different relay types.

Due to the limited number of values given, it is of no use to suggest some new values. Some of the referred publications give recommended values that seem to be reasonable [e.g. Heising and Patterson, 19891. I would suggest 250 to 1000 years for electromagnetic relays; 100 to 200 years for electronic relays (single function) and 10 to 30 years for electronic relay systems. The life time of modern computerized relays is not clear.

Section Reference ETTF Remarks 4.1.1 IEEE 1983 2850 spurious operation

200 overcurrent 250 thermal overload 75 overload- time delay 90 undervoLtage" time delay 50 undervolt~ge' instant 200 overvoltage

4.2.2 Connor and Parkins 1966 89 4.2.3 IEEE 1974 5000 4.2.4 Patterson and Teague 1974 4·30 new type of solid-state relay 4.2.6 Yoguchi et al. 1984 7.5 phase COll1l8"; son

10.4 directional comparison 70 transformer differential 20 bus differentiaL

4.2.8 Chan 1988 25 mal-trip

4.2.9 Helsing and Patterson 1989 1000 electromagnetic' time overcurrent 300 electronic' single function 20 electronic relay system

Table 26. Summary of relay life times; recommended values and data from surveys.

Section Reference ETTF Remarks

4.3.1 Snaith 1977 100 undervoltage" unrevealed fault 4.3.2 Allan and Andraktas 1982 250 fault detector

2000 relay 300 triD signal device

4.3.5 Anderson et al. 1987 800 mal-triD of circuit breaker 4.3.6 Allan 1988 100 4.3.7 Fransen 1989 140 mal-trip due to relay or operator 4.3.8 Mohan Roa and Sekhar 1990 1700 mal-trip due to relay 4.3.13. Kjolle and Sand 1991 20 control eQUipment" 22 kV rural network

60 control~icment· l' tv urban network

Table 27. Summary of relay life times; data used in reliability studies.


5. FUSES


5.1.1. IEEE standard 500 [IEEE, 1977] gives reliability data for components of nuclearpower stations. The data has been derived by a Delphi-method combined with the results form several surveys and data bases. For fuses the following values are recommended (catastrophic failures only)

All types Fuses (open) below rating Fails to interrupt

Up to 1000 Volts Fuses (open) below rating Fails to interrupt

: ETTF = 8800 years ; range = < 80, 30000 > : ENTF > 100,000 cycles

: ETTF = 5500 years ; range = <550, 19000> : ENTF > '00,000 cycles

Above 1000 Volts to 36.6 kV, indoor Fuses (open) below rating : ETTF = 10,000 years; range = <700,30000) Fails to interrupt : ENTF > 100,000 cycles

From 2.3 to 138 kV outdoor Fuses (open) below rating Fails to interrupt

: ETTF = 12,500 years ; range = < 600, 40000) : ENTF > 100,000 cycles

-60- Fuses.


5.2.1. An IEEE sponsored survey of electrical equipment reliability in industrial plants was completed during 1972 [lEEE,1974). This survey included a total of 1982 equipment failures that were reported by 30 compagnies covering 68 plants in nine industries in the United States and Canada. The survey resulted for fuses in the following values:

ETTF = 500 years, c.i. = [250,3000).

5.2.2. The PNEM-survey [van Amelsfoort et aI., 1986) covering the period 1980 - 1986, resulted for fuses in the following data:

ETTF = 4000 years, c.i. = [2500, 13000).

-61- Fuses.


5.3.1. Dialynas and Papadopoulos (1989) use a value of ETTF = 250 years for a 20 kV fuse-cutout.

5.3.2. Whiting (1989) determines the reliability of power supplies to broadcast transmitting stations. Component data are taken from various sources. For fused switches they use:

ETTF = 500 years repair time = 24 hours

5.3.3. Saliam et al. (1990) calculate, as an example, reliability indices for the MVnetwork of Port. Fouad, Egypt. For fuses they use:

ETTF = 430 years repair time = 1.1 hours

5.3.4. Volkmann et al. (1991) give a value of ETTF = 250 years for a fuse in an underground distribution system. For the source of their data they refer to two internal reports of Pacific Gas and Electric. For a fuse in an overhead distribution system they give

ETTF = 220 years ETTF = 270 years

(rural feeder) (urban feeder)

These values were computed form historical outage records for a group of 85 rural and 95 urban feeders, over a period of approximately 5 years. The repair times are.

rural overhead fuse urban overhead fuse rural underground fuse urban underground fuse

: 155 hours : 130 hours : 200 hours : 74 hours

-62- Fuses.

5.4. Ageing data

No ageing data for fuses has been found.

-63- Fuses.

5.5. Conclusions

Table 28 summarizes the values for fuse life time given in this chapter. From [IEEE, 1977) it becomes clear that the probability that a fuse fails to interrupt is very small. The values in the table are therefore, probably, only for "fuse mal-trips".

The values used in the reliability studies appear too low. I would suggest values between 1000 and 5000 years. One should keep in mind however that the probability of an incorrect trip (or fail-to-trip) because of choosing the wrong fuse size, can be much higer.

Section Reference ETTF Remarks 5.1.1 IEEE 19n 5500 <100OV

10000 >1000V Indoor 12000 >1000V outdoor

5.2.1 IEEE 1974 500 5.2.2 van Amelsfoort et al. 1986 4000 5.3.1 Oletyne. end Papadopoulos 1989 250 fuse cutout 5.3.2 Whiting. 1989 500 fused aw; tch

5.3.3 Saltern et al. 1990 430 5.3.4 Volkmann et at. 1991 220 rural feeder

270 urban feeder

Table 28. Summary of fuse life times.

-64- Fuses.

6. VOLTAGE AND CURRENT TRANSFORMERS


6.1.1. IEEE standard 500 [IEEE,1984] gives reliability data for components of nuclearpower stations. The data has been derived by a Delphi-method combined with the results from several surveys and databases. For instrument transformers the following values are recommended (catastrophic failures only): voltage transformers:

0-10 kV : ETTF = 325 years ; range = <145,425> open circuit : ETTF = 1600 years ; range = < 1300, 2000>

<200,550> short circuit : ETTF = 400 years ; range = over 10 kV : ETTF = 170 years ; range =

open circuit : ETTF = 1000 years ; range = < 110, 300> <600, 1700> <140,400> short circuit : ETTF = 200 years

Current transformers 0-10 kV

open circuit short circuit

over 10 kV open circuit short circuit

: ETTF = 500 years : ETTF = 2300 years : ETTF = 650 years : ETTF = 350 years : ETTF = 1500 years : ETTF = 420 years

; range =

; range = ; range = ; range = ; range = ; range = ; range =

<250, 1000> <1100,4500> <350,1400> <280, 1000> <300,4200> <350,1200>

6.1.2. Kloeppel et al. [1990] give recommended values of component data for reliability studies. They were based on data from several industries in Eastern Germany as well as from the public supply. The following values are recommended for voltage and current transformers.

6 kV : ETTF = 1000 years; : repair time = 4 hours ; range = <2,6>

10,30 kV : ETTF = 500 year ; range = <250,1000> : repair time = 7 hours ; range = <3,10>

110 kV : ETTF = 230 years ; range = <150,350> : repair time = 24 hours ; range = <12,48>

-65- Voltage and current transformers.


6.2.1. Connor and Parkins [1966] report about a 14-year survey of faults in networks with nominal voltages between 2 and 33 kV. Some results are given below:

current transformers : ETTF = 2700 years; voltage transformers : ETTF = 17,000 years.

6.2.2. Cigre Working Group 23.07 performed a study after the reliability of instrument transformers rated at 72.5 kV and above that consist of a paper-oil system [Cigre, 1989]. The survey covered a total of 2.31 million transformer-years (136.033 transformers during 17 years).

The results of the survey are given in Table 29. All ETTF-values and confidence intervals are in years:

violent failures non-violent failures ETTF c. i. ETTF c. i .

current transformers 7500 [6000 10 000) 4000 [3800 4800)

magnetic voltage transformers 5300 [4000 7500) 3500 [3000 4300)

capacitive voltage transformers 20 000 [15000 33000) 3000 [2600 3500)

Table 29: failure data according to Cigr~ survey [Cigre, 19891.



6.3.1. De Clerq et al. (1985) compare two methods for HV/MV substation reliability.

MV voltage and current transformers ETTF = 500 years repair time = 4 hours


6.4. Ageing data

6.4.1. Biir et al. (1990) use a Delphi-method to determine the position of the knee in the bath-tub curve, i.e. the age where the wear-out phase starts to become important. For current transformers in an MV/LV substation they find, in case of

good circumstances : 36 years; average circumstances : 34 years; bad circumstances : 24 years.

For measurement transformers in an MV substation they find, in case of good circumstances : 32 years; average circumstances : 29 years.


6.5. Conclusions

Table 30 summarizes the results of this chapter. From both surveys the conclusion is that the lifetime of voltage and current transformers is several thousands of years. The data from other sources is not convincing enough to overrule this conclusion. I therefore suggest an ETTF value between 2000 and 3000 years.

Section Reference vol tage current Remarks 6.1.1 IEEE 1984 325 500 < 10 kV

170 350 > 10 leV

6.1.2 Kloeooel et at. 1990 1000 1000 6 kV 500 500 10 30 kV 230 230 110 kV

6.2.1 Connor and Parkins 1966 17000 2700 2 - 33 kV 6_2.2 CIGRE 1989 2300 7500 > n.5 kV 6.3_1 Oe Clerq et al. 1985 500 500 Hedilll1 Voltage

Table 30. Summary of voltage and current transformer lifetimes.


7. GENERATORS


L.1...1 Green and Bourne [1972] give average component failure-rates for electrical components. For generators they give:

AC, general : ETTF = 16 years DC, general : ETTF = 13 years Tachometers : ETTF = 23 years Synchros : i:TTF = 14 years

7.1.2. IEEE standard 500 [lEEE,1984] gives reliability data for components of nuclearpower stations. The data has been derived by a Delphi-method combined with the results from several surveys and databases. For generators the following recommended values are given (catastrophic failures only):

Steam turbine driven fails once started fails to start

Gas turbine driven fails once started fails to start


: ETTF = 2.2 years : ETTF = 8 years

; range = <75,3000> ; range = < 120, 6000 >

; range = <0.5, 250> ; range = <1.5,1000>

7.1.3. The IEEE gold book [IEEE, 1991] gives recommended values for the components of industrial and commercial power systems. For generators it recommends:

steam turbine driven generators in continous service ETTF = 5.9 year

emergency and standby units (reciprocating engine driven) ETTF = 185 hours failure rate per start attempt: 0.0135

-70- Generators.


7.2.1. Dickinson (1962] presents the results of an AlEE survey after the reliability of electrical equipment in industrial plants. The survey was held in 1959. For steam turbine generators the results are:

ETTF = 700 days, c.L <600,900 days>

The failure data appears to be different for different locations, therefore some detail is presented below:

USA and Canada : ETTF = 1800 days Latin American : ETTF = 365 days Middle East pipeline : ETTF = 270 days

The repair time reported is 111 hours.

;c.i. = <1300,2900days> ; c.L = <260,600 days> ; c.L = <210,360 days>

7.2.2. An IEEE sponsored survey of electrical equipment reliability in industrial plants was completed in 1972 (lEEE,1974]. This survey included a total of 1982 equipment failures that were reported by 30 compagnies covering 68 plants in nine industries in the Unites States and Canada. For generators the following data were found:

Steam turbine driven Gas turbine driven driven by motor, diesel or gas engine

: ETTF = 32 year : ETTF = 1.5 year : ETTF = 15 year

; c.L = (16,45] ; c.L = (1.1,2.3] ; c.L = (6,00 >

7.2.3. Dopazo et al. (1976] have collected data to find the relation between unavailabilty and time to maintenance. The data was derived from supercritical and cyclone-fired generating units, in the period 1970-1975. Figure 9 shows the duration of maintenance as a function of the time to maintenance. Figure 10 shows the forced unavailability as a function of the time to maintenance.

7.2.4. O'Donnell [1980] describes the results of a survey after the reliability of generators in industrial plants and commercial buildings. He gives the following data:

Continuous units (steam turbines)

Continuous units; age :s 10 years

Continuous units; age > 10 years

: ETTF = 6 years; c.L = <4.5, 8.5 > : repair time = 32.7 hours

: ETTF = 1.7 years; c.i. = <1,2.5>

: ETTF = 8.3 years; c.L = <6,13>

Emergency standby units (reciprocating engines) : ETTF = 185 hours; c.L = < 170,200 > : repair time = 478 hours : probability of fail to start: 1.3 %, c.i. = <0.8, 1.9>

The survey was too small to draw any additional conclusions.

-71- Generators.

iii ?i e w 60

'" ~ ~ 0

"- 60 0 .... X

'" Z W ...J 40

30

20

10

O~ ____ -L ______ L-____ -L ____ ~ ______ ~ ____ ~ ______ ~ ____ __

o 100 200 300 400 600 600 700

LENGTH OF PRECEDING RUN (DAYS)

Figure 9. Duration of maintenance as a function of the time time maintenance, according to Dopazo et al. [1976J.

-72- Generators.

> 20% >-:::; iii 18%

" ~ ! 16% z ::>

0 14% w

~ 0 12% 0 ,.

10%

8%

6%

~. ...--- 15% CONFIDENCE INTERVAL

4% L>.

2%

O%~~--~--~--~--~--~~--~--~------o 1·3 4-6 7-9 10-1213-'616-1819-2122-2425-27

MOUTHS SINCE PlANNES OUTAGE

Figure 10. Forced unavailability as a function of the time to maintenance, according to Dopoza et al. [1976J.

7.2.5. Schilling et al. [1988] give failure data for 10 power plants in Brazil: four thermal oil units and six thermal coal units. The data for all units cover the period since their initial commercial commitment, thus representing their real commercial age. Some results for these 10 units are given in Table 31. Note that the ETTF is given in days I.

Age X util. ETTF c. i. (Years) (days)

sel 15.92 30.4 X 110 <85 150>

se2 15.25 32.2 X 80 <65 110> se3 10.58 23.3 X 50 <40 65>

se4 10.00 24.3 X 50 <40 65> jll 8.67 80.6 X 32 <27 40> jl2 8.67 81.6 X 29 <24 36>

H3 8.67 86.3 X 50 <40 65> il4 8.33 81.2 X 50 <40 65>

~15 3.25 67.9 X 25 <20 40> j 16 2.5 71.6 X 31 <23 50>

Table 31: Failure data of 10 power plants according to [Schilling et al., 1988J.

The first column gives a code for the unit, sc1 - sc4 are oil units (2x81 MW, 2x218MW), jl1 - jl6 are oil units (2x50MW, 2x66MW, 2x125MW). The third column gives the percentage of time that the unit was in operation.

-73- Generators.

7.2.6. Smith [1989) gives equivalent unavailability factors for cogeneration on power plants. For utility cogeneration plants they find EUF = 13.4 %, c.i. = [8,24). The planned unavailability is 9.0 %, the unplanned unavailability 4.4 %. For industrial cogeneration plants they find EUF = 4.4 %, c.i. = [2,6). The planned unavailability is 2.8 %, the unplanned unavailability is 1.6 %. All plants under study were put into operation in the 1980's, had relatively high service factor, and an electrical power between 75 and 100 MW.

(EUF = Expected Unavailability Factor: the percentage of time that a component is not available due to forced as well as scheduled outages.)

7.2.7. Morzelle et al. [1989) have analysed the reliability of 76 UD-45 emergency sets installed before 1980 and 175 sets installed between 1980 and 1988 and compared the results with data obtained from sets installed before 1980.

Before 1980 failure to start failure while running

1980-1988 failure to start failure while running

: ENTF = 600 ; sample too small. : ETTF = 425 hours ; c.i. = <300, 650>

: ENTF = 28,000 ; sample too small : ETTF = 1000 hours ; c.i. = <800, 1400>

7.2.8. Ficek and Grolich [1989) followed about 90 steam turbines from one manufacturer during 12 years (1974-1985). The authors conclude that these turbines have a high availability. They give the following data:

Availability (ratio beteen the time of operation and the desired time of operation): 98.3 ± 0.3%

Technical availability (percentage of time spend on normal operation, stand-by, and non-operation due to external causes): 87.8 ± 2.0%

Usefullnessfactor (percentage of time spend on normal operation and stand-by): 86.1 ± 2.5%

Technical use factor (percentage of time spend on normal operation): 63.2 ± 2.0%

Failure halt factor (percentage of time spend on non-operation due to failures): 1.0 ± 0.2%

Stand-by halt factor (percentage of time spend on stand-by): 22.3 ± 1.1 %

Maintenance halt factor (percentage of time spend on maintenance): 9.6 ± 1.8%

7.2.9. Farmer [1989) presents some results of a study of diesel generator ageing in nuclear power plants. The following subdivision of ageing failures, with resoect to the cause, has been made:

vibration and shock poor manufacturing

:28% : 18%

-74- Generators.

adverse environment maintenance errors

: 17% : 9%

The following subdivision with respect to the location of the failure has been made: governor control system : 30% fuel oil supply system : 13% the diesel engine itself : 10%

One of the major potential contributors to ageing turned out to be the fast start requirements imposed by technical specifications for nuclear plants.

7.2.10. Smith et al. (1990) present the results of a reliability survey of 600 to 1800 kW diesel and gas-turbine generating units. The survey was performed in 1988 and sponsored by the U.S. Army Engineering and Housing Support Center. The data was needed to support the analysis of power systems at command, control, communications, and intelligence installations worldwide. Plants were selected from a wide variety of applications (e.q. electric utilities, cogenerators, hospitals, airfields, military installations, and computer and control facilities). Twenty-two plants participated in the study, providing data on 708 unit-years of operating experience.

A distinction is made between failure rates based on period hours (calendar time) and operating hours (power production time). Based on period hours the results are:

Diesel auxiliary continous : ETTF = 93 days ; c.i. = <80, 105 days>

: repair time = 2.9 hours standby : ETTF = 290 days ; c.i. = <260,330 days>

: repair time = 2.8 hours

Diesel package continous: ETTF = 83 days ; c.i. = <75,90 days>

: repair time = 6.4 hours standby : ETTF = 380 days ; c.i. = <320,460 days>


Gas turbine continous: ETTF = 80 days ; c.i. = <70,95 days>

: repair time = 7.2 hours standby : ETTF = 1160 days ; c.i. = < 900, 1500 days>


Based on operating hours the following results are given:

Diesel auxiliary - continous - standby

Diesel package - continous - standby

: ETTF = 11 days : ETTF = 68 days

: ETTF = 31 days : ETTF = 23 days

-75- Generators.

Gas turbine - continous - standby

: ETIF = 49 days : ETIF = 8 days

The high failure rates based on operating hours can be attributed to the relatively low utilization of these units.

The mean time between planned outages (MTIPO) and the duration of planned outages are given below, based on period hours

Diesel auxiliary continuous standby

Diesel package continous standby

Gas turbine continous standby

: MTIPO = 28 days : MTIPO = 286 days

:MTIPO = 62 days : MTIPO = 180 days

: MTIPO = 66 days : MTIPO = 390 days

; duration = 1.3 hours ; duration = 3.8 hours



In the discussion to their paper, the authors give data on the probability of fail-to-start. Here they refer to a number of other publications. I will reproduce their conclusions below; where ENTF = Expected Number of starts To Failure. The results of a number of gas-turbine starting reliability studies are given in Table 32. Some results of diesel starting reliability studies are given in Table 33. The references are given after Table 33.

St~ ENTF c.; . 1 210 <140 400>

2 155 <125 200> 3 125 <105 150> 4 128 <105 160>

Table 32: results of gas-turbine starting reliability studies.

Study ENTF c. ; . 1 33 not available 5 270 <220 350>

60 55 <35 130> 6b 200 ..... 1. too small

6c 300 sampl. too small

6d 55 <40 90> 6e 100 sample too small

7 11 not available 8 58 not eva; labLe

Table 33: results of diesel starting reliability studies.

1 ARINC Research Corporation. Final Report - RAM Study of Diesel and Gas-Turbine Generator Sets. Publication 4219-03-01-4803, October 1988. 2 Booz, Allen Applied Research. Small Gas Turbine Start Investigation, April 1970.

-76- Generators.

3 Kongsberg Dresser Power. Internal Study Comparing Diesels with Gas-Turbine Engines (unpublished). 1984. 4 AT&T. Internal Study for Gas-Turbine Reliability (unpublished). 1980. 5 Electric Power Research Institute. Reliability of Emergency Diesel Generators at U.S. Nuclear Power Plants. NSAC 108. September 1986 6 U.S. Nuclear Regulatory Commission. Nuclear Computerized Library for Assessing Reactor Reliability (NUCLARR). NUREG/CR-4639 EGG-2458. Volume 5. RX. June 1988. 6a Consumer Power Compagny - Big Rock point 6b Northeast Utilities - Milstone 6e Northeast Utilities - Connecticut Yankee 6d Commenwealth Edison Compagny - Zion 6e Consolidated Edison Compagny of New York Inc. - Indian Point 7 Institute of Nuclear Power Operations. Nuclear Plant Reliability Data Systems. 1982 Annual Report. 1983. 8 Electric Power Research Institute. Diesel Power Reliability at Nuclear Power Plants: Data Prelimanary Analysis. NP-2433. June 1982

7.2.11. Yotsumoto et al. (1990) give data for origin of failure in an engine-generator set. Actual statistical data from telecommunications offices are used. The functional block diagram of an enginegenerator set. as they use it. is shown in Figure 11.

For this configuration they give the following probabilities of these blocks contribution to a failure:

Main block failed during operation failed to start

Common block failed during operation failed to start

Detecting block failed during operation failed to start

Controlling block failed to start

: 13 % : 22 %

: 5 % : 16 %

:4% : 15 %

: 25 %

Detecting Block Main Block • Temperature • Fuel system • Lubricating oil • Lubrication sYstem

pressure • Cooling system • Water flow • Generating system • Output voltage • Speed I

l Common Block Controlling Block • ~tart-up system

• Distribution board • Fuel system

f- • Cooling systefTl • Control board

Figure 11: Functional block diagram of Engine-Generator set used by [Yutsomoto et al., 19901.

7.2.12. Verplanke (1991) studied failures that occured in the power system of a chemical industry in The Netherlands between 1970 and 1991.

Generators 11 kV ETTF = 2 years; c.i. = <1.5.3.5>

-77- Generators.


7.3.1. Snaith [1977] uses the following values for the electricity supply of a nuclear power station.

Diesel generator probability of failure to start : 3% ETTF, while running : 300 hours

Diesel generator circuit breaker probability of failure to close on command : 1.6%

7.3.2. Magnon et al. [1977] use a fault tree to determine the reliability of stand-by diesel generator sets in nuclear plants. From data for components of the diesel generator (3000 kW, 1500 rpm) they find the following reliability data:

probability of set failure in idle phase revealing at start up: 1.6% (the set is tested every 15 days)

probability of failure to start owing to a fault at attempted start-up: 0.36%

failure in the steady running phase: ETTF = 15 days.

7.3.3. Allan et al. [1980] use, in a reliability study of electrical auxiliary systems of a power station, the following data for standby generations (23.5 kV):

ETTF = 30 days (while running) average repair time = 78 h. probability of failing to start = 10 %

7.3.4. Dialynas and Allan [1986] use the following generating units data for 1 MVA, 2 MVA and 10 MVA units in 33 kV and 11 kV distribution networks:

ETTF = 10 years starting failure probability = 10% (20% for 1 MVA unit) time to maintenance = 1 year durating of maintenance = 168 hours stuck breaker probability = 0.2%

7.3.5. Dialynas and Allan [1987] describe a reliability model for a power distribution network with local generation. They use the same values as in [Dialynas and Allan 1986]

7.3.6. Whiting [1989] determines the reliability of power supplies to broadcast transmiting stations. Component data are taken from various sources. He uses for 415 V, diesel standby generators:

ETTF = 320 days repair time = 78 hours

-78- Generators.

7.3.7. In a study on energy planning for Curacao [Government, 1989] the following nonavailability values are used for generating units.

7.5 MW steam unit: planned maintenance forced outages

25 MW steam unit: planned maintenance forced outages

18 MW gas turbine; 3.1 planned maintenance forced outages

: 8.2% : 13.8%

: 12.3% : 9.7%

and 3.75 MW steam units: : 7.0% : 3.0%

For diesel units the data are given in Table 34 (based on observation):

capacity in operation since planned maintenance forced outages 4.5 MW 1971 13.4% 8.4%

1.1 MW 1967 4.5% 13.8%

2.6 MW 1967 12.6% 13.8%

4.5 MW 1968 13.1% 22.4%

4.5 MW 1970 14.4% 23.4%

2.0 MW 19n 0.0% 9.4%

2.0 MW 19n 35.5% 6.2% 2.0 MW 1979 2.1% 17.3%

2.0 MW 1979 O.OX 3.3%

Table 34. Failure date for diesel units on Cura9ao {Government, 1989J

7.3.8. Billinton and Bebnath [1990] present an alternative four-state model for peak lead units. They give the following non-availability data, based on 1989-1984 ERIS data. The non-availability is defined as the chance that the unit is forced out when it needs to be available.

(ERIS = Canadian Electrical Association, Equipment Reliability Information System.)

Combustion Turbine Units 1-9MW :8.41% 10-24 MW : 9.24% 25-49 MW : 16.73% > 50 MW : 31.90%

Diesel generating units 1-4 MW : 7.72% > 5 MW : 30.60%

7.3.9. Allan and Inga-Rojas [1990] describe a method for distribution system reliability. In an example they use for standby generators:

probability of failure to start: 3 %, 4%, 5%

-79- Generators.

failure while operating: ETTF = 14 days.

7.3.10. Dialynas and Koskolos (1991) study an industrial power system with 5 local generators: a parallel steam unit (6.88 MVA), two parallel gas units (6.47 and 6.22 MVA), and two stand-by diesel units (0.50 and 0.70 MVA). For all units they use a forced outage rate (FOR) of 10 %. Maintenance is performed once a year during 168.0 h. For the stand-by units a starting failure probability of 20 % and a stuck breaker probability of 0.2 % are used.

The FOR is defined as the percentage of time that the component is out of operation due to failures (i.e. due to forced outages). The time that the component is out of operation for maintenance (scheduled outages) is not included in the FOR.

7.3.11. Prescott et al. (1991) use a value of ETTF = 10 years for the failure of standby generation to a large computer and communications installation. The repair time is 8 hours.

7.3.12. Patton [1992] investigates the effect of the duty cycle experienced by a generating unit on the unit's failure rate and availability. He uses the following failure data:

800 MW fossil-fired 600 MW fossil-fired 400 MW fossil-fired 200 MW fossil-fired 50 MW combustion turbine

: ETTF = 290 hours; : ETTF = 290 hours; : ETTF = 280 hours; : ETTF = 540 hours; : ETTF = 680 hours.

The following relation is used between the failure rate (per year) and the number of starts per year (5):

A = Ao + 1.315 S ,

where "0 is the failure rate for zero number of starts (according to the above-given values). The number of starts appears to have a considerable influence on the failure rate. One start per month already more than doubles the failure rate.

The above relationship is based on an EPRI-study after the reliability of combined-cycle units [Brown, Gardner, 1982)

-80- Generators.

7.4. Ageing data

7.4.1. Schilling et al. (1987) obtained failure data for large thermal generators. They fit a number of expressions for the failure rate to the times-to-failure assuming a repair process that is "as-bad-as-old". Assuming a Weibull distribution, they obtain.

char. life-time = 2.353 months shape factor = 1.709

7.4.2. Simpson and Stoll (1989) give a lot of data on availability and outage factors for 50 MW to 400 MW oil/gas fired subcritical units. Figures 12 through 15 give some of the results obtained from the utility industry (North American Reliability Council). Figure 16 gives the forced outage rate as a function of the unit's age. A significant increase occurs for units older than 25 years. This relation is obtained by using utility industry data for units smaller than 200 MW.

PER CENT FORCED OUTAGE FACTOR 10r----------------------------------------------,

-- 10 HRS/ST

8 ~ 300 HRS/5T

6

4

2 t)

25 HRS/ST

800 HRS/ST

120 HRS/ST

1500 HRS/ST

O~--~----~--~~---L--__ ~ ____ ~ __ _L ____ ~

o 2 3 4 5 6 7 a THOUSANDS OF SERVICE HOURS

Figure 12. Forced non-availability versus annual service hours, for different values of the service hours per start, according to Simpson and Stol/ [1989J.

7.4.3. Vesely [1990) determined ageing parameters for components of nuclear plants. For fail-to-start of a diesel generator he finds.

ETTF = 0.96 years c.i. = <0.07,16> shape factor = 1.7 c.i. = <1.2,2.4>

-81- Generators.

30

25

20

15

10

5

rP~E~R_C~E~N_T~F~O~R~C~E~D~O~U~t~A~G~E~R~A~T~E~ ____________________ ~

5 STARTS

~ • STARTS

10 STARTS

80 STARTS -:-20 STARTS

150 STARTS

O~--~-----L----~--~~--~----~--~ o 2 3 4 5 6 7

THOUSANDS OF SERVICE HOURS

Figure 13. Failure rate as a function of annual service hours, for different values of the number of starts per year, according to Simpson and Stoll {1989J.

20

15

10

5

rP_E_R_C_E_N_T~S~C_H~E~D~U~LE~D~O~U~t~A~G~E~F~A=C~TO~R __________________ ~

5 STARTS

~ .0 STARTS -10 STARTS

80 STARTS

20 STARTS

.. -:- 150 STARTS

oL---~ ____ ~ __ _L __ ~ ____ ~ __ ~ ____ ~ __ ~

o 23456 THOUSANDS OF SERVICE HOURS

7 8

Figure 14. planned unavailability as a function of annual service hours, for different values of the number of starts per year, according to Simpson and Stoll {1989J.

-82- Generators.

PER CENT AVAILABILITY 95,-------~~~------------------------_,

90

85

80

5 STARTS

40 STARTS

10 STARTS

80 STARTS

20 STARTS

150 STARTS

75L---~----~ __ ~ ____ ~ __ ~ ____ ~ __ ~ ____ ~

a 1 2 3 4 5 6 THOUSANDS OF SERVICE HOURS

7 8

Figure 15. Availability (forced and planned) versus annual service hours, for different values of the number of starts per year, according to Simpson and Stoll [1989J.

PER CENT AVAILABILITY PER CENT FORCED OUTAGE RATE 90 14

-- AVAILABILITy···· FORCED OUTAGE RATE

12

85 10

/ B

80

.' 6 .'

". '-. -. -----_. 4 75

2

70L---~----~----~--~----~--~----~--~0 o 5 10 15 20 25 30 35 40

YEAR

Figure 16. Availability and failure rate as a function of age, according to Simpson and Stoll {1989J.

-83- Generators.

7.5. Conclusions

Table 35 and table 36 give the ETTF values for generators. as presented in this chapter. The values fall into two groups: life times of several days and life times of several years. From the description of the different studies I concluded that the low value holds for stand-by units and the high one for continous units. A serious descripancy in this are the values for conituous units presented by Smith et al. [1990].

From the values presented here I would suggest 5 to 20 days for stand-by units and 1 to 3 years for continuous units.

Table 37 and Table 38 give the expected number of starts to failure (ENTFI. according to the different sources. From the surveys the fail-to-start probability appears to vary between 0.5 and 2%. The probability values used in reliability studies are in general higher: 2 - 20 %.

Section Reference ETTF Remarks 7. 1. I Green and Bourne 19n 13 - 23 vears

7.1.2 IEEE 1984 140 years steam-turbine driven 2.2. years gas-turbine driven

7.1.3 IEEE 1991 5.9 years continous service 185 hours stand· by

7.2. I Dickinson 1962 2 years steam-turbine driven 7.2.2 IEEE 1974 32 years steam-turbine driven

1.5 Years RBs-turbine driven 15 years engine driven

7.2.4 O/Donnell 1980 1. 7 vears continuous- age! 10 years 8.3 years continuous" age> 10 years 185 hours stand'by

7.2.7 Morzelle at al. 1989 425 hours stand· by: before 1980

1000 hours stand·b)r; 1980-1988

7.2.10 Smith et at. 1990 II • 49 days continuous

8 - 68 days stand'by 7.2.12 Ver"lanke 1991 2 vears

Table 35: Summary of generator life times: recommended values and data from surveys.

Section Reference ETTF Remarks

7.3. I Snaith 19n 300 hours 7.3.2 Magnon et a1. 19n 15 dayS stand'by

7.3.3 Allan et a1. 1980 30 dayS stand'by

7.3.4 Dialynas and ALlan 1986 10 years 7.3.6 WhHing, 1989 320 daYS stand· by

7.3.9 Allan and IrI9!I'Rojas 1990 14 days stand·by_ 7.3.11 Prescott et a1. 1991 10 years stand· by

Table 36. Summary of generator life times: data used in reliability studies.

-84- Generators.

Section Reference ENTf Remarks 7.2.4 O'Donnell 1980 75 7.2.7 Morzelle et al. 1989 600 before 1980

28 000 1980·1988 7.2.10 Smith et at. 1990 210 gas· turbine driven

155 gas· turbine driven 125 gas· turbine driven 128 Aas·turbine driven 33 diesel driven 270 diesel driven 55 diesel driven 200 diesel driven 300 di esel driven 55 diesel driven 100 diesel driven 11 diesel driven 58 diesel driven

Table 37. Summary of fail-to-start data: data from surveys.

Section Reference ENTF Remarks 7.3.1 snaith 1977 33 diesel generator

63 c i rcui t breaker 7.3.2 Magnon et a l. 1977 51 7.3.3 Allan et al. 1980 10 7.3.4 Dialynas and Allan 1986 10 2 10 MVA

5 1 MVA 7.3.9 Allan and (noa-Roias 1990 20 - 30 7.3.10 Dialynas and Koskolos 1991 10

Table 38: Summary of fail-to-start data: data used in reliability studies.

-85- Generators.

8. "UNINTERRUPTABLE" POWER SUPPLIES

8.1. Recommended yalues

8.1.1. IEEE standard 500 [IEEE,1984] gives reliability data for components of nutlearpower stations. The data has been derived by a Delphi-method combined with the results from several surveys and databases. For uninterruptable power supplies the following data are recommended.

All types No output Fail to transfer Degraded Incipient

: ETTF = 30 years ; range = <4, 600> : ETTF = 20 years ; range = <3,450> : ETTF = 57 years ; range = < 8, 1300 > : ETTF = 57 years ; range = <8,1100>

Single-phase static inverter No output : ETTF = 110 years ; range = < 10, 400 > Fail to transfer : ETTF = 85 years ; range = < 7, 300 > Degraded : ETTF = 350 years ; range = < 30, 1100 > Incipient : ETTF = 165 years ; range = < 14, 600 >

Three phase static inverter No output : ETTF = 40 years ; range = < 4, 600 > Fail to transfer : ETTF = 30 years ; range = < 3, 450 > Degraded : ETTF = 55 years ; range = < 6, 900 > Incipient : ETTF = 120 years ; range = < 10, 2000 >

8.1.2. The IEEE gold book [IEEE,1991] recommends:

rectifiers : ETTF = 26 years.

inverters : ETTF = 291 days.

-86- 'Uninterruptable power supplies'.


8.2.1. The IEEE sponsored survey completed in 1972 [lEEE,1974) results in:

rectifiers : ETTF = 26 years ; c.L = [15.65).

inverters : ETTF = 300 days ; c.i. = [200,600 days).

-87- ·Uninterruptable power supplies·.


8.3.1. The IEEE orange book [IEEE, 1987) states that large non-redundant UPS systems can have an ETTF of 20,000 hours (2 years) using handbook reliability data; but field experience indicates ETTF's in the order of 40,000 hours. A redundant system is 2 - 4 times more reliable than a nonredundant system.

8.3.2. Suntio and White [1988) use different types of UPS to optimize the power supply. They use the following data:

Traditional Battery ETTF = 100 years maintenance checking one a year

SLA battery ETTF = 50 years maintenance checking once every 3 years

Rectifier ETTF = 1 5 years repair readiness 24 hours maintenance checking once every year

8.3.3. Van der Vaart en Bouwkneght [1989) give data for the failure of UPS-systems. They refer to this data as ·generally accepted figures·. rectifier : ETTF = 4.5 years; battery : ETTF = 11 years; inverter : ETTF = 3 years; defective set isolator : ENTF = 20, N = number of inverter failures; bypass system : ENTF = 20, N = number of inverter failures;

8.3.4. Schneider [1989] presents reliability parameters for components of UPS-systems. Load-bus : ETTF = 8000 years ; repair time = 4 hours DC-bus : ETTF = 8000 years ; repair time = 4 hours Static switch : ETTF = 12 years ; repair time = 4 hours Inverter : ETTF = 4 years ; repair time = 4 hours 1 battery : ETTF = 4 years ; repair time = 39 hours 2 batteries in parallel : ETTF = 1800 years ; repair time = 19 hours

8.3.5. Suntio et al. (1989) give the following data for a standby UPS: Inverter : ETTF = 10.3 years Battery charger : ETTF = 17.1 years Transfer switch : ETTF = 22.8 years

For a true UPS they give

Inverter : ETTF = 3.5 years Rectifier : 6.2 years Bypass switch: 22.8 years


Figure 17 shows the difference between a standby UPS and a true UPS.

Public p"""r L.i ...

-

StatJ.c Dypua Switch

R.cti- Inver-tier 1-,- tor

aattery S .. _

Transrer Swi tc:h

CRITICAL AC

LOAD Batt. In-Char- _~

pI" ter

CRITICAL AC

LOAD

Figure 17: True UPS (left) and standby UPS (right); from Suntio et al. [19891.

8.3.6. Bakker [1989] mentiones the following data, as given by compagny A for their UPS.

Rectifier/charger : ETTF = 9 years. Battery : ETTF = 14 years

Inventer : ETTF = 6 years Static interrupter (ultra rapid stoppage) : ETTF = 60 years static by-pass switch : ETTF = 60 years

For Compagny B the following data were given:

Rectifier/charger

Battery

Inverter

: ETTF = 7.6 years : repair time = 12 hours : ETTF = 9.5 years : repair time = 12 hours : ETTF = 3.6 years

Static by-pass switch : repair time = 12 hours : ETTF = 16.5 years

DC bus

AC bus

: repair time = 12 hours : ETTF = 11.4 years : repair time = 2 hours : ETTF = 43.1 years : repair time = 2 hours

From this data he determines ETTF-values for emmergency supplies from 3 manufacturers. The results are given in Table 39.

Manuf. A. Manuf. B Manuf. C

Static Static Dvnamic Standard UPS 24E" 20 yr 23 yr Extra inverter 41 yr 29 yr 2 UPS chains 60 yr 33 yr 132 yr 3 UPS chains 60 yr 29~ 59 -"ir

Table 39: ETTF for different UPS types from three manufacturers [Bakker, 19891.


8.3.7. Fiorina and Chevalier [1990] compare two methods for the reliability analysis of UPS-systems. They use the following data (derived from Merlin Gerin studies on elements):

Bypass switch : ETTF = 50 years Inverter : ETTF = 10 years Battery : ETTF = 14 years Rectifier : ETTF = 8 years

;repair time = 6 hours. ; repair time = 6 hours. ; repair time = 6 hours. ; repair time = 6 hours.

8.3.8. Dialynas and Koskolos [1991] use data for failure of UPS for a reliability study of an industrial power system. Redundant UPS's are used. They use the following data.

rectifier : ETTF = 27.4 yr ; repair time = 1 hour; inverter : ETTF = 5.5 yr ; repair time = 1 hour; main time to discharge battery: 8 hours; probability of static interrupter not successfully isolating faulted inverter: 1 %.

8.3.9. Prescott et al. [1991] use a value of ETTF = 35 years for the failure of a UPS plant.

8.3.10. Lee [1991] compares a dual-conversion UPS with a single-conversion on-line UPS. Figure 18 shows both types. He states that for a dual-conversion UPS:

ETTF - 400 days

This low value is due to the constant heating and stress produced in the power semiconductors.

The new single-conversion on-line designs are said to have a higher reliability:

ETTF - 2000 days.

-'" SWkswiU'h

,-------lC '" - - '"

~=----a- I! -'--_...J--- SappiJ1'DaCewimucWtyanUable

• - Silpplrnme wIda taiUtJ fIiJun! .- ~

-. l T

- Supply rowt wttb oUlky .,aiiIbIt

• - Supply I"Mlte wid! ~. faihlre

Figure 18: Single conversion UPS (left) and dual conversion UPS (right); from Lee [1991J.


8.3.11. Warren [1992] examines different UPS system configurations to formulate the most reliable system. The failure rates used are based on field statistics and MIL-HDBK-217-E. He uses the following data:

Rectifier Battery Inverter Static Switch Electromagnetic transfer switch

: ETIF = 20 years; : ETIF = 11 years; : ETIF = 6 years; : ETIF = 45 years; : ETIF = 22 years;


8.4. Ageing data

No ageing data for UPS's has been found.


8.5. Conclusions

Table 40 summarizes the life time values for components of Uninterruptable power supplies. Table 41 summarizes these values for complete UPS systems. For batteries, switches and busses no survey data were available. This makes it hard to judge the worth of the different values used in reliability studies.

The only survey available gives a lifetime of about 1 year for the inverter, making it by far the most sensitive part of the UPS. Remarkably, all reliability studies use a higher reliablity for the inverter: ETIF = 3 - 10 years. The lower values is however confirmed by [IEEE, 1987) and [Lee, 1991) who give UPS lifetimes of 2 and 1 year, respectively. All other studies use UPS lifetimes that are probably much too optimistic.

From the available information I would suggest 0.5 to 2 years for the inverter and 10 to 30 years for the rectifier.

Section Reference inverter rectifier battery bypass load-bus DC· bus Remarks switch

8.1.2 IEEE 1991 291 d 26 yr

8.2.1 IEEE 1974 300 d 26 yr

8.3.2 suntio and White 1988 15 yr 50·100 yr

8.3.3 van der Veart 1989 3 yr 4.5 yr 11 yr 10 • 8.3.4 Scheider 1989 4~ 4 yr 12 yr

8.3.5 suntio et ale 1989 3.5 yr 6.2 yr 22.8 yr true UPS

10.3 yr 17.1 yr 22.8 yr stand-by UPS 8.3.6 Bakker 1989 6 yr 9E 14 yr 30 yr manuf. A

3.6 yr 7_6 yr 9.5 yr 16.5 yr 43.1 yr 11.4 yr manuf. B

8.3.7 Fiorine et al. 1990 10 yr 8 yr 14 yr 50 yr

8.3.8 Dialvnas et at. 1991 5.5 yr 27.4 yr 100 * 8.3.11 Warren 1992 6 yr 20 yr 11 yr 22-45 yr

Table 40: Summary of UPS component lifetimes.

Section Reference ETTF Remarks 8.1_ 1 IEEE 1984 30 years no output

20 years fail to transfer 57 years de.raded 57 years inci2!ent

8.3.1 IEEE 1987 2 - 4 years nonredundant 4 - 16 years redundant

8.3.6 Bakker 1989 20 - 23 years standard UPS 29 - 41 years extra inverter 33 - 132 years 2 UPS chains 29 - 60 years 3 UPS chains

8.3.9 Prescott et ale 1991 35 years

8.3.10 Lee 1991 400 days dual conversion 2000 days single conversion

Table 41: Summary of UPS lifetimes.


9. CABLES AND ACCESSORIES

All values in this chapter are for 1000 meter of cable, unless otherwise noted I


9.1.1, Green and Bourne [19721 give average component failure-rates for electrical components. For cables they give:

less than 1 kV ETTF 1 to 33 kV: ETTF 33 kV to 275 kV

: 190 years : 26 years : ETTF = 1 5 years

9.1.2. Kloeppel et al. [19901 give recommended values of component data for reliability studies. They were based on data from several industries in Eastern Germany, as well as from the public supply. They recommend the values below:

1 KV : ETTF = 17 years, range = <7,60> : repair time = 12 hours, range < 1 0, 30 >

6, 10 kV : ETTF = 14 years, range = <7,50> : repair time = 12 hours, range = < 10, 30>

20, 30 kV : ETTF = 11 years, range = <8, 50> : repair time = 30 hours, range = < 15, 50 >

110 kV : ETTF = 11 years, range = < 10, 100> : repair time = 40 hours, range = <30, 100>

9.1.3. The IEEE gold book [IEEE, 1991 1 recommends the ETTF values given in Table 42.

Type of cable ETTF (ye.rs) average median repair time repair time

(hours) (hours)

Above ground and aerial o • 600 Volt 216 457.0 10.5 600 - 15 000 Volt

All 21.6 40.4 6.9 In trays above ground 33.0 8.9 8.0 In conduit above ground 6.2 140.0 47.5 Aerial cable 2.1 31.6 5.3

Below ground and direct burial o - 600 Volt 78.6 15.0 24.0 600 - 15 000 Volt

All 49.4 95.5 35.0 In duct or conduit 49.7 96.8 35.0

Above 15 000 Volt 90.7 16.0 16.0 thenmoDlastic 600 • 15 000 Yolt 78.8 44.5 10.0 thermosetting 600 - 15 000 Volt 34.3 168.0 10.0

lpaper insulated lead covered 600 V - 15 kV 33.4 48.9 26.8 other type of insulation 600 - 15 000 Volt 16.6 16.1 28.5

Table 42: Recommended values according to IEEE Gold Book [IEEE, 1991 J.

-94- Cables and acessoiries

The last two colums give the industry average and the median plant average of the actual downtime per failure.

The IEEE gold book recommends the following ETTF values for for cable joints with rated voltages between 600 and 15,000 Volts ..

all types of insulation; in duct or conduit below ground ETTF = 1150 year; average repair time = 36.1 hours; median repair time = 31.2 hours

thermoplastic ETTF = 1325 year; average repair time = 15.8 hours; median repair time = 8.0 hours.

paper insulated lead covered ETTF = 950 year; average repair time = 31.4 hours; median repair time = 28.0 hours.

The IEEE gold book recommends the following ETTF-values for cable terminations for rated voltages between 600 and 15,000 kV.

thermoplastic ETTF = 239 year; average repair time = 10.6 hours; median repair time = 11.5 hours.

thermosetting ETTF = 3260 year; average repair time = 451.0 hours; median repair time = 11.3 hours.

paper insulated lead covered ETTF = 1280 year; average repair time = 68.8 hours; median repair time = 29.2 hours.



9.2.1. Dickinson (1962) gives the results of an AlEE survey held in 1959 after the reliability of electrical equipment in industrial plants. the survey covered 33 compagnies with 58 plants. The results for power cables are reproduced below.

Lead-covered power cable aerial : ETTF = 25 yr above-ground conduit : ETTF = 410 days underground conduit : ETTF = 12 yr direct burial : ETTF = 22 yr

Non-leaded power cable aerial above-ground conduit underground conduit direct burial

: ETTF '" 5% yr : ETTF '" 12 yr : ETTF = 8 yr : ETTF = 6 yr

The following repair times are reported:

Lead-covered power cable. aerial : 5 hours above-ground conduit : 5 hours underground conduit : 111 hours direct burial : 21 hours

Non leaded power cable aerial above-ground conduit underground conduit direct burial


For cable joints and terminations results are:

; sample too small for confidence interval ; c.i. = <300,750 days> ; c.i. = <9,15 yr> ;c.i. = <17, 34yr>

; c.i. = <4, 7% yr> ; c.i. = < 10, 16 yr> ; c.i. = <7,9 yr> ; c.L = <3, 12 yr>

leaded : ETTF = 550 year ; c.L = < 450, 700 yr> non-leaded: ETTF = 310 year; c.i. = <260,380 yr>

The repair times reported are: leaded joints! terminations : 15 hours non-leaded joints/terminations : 17 hours

9.2.2. Connor and Parkins (1966) report about a 14-year survey of failures in networks with nominal voltages between 2 and 33 kV. The records of some 130,000 km.years were available. For underground cables the following values are found:

2-5 kV solid or resistance earthing: ETTF = 55 years; 6.6 kV solid or resistance earthing : ETTF = 19 years; 11 kV arc-suppression coil : ETTF = 19 years; 11 kV solid or resistance earthing : ETTF = 23 years;


22 kV solid or resistance earthing : ETTF = 35 years; 33 kV solid or resistance earthing : ETTF = 58 years.

Table 43 gives the relative contributions of the different faulure causes to the failure rate:

S.YS.tem vol tege 2-5 kV 6.6 kV 11 kV 11 kV 22 kV 33 kV method of earthing sol id/res sol id/res arc sup co; 1 sot id/res sol id/res sol id/res total hlll16n agency 49% 42X 27% 29% 43X 21X 30X mechanical damage to sheath 4X 7% 3X 4X 5X 4X 4% corrosion 4X lX 2X insulation failure l1X l1X 20X 17% 7% 29% 17%

1PQ.~e-box failure 5X 17% lOX lax 7% 11% termination failure 7% 16X 20X 15X 20X 4X 16X

I joint fai lure l1X 13X ax 13X 2X 25X l1X I Around subsidence 7% 1X 3X 5X 4% 4X miscellaneous 7% 5X 4% 8X 7% 5X

Table 43. Contribution of different causes to power cable failures, according to Connor and Parkins [1966J

The most serious single fault cause is human agency. This is (according to the authors) mainly due to the increasing use of excavators and other mechanical aids. Since 1951 the fault rate due to human agency has increased in both urban and rural areas. The most likely times for this type of fault to occur are round 9.00 am and round 3.00 pm.

For failures in the underground cable itself, follows from the above data:

2-5 kV solid or resistance earthing : ETTF = 67 years; 6.6 kV solid or resistance earthing: ETTF = 27 years; 11 kV arc-suppression coil : ETTF = 26 years; 11 kV solid or resistance earthing : ETTF = 32 years; 22 kV solid or resistance earthing : ETTF = 45 years; 33 kV solid or resistance earthing : ETTF = 82 years.

The lower failure rate of 33 kV cables, in comparison with 11 kV cables, is said to be due to the fact that 33 kV cables are generally larger, mechanically stronger, and usually buried deeper than 11 kV cables. The main failure causes for 33 kV cables are: insulation failure and joint failure.

For cable joints they found:

11 kV 33 kV

: ETTF = 1100 years; : ETTF = 1100 years.

For cable terminations the results are:

11 kV 33 kV

: ETTF = 1000 years; : ETTF = 2500 years.


9.2.3. An IEEE sponsored survey of electrical equipment reliability in industrial plants was completed during 1972 IIEEE,1974J. This survey included a total of 1982 equipment failures that were reported by 30 compagnies covering 68 plants in nine industries in the United States and Canada. The results for cables are given in Table 44.

The last four columns give, respectively: the industry average of the actual downtime per failure; the median plant average of the actual downtime per failure; the average estimated time to fix the failure during 24 hour work day in case of repair of the failed components and in case of replace with a spare.

Type of cable ETTF c. i. Bvesrege actual down time estimated restore (years> (hours) time (hours)

;ndust~ plant recall' replace

Cable· all type of insulation 50 [40 60]

Above ground and aerfal o • 600 Volt 200 [100 2000] 457.0 10.5 20.8 39.7 600 . 15 000 Vol t

All 20 [15 30] 40.4 6.9 26.8 60.4 In trays above ArOlSld 30 [20 65] 8.9 8.0 49.4 119.0 In conduit above ground 6 [319] 140.0 47.5 19.8 Aerial cabLe 20 [15 30] 31.6 5.3 10.6 28.0

Below growd and direct burial

o - 600 Volt 75 [40 700] 15.0 24.0 26.8 600 - 15 000 Volt

All 50 [40 60] 95.5 35.0 20.4 26.8 In duct or conduit 50 [40 60] 96.8 35.0 20.9 26.8

Above 15 000 Volt 90 [50 450] 16.0 16.0 16.0 Cable - thenmoplastic

600 - 15 000 Volt 75 [50 140] 44.5 10.0 22.5 29.3 Cable - thermosetting

600 - 15 000 Volt 35 [25 50] 168 26.0 27.2 55.2 Cable - paper insulated lead covered

600 - 15 000 Volt 35 [25 50] 48.9 26.8 17.3 18.3 CabLe - other type of insulation

600 - 15 000 Volt 17 [10 35] 16.1 28.5 23.2 44.8

Table 44: Cable data according to 1972 IEEE survey [IEEE,19741.

The survey resulted in the ETTF-values and corresponding confidence intervals for cable joints for rated voltages between 600 and 15,000 Volts, as shown in Table 45.

type of joint ETTF c. i. average actual down time estimated restore time (years) (hours) (hours)

industry i plant repair restore

All tvoes of insulation

all 1000 [800 2000] above ground and aerial 1200 [600 7500] in duct or conduit below ground 1150 [850 1800] 36.1 31.2 14.7 5.5

thermoelastic 1300 [900 2300] 15.8 8.0 12.6 22.0 thermosetting 800 [400 5000] paper insulated lead covered 950 [600 2250] 31.4 28.0 30.0

Table 45. Data for cable joints according to the 1972 IEEE survey [IEEE, 19741


From the confidence intervals one can conclude that it is not realistic to consider different ETTF-values for the different types of cable joints.

The survey result for cable terminations are given in Table 46.

type of termination ETTF c. ;. average actuaL down time estimated restore time (vears) (hours) (hours)

industry 'plant reoair restore

All types of insulation 1650 [1350 2100] o • 600 VoLts

Above ground and areal 7900 [4500 32000] 3.8 4.0 8.0 8.0 600 - 15 000 Volt 1130 [850 1700] 198 11.1 34.6 40.6

Above ground and areaL 1130 [850 1700]

in trays above ground 3000 [1700 12000] 8.0 9.0 48.8 58.3

in condui t above ground 780 [435 4000]

aedal cable 540 [380 1000] 48.5 11.3 15.3 18.0 duct or conduit below ground 3300 [1850 13000] 25.0 23.4 28.8 30.0 thennoDLastic 240 [150 600] 10.6 11.5 12.0 12.0 thermosetting 3300 [2100 7000] 451 11.3 30.2 42.8 paper insulated lead covered 1300 [850 2500] 68.8 29.2 39.0 30.0

Table 46. Data for cable terminations according to the 1972 IEEE survey [IEEE, 19741

9.2.4. Braun [1986] presents some results of the failure data kept by the Northwest Underground Distribution Committee of the Northwest Electric Light and Power Association. Their annual report on component reliability is only concerned with natural failures. All failures caused by abnormal external means, such as through dig-ins or damage prior to installation are not intended to be included in the data. The author mentiones that there are still problems with field people not reporting the material failures. All failure rates reported should therefore he considered on the low side. For 15 kV-cable he finds the ETTF-values in Table 47.

Year HMWPE XLPE 175mn2 220 1J1112 175 nm2 220 nm2

1973 179 yr. 115 yr. 596 yr. -1974 392 yr. 114 yr. 304 yr. 287 Yr. 1975 223 yr. 107 yr. 488 yr. -1976 135 yr. 126 yr. 342 yr. 94 yr.

19n 129 yr. 155 yr. 413 yr. -1978 78 yr. 233 vr. 152 vr. 2011 vr. 1979 60 yr. 179 yr. 237 yr. 1341 yr.

1980 47 vr. 248 yr. 5363 vr. -1981 30 yr. 169 vr. 1609 yr. 3218~.

1982 34 yr. 95~. 2299 yr. -1983 37 yr. 93 yr. 304 yr. -

Table 47: Data for 15 kV cables according to Braun £19861.

The results for low-voltage (oS 600 Volts) cables are given in Table 48.

Figure 19 shows the ETTF for 175 mm2 and 220 mm2, 15 kV, HMWPE-cables plus the confidence interval for the consecutive years. The author only gives the size of the population for 1983. It is assumed that the size of the population did not change. There is a clear decrease in ETTF for 175 mm2 as a function of time. No explanation for this


decrease is given. There is no significant change in ETTF for 220 mm2.

year ETTF year ETTF year ETTF 1969 107 yr. 1974 322 Yr. 1979 220 yr. 1970 129 yr. 1975 413 yr. 1980 335 yr. 1971 217 yr. 1976 304 yr. 1981 201 yr.

1972 260 Yr. 19n 220 yr. 1982 230 Yr. 1973 460 yr. 1978 227 yr. 1983 206 yr.

Table 48: Data for low-voltage cables according to Braun [1986J.

'~r---------------------~

!

5 ,.

1~----------~----------~ ,L-__________ ~ ________ ~

1In 1171 1113 1171 117. 1113

ob .. rved y .. r ob .. fwd , •• r

Figure 19. ETTF for 175 mm2 (left) and 220 mm2 (right) 15 kV HMWPE cables, according to Braun [1986J. The shaded areas represent the 95% confidence intervals.

From the 1983 data, for which the population size has been given, confidence intervals can be determined:

HMWPE, 175 mm2 15 kV HMWPE, 220 mm2 15 kV XLPE, 175 mm2 15 kV HMWPE, 25 kV XLPE, low voltage

: ETTF = 37 years, c.i. = <33,41 > : ETTF = 93 years, c.i. = < 73, 130 > : ETTF = 300 years, c.i. = < 200,500> : ETTF = 18 years, c.i. = <15,23> : ETTF = 180 years, c.i. = < 160, 200>

For primary splices, 15 kV, molded rubber, the 1983 data results in: ETTF = 470 years; c.i. = <400,550 years>.

For primary splices, 25 kV, molded rubber: ETTF = 3000 years,

For primary s[lices, 35 kV, molded rubber: ETTF = 2200 years.

In the two latter cases, there is not enough data for a confidence interval.


9.2.5. Ferran et al. [1986) give failure data for underground cables in the EdF-network during the first half of the eighties. Table 49 gives the ETTF for 1 km of underground MV network. The network under study had a total length of about 75,000 km. From the text one can conclude that these values include all failures in the network, i.e. not only those in the underground cable. The increase of the ETTF is significant.

year EnF c.; . 1982 11.2 yr. <10.9 11.4>

1983 11.9 yr. <11.6 12.2> 1984 13.4 yr. <13.0 13.7.

'82-'84 12. I yr. <11.9 12.3>

Table 49: Data for underground MV networks according to Ferran et al. [1986J.

The results for about 70,000 km of underground LV network are given in Table 50. From the text one can conclude that these values include all failures in the network, i.e. not only those in the underground cable.

year EnF c. i .

1982 12. I yr. <11.8-12.4>

1983 12.1 yr. <11.8·12.4.

1984 13.2 yr. <12.8·13.5.

1985 12.5 yr. <12.3'12.6.

Table 50: Data for underground L V networks according to Ferran et al. [1986J.

Table 51 gives the ETTF-values for underground MV-cables.

lQ)rignated paper isolation

Year EnF c. i. 1980 27.3 yr. <26.2·28.6. 1981 32.9 yr. <31.4·]4.5. 1982 32.3 yr. <30.8·33.8> 1983 44.3 yr. <42.0-46.8> 1984 53.2 vr. <50.3-65.4>

'80- '84 36.0 yr. <35.3·36.8. XLPE insuLation

Year EnF c. i. 1980 30.3 yr. <28.4·32.3. 1981 35.1 yr. <33.2,37.2. 1982 42.0 yr. <39.8-44.4> 1983 50.8 yr. <48.3,53.6. 1984 54.0 vr. <51.4·56.9>

'80- '84 43.8 yr. <42.8-44.8>

Table 51: Data for underground MV cables according to Ferran et al. [1986J.

I.

--.-5. 00 00 000 000 00 .. ~ .. /

e /

• /

• ~ • .! 40 00 000 .~ . 000 -Imprlgnated pap.,

/

" / • "-XLPE ~ _r -

31 Y -. - .. .. _. _. 00 000

2. 1980 1 •• 1 U82 1983 1984

obMrved y .. ,

Figure 20. Trent in cable life time, according to Ferran et al. [1986J.

The trent in reliablity is shown in Figure 20. It can easily be seen that the reliability of cables increases. As the values for both isolation type are about the same and remain about the same, one can conclude that the failure mechanism may have nothing to do with the type of isolation. Before 1980 the newly installed cables were mainly paper-


insulated lead-covered; After 1980 mainly XLPE insulated 3-core twisted.

The values found for cable joints are shown in Table 52.

XLPE/XLPE ~oints Paper/XlPE joints

observed year ETTF observed year ETTF 1980 1600 yrs 1980 830 yr. 1981 1800 yr. 1981 760 yr. 1982 2000 yrs 1982 910 yr. 1983 4800 yr. 1983 no yr. 1984 14 000 yr. 1984 760 yrs

Table 52: Cable joint data according to Ferran et al. (1986J.

For cable terminations they give:

1980: 1981 : 1982: 1983: 1984:

ETTF = 6,000 years ETTF = 20,000 years ETTF = 50,000 years ETTF > 20,000 years ETTF > 200,000 years

9.2.6. One of teh Dutch regional electricity authorities, NV PNEM. performed a survey after the reliability of components in their 10 kV network, covering the period 1980 -1986 [van Amelsfoort et al.,1986). It resulted for cables in:

ETTF = 100 years, c.i. = [90,110J.

9.2.7. Waumans [19861 presents the result of failure registration in a number of medium and low-voltage networks in the Netherlands, during 1979 through 1984. The results are summarized in Table 53.

polymer insulated PBl:Ier fnsulated all twe. non· armored armored lead covered includinLl9ints and terminations

mediun voltage 103_yeafs ,. 1000 years 70_ years 5S years low voltage 13 years 60 years 32 years 21 years

house service connection 48 years 42 year. 18 years

Table 53. Data for underground cables, according to Waumans {1986J.

9.2.8. During 1987 VDEN registered all failures of power system equipment in Dutch medium-voltage and low-voltage distribution networks [VDEN, 19881. For the 400 Volt networks they registered 2006 failures in 69,692 km of underground cable, leading to:

ETTF = 34.7 yrs, c.i. = [33,371.

Detailed results are given in Table 54.

For 10-12.5 kV cables the survey results in:

ETTF = 79 years c.i. = [74,861 (not including failure of a joint)


ETTF = 59 years c.i. = [55,63) (including failure of a joint).

For 10-12.5 kV the survey results in a subdivision of failure causes for underground cables, joints and terminations.

Manufacture and assembly Excavation

: 9 % : 43 % : 7 % Soil warping

Moisture Pollution and wear Others external Others internal

: 5 % :4% : 14 % : 16 %

~r insulated lead covered All hi lures Excavations Others

oolvmer insulated armoured

All failures Excavations others

polymer insulated non-armoured

All failures Excavations Others

ETTF (yrl c.;. (yrl

38 [35 42]

IT [70 86]

IT [70 86]

71 [60 88]

110 [90 150]

200 [150 300]

20 [18 22]

32 [28 36]

52 [45 62]

Table 54. Data for 400 Volt cables according to VDEN survey (VDEN, 19881.

9.2.9. Pijls (1988) analysed failures in the electricity supply to a number of chemical plants during 8 years. For a 10kV cable connection he finds

ETTF = 90 year, c.i. = <65,140 year>.

The typical cable length in the system under study is one kilometer.

70% of the failures occured in the cable itself; 20% in the cable connectors; 10% in the cable terminators.

This implies for failure in the underground cable itself:

ETTF = 125 years; c.i. = <80,275 years>.

From the failure data collected we find for cable terminations of 10 kV cables:

ETTF = 1850 years, c.L = < 900, ->

9.2.10. A group of experts on synthetically insulated MV-cables performed a survey after the reliability of MV-cables with different types of insulation IBlechschmidt,1989).


Results were obtained from a population of almost 500,000 km.year.

The results of the survey over the period 1980-1986 are given in Table 55. The results of the survey over the period 1980-1988 are given in Table 56 for LOPE and XLPE insulated cables [Blechschmidt et al.,19911. Table 57 gives the results of the survey over the years 1987 and 1988, i.e. the difference between the two tables above. The differences between these three tables are considerable.

10 kV 20 kV 30 kV ETTF c. i. ETTF c.i. ETTF c. i ..

XLPE 260 [240 285] 330 [305 360] 38 [33 45J

LOPE 112 [100 128] 34 [32 36) 26 [23 30)

PVC 25 [23 28) 18 [15 23) 7.3 [6 9)

EPR 32 [28 36) 80 [60 115) 39 [33 46)

Table 55: Cable data over 1980 - 1986, according to Blechschmidt [19891.

10 kV 20 kV 30 kV

ETTF c.L ETTF c. i .. ETTF c. i ..

XLPE 325 [30OJ50) 250 [240 270) 35 [3~42)

LOPE 150 [135 170) 27 [26 28) 23 [21 26)

Table 56: Cable data over 1980-1988, according to Blechschmidt et al. [1991].

10 kV 20 kV 30 kV ETTF c. i .. ETTF c. i. ETTF c. f ..

XLPE 540 [470 650) 183 [170 200) 33 [27 41)

LOPE 270 [200 400) 16 [15 In 17 [14 20)

Table 57: Cable data over 1987 and 1988 (difference of Table 56 and 57).

9.2.11. The Rural electrical Association (a credit agency of the U.S. Department of Agriculture) conducted a survey after failures of cables in 1988 and 1989 [Dedman and Bowles, 19901. By observing a population of about 400,000 km.year and over 9,000 cable failures they find:

ETTF = 44 years.

They also registered the year of installation of the cable. This made it possible to determine

25o,----------------------------------,

200 1---------------t'I:--I--'I-------l

:lS0r--------~r--~----~----~ • b

~Ioor----------.~+-~------~.-~

50r-------7---~~-----~~

~~0--~7~2--~7-4-~76---78---80---82---84---8-6--~88

year of installation

the average failure rate during Figure 21. ETTF for cables of different year of the passed life time of cables installation, according to Dedman and Bowles [19901. as a function of the year of installation. The results are given in table 58. Figure 21 gives the inverse failure rate (ETTF) of cables of different year of installation, during 1988 and 1989. The value for 1985 (ETTF = 651 years) has


been omitted for clearity. In general, older cables show more failures than younger cables; but for very young cables the trent appaers to reverse.

year of observed ETTF (1 kin) c.i. (years) installation kin. year

1970 13 298 36 vr <33 40>

1971 25 544 17 vr <16 18>

1972 26724 20 vr <19 21>

1973 31 588 42 vr <39 45>

1974 36 347 22 vr <21 23>

1975 36560 40 Yf <37 43>

1976 31 522 38 yr <35 41>

1977 36 569 72 vr <65 80>

1978 45 203 161 vr <140 180>

1979 39 710 89 yr <80 100>

1980 26 370 56 yr <51 62>

1981 19 823 215 yr <180 270>

1982 14 867 162 yr <135 205>

1983 13 303 242 yr <190 330>

1984 11 166 115 yr <95 145>

1985 10 420 651 yr <430 1300>

1986 6 526 96 yr <75 130>

1987 5 738 25 yr <22 29>

1988 2079 40 yr <30 55>

All 433 357 44 yr

Table 58: "Ageing" data according to REA survey [Dedman, Bowles, 19901.

9.2.12. Franke [1990) observed, during 12 years, failures in a chemical plant in Eastern Germany. Early failures in the wear-in phase (1.5 .. 2 years) have been removed from the data. For 6 kV cables he finds:

ETTF = 45 years; c.i. = < 30.70>

Included in this value are

single and multi-phase shortcircuits short-circuits in cable joints short-circuits in cable terminations

9.2.13. Horton et al. [1990) provide estimates of the failure rates of underground distribution system components. Data on molded rubber splices was available since 1982 from NELPA (Northwest Electric Light and Power Association) surveys:

15 kV splices : ETTF = 325 years 25 kV splices: ETTF = 550 years 35 kV splices : ETTF = 400 years

; c.i. = <300,350> ; c.i. = <450,700> ;c.i. = <300,650>


9.2.14. Heiss and Schweer [1991] present the analysis of failure events during 5 years in 3000 km of underground cables in compensated 20 kV networks. From this they find for underground cables:

ETTF = 47 years; c.i. = <42,53 years>

70 % of the faults were caused by foreign influences (dredges, trench work and stones), 10% by atmospheric overvoltages (the systems under study contained some 5000 km of overhead lines), 20% by material faults

74% of all fault locations concerned the underground cables, 6% the cable joints, 20 % the cable terminations.

This implies for failures in the undergroudn cable itself:

ETTF = 63 years; c.i. = < 56,73 >

9.2.15. Morrison and Arhart [1991] give data for cable system performance in the period 1961-1990. Data has been collected from 36 plants of a major industrial manufacturer. Their results are given in Table 59. A subdivision in failure location is given in Table 60. Incorporating only failures in the cable itself leads to the lifetime expectations given in Table 61.

ETTF c.; . All cables 41 years <35 49>

EPR 29 years <Z3 41>

HMllPE 26 years <ZO 38>

other 66 ye.rs <51 91>

Table 59: Cable lifetimes, according to Morrison and Arhart [1991J.

cable joints term EPR 98X ZX . HMllPE 73X 10X 17X

Other BOX 10X 10~

Table 60: Failure location, according to Morrison and Arnhart [1991J.

ETTF c.;'

All cables 49 vrs [41 59]

EPR 30 yr. [Z3 4Z]

HMWPE 35 yr. [Z6 56]

Other 8Z vr. [6Z 1Z0]

Table 61: Only failures in the cable itself, according to Morrison and Arnhart [1991J.


The authors made a comparison between cables in a wet environment and cables a dry environment. The reliability of cables in a wet environment is significantly lower than that of cables in a dry environment. The holds expecially for EPR insulated cables where 54 failures occured in 43 circuit-miles of wet environment but none in 23 circuit miles of dry environment. Some results are given in Table 62.

wet env; rorrnent dry env;rorment ETTF c. i. ETTF c. i .

EPR 14~. [11 19] . .

HMWPE 23 yr. [17 35] 26 yrs [14 156]

others 73 yrs [53 115] 55 yrs [39 m all cables 34 yr. [29 42] 73 yrs [53 116]

Table 62: Influence of environment, according to Morrison and Arnhart [1991].

9.2.16. Bovy et al. (1991) analysed the outage occurances in some 4000 km of underground 10kV cable of a Dutch utility, during 6 years. For 1 km of underground cable they find:

ETTF = 77 years, c.L = < 68,87 years> repair time = 12.4 hours ETTF (phase-to-earth faults) = 1100 years, c.i. = < 1500, 2500 years> probability of multiple fault following phase-to-earth fault: 26%

9.2.17. Verplanke (1991) studied failures that occured in the power system of a chemical industry in The Netherlands between 1970 and 1991.

50 kV and 11 kV cable connections:

ETTF = 180 years; c.L <120,300.

25 % of the failures occured in the cable joints, 75 % in the cable itself.

This implies for failures in the cable itself:

ETTF = 240 years.



9.3.1. Dickinson [1957) gives a typical value of ETTF = 12 years for 1 km of cable 2,400 volt and up. and a typical value of ETTF = 22 years for 1 km of cable 600 volt and below. Typical repair times are 28 hours if a spare length is on hand and 41 hours if the run is too long for the spare length.

For cable joints in industrial power systems he gives a typical value of:

ETTF = 200 years.

For the repair time after failures in cable terminations he gives:

2400 volt and below 6900 volt and below 26,400 volt

: 10 hours : 14 hours 35 hours and more:

9.3.2. Dickinson [1960) uses the following values for power distribution systems for refinery process units. The values are based on several published papers and on records obtained from refineries.

straight jOint : ETTF = 1000 years ; repair time = 40 hours.

tee joint. leaded : ETTF = 500 years ; repair time = 40 hours.

tee joint. rubber or equal : ETTF = 1000 years ; repair time = 30 years.

9.3.3. Capra et al. [1969) use the following data for a reliability analysis of 12 kV underground distribution systems of Pacific Gas and Electric.

three-phase cable : ETTF = 32-160 years; single-phase cable : ETTF = 80 - 320 years

For 12 kV cable terminations they use the following data:

600 A terminations : ETTF = 670 - 1250 years; 200 A terminations : ETTF = 400 - 1250 years.

They use the following estimated time range for different operations in underground distribution systems:

receive alarm. travel to open substation breaker. or open main-line interruptor

receive trouble call. travel to open line protective device identify new sectionalizing location read fault locators for single-phase system read fault locators for three-phase system remove or replace grate operate switch or automatic interruptor

-108-

: 15 - 30 minutes; : 40 - 90 minutes; : 3 - 6 minutes; : 3 - 6 minutes; : 6 - 12 minutes; : 6 - 12 minutes; : 2 minutes;

Cables and acessoiries

remove 200 A terminations replace 200 A terminations drive between adjacent sectionalizing points:

single-phase system three-phase laterals three-phase main

operate nonload-break disconnects call for test

: 9 - 18 minutes; : 8 - 17 minutes;

: 2 - 3 minutes; : 3 - 6 minutes; : 6 - 9 minutes; : 3 minutes; : 6 - 12 minutes.

9.3.4. Bocker and Kaufmann [1977] use the following values for 10 kV cable in a public distribution network:


9.3.5. Lonsdale and Hitchen [1977] use the following value for underground cable in a public distribution network in the Nothwest of England. The data were based on an examination of system performance over recent years:

ETTF = 42 years.

9.3.6. Chang [1977] uses the following data for single-phase lateral cable to evaluate distribution system design:

ETTF = 40 years;

For cable terminations he uses:

ETTF = 1000 years;

average repair time required in restoration work: receive call, travel to location : 45 minutes; interrogate fault indicators : 36 minutes; operate load break connector/switch : 15 minutes; locate fault with fault locator : 1 hour; repair cable : 5 hours. replace cable terminations : 6 hours.

9.3.7. McNab [1977] uses the following values for 33 kV cable in a public distribution network in the South of Scotland:


9.3.8. Allan, Dialynas and Hormer [1979] use the following data for cables in a reliability study of a distribution system:

ETTF = 31 years; repair time = 33.5 hours; time to maintenance = 4 years; maintenance time = 8 hours.


9.3.9. Allan et al. [1980] use, in a reliability study of electrical auxilary systems of a power station, the cable reliability data shown in Table 63.

ETTF repair time

23.5 kV 200 yr 120 h

11 kV 200 yr 48h

3.2 kV 200 yr 36h

Table 63: Cable data used by Allan et al. [1980}.

9.3.10. Nelson and Johnson [1982] compare three distribution voltages for the power supply to a gas centrifuge uranium enrichment plant. They use the following reliability data (based on 1974 IEEE survey and on other available data) for cables:

13.8 kV: ETTF = 50 years; 24.9 kV: ETTF = 45 years; 34.5 kV: ETTF = 42 years.

For cable joints they use:


For cable terminations they use:


9.3.11. Ruoff and van Meeteren [1983] use a value of ETTF = 53 years for shortcircuits in underground distribution cable.

9.3.12. Williams and Mudge [1983] use a value of ETTF = 25 years for an 11 kV incoming cable and of ETTF = 20 years for a feeding cable. The repair time used is 30 hours. The values have been drawn from distribution system statistics.

9.3.13. Billinton and Goel [1986] use the following data for cable sections in a reliability analysis of an existing 14.4 kV distribution network in Saskatchewan, Canada:


9.3.14. Dialynas and Allan [1986) use the following values for cables, when including local generation in the reliability analysis of power distribution systems:

ETTF = 31 years; repair time = 33.5 hours; time to maintenance = 4 years; maintenance time = 8 hours.


9.3.15. Dialynas and Allan [1987) describe a reliability model for a power distribution network with local generation. For cables they use:


9.3.16. Goldberg et al. [1987) use the following values for an underground 21 kV feeder of Pacific Gas and Electric:

HMWPE cable after 10 years of service: ETTF = 32 years

XLPE cable in the first year of operation: ETTF = 530 years

9.3.17. Billinton [1987) uses for cable sections in the Annaheim distribution system of the Saskatchewan Power Corporation:


9.3.18. Dialynas [1988) uses a value of ETTF = 120 years for 150 kV cable.

9.3.19. Brzozowska-Theil and Theil [1988) incorparate reliability in the switching of medium voltage networks. For 20 kV-cable they use the following data:

ETTF = 7 years; repair time = 2.0 hours; switching time = 0.5 hours.

9.3.20. Dialynas and Papadopoulos [1989) use a value of ETTF = 20 years for 20 kV cables.

9.3.21. Fransen [1989) uses a value of ETTF = 195 years for short circuits in cables. The value is based on 7 Y. years of failure data in the power system of a large chemical plant. The author does not relate the ETTF value to a length of cable. Typical cable lengths in the system under study are between one and two kilometers. The repair time used is 24 hours. The author mentions 19 failures in 7 Y. years in about 500 cables. This leads to c.L = < 125. 375 years>

9.3.22. Roos [1989) compares the supply reliability for different configurations of medium voltage power systems. For cable strings he uses the following values:

Independent failure of cable string due to failure in cable (for 1 km of cable): ETTF = 31 years; repair time = 15 hours

Independent failure of cable string due to failure in station (for 1 cable string): ETTF = 170 years; repair time = 6.5 hours


9.3.23. Whiting [1989] determines the reliability of power supplies to broadcast transmitting stations. Component data are taken from various sources. For single-phase cable he uses:

11 kV lead-covered cable: ETTF = 180 years repair time = 48 hours

11 kV pvc - insulated cable ETTF = 400 years repair time = 36 hours

415 V lead-covered cable ETTF = 180 years repair time = 24 hours

415 V pvc-insulated cable ETTF = 400 years repair time = 12 hours

9.3.24. Horton et al. [1989a] determine the reliability of service of an underground 21 kV feeder of Pacific Gas and Electric. They use the following data:

HMWPE cable after 10 years of service ETTF = 32 years

XLPE cable in the first year of operation ETTF = 530 years

9.3.25. Horton et al. [1989b] use their Distribution Reliability Assessment Model (DREAM) to compute reliability indices for distribution feeders. They use the following cable data:

HMPWE cable ETTF = 32 years repair time: 90 minutes

XLPE cable ETTF = 530 years repair time = 90 minutes

9.3.26. Allan and Inga-Rojas [1990] describe a method for distribution system reliability. In an example they use for cable connections:

ETTF = 10 years.

9.3.27. Sallam et al. [1990] calculate. as an example. reliability indices for the MVnetwork of Port-Fouad. Egypt. For 3-phase cable they use the followinf values:

ETTF = 7.8 years repair time = 1.09 hours


For cable terminations at 13.8 kV they use:

ETTF = 550 years repair time = 25.0 hours

9.3.28. Mohan Rao and Sekhar [1990] compare the reliability of a number of distribution systems. They use the following data:

13.8 kV cable, conduit below ground ETTF = 50 years repair time = 26.5 hours

480 V cable, conduit above ground ETTF = 230 years repair time = 11 hours

cable terminations, 13.8 kV ETTF = 550 years repair time = 25 hours

cable terminations, 480 V ETTF = 5000 years repair time = 4 hours

9.3.29. Dialynas and Koskolos [1991] use a value of ETTF = 120 years for 1 km of cable. The repair time used is 96.0 hours. The cable in their system is operated at 20 kV.

9.3.30. Volkmann et al. [1991] use a value of ETTF = 32 years

for underground HMWPE-cable, and a value of ETTF = 535 years

for underground XLPE-cable.

The average repair time, from historical outage data, is 302 minutes for cables in a rural system and 229 minutes for cables in an urban system.

They refer to two internal reports of Pacific Gas and Electric as a source for these values.

9.3.31. Kjelle and Sand [1991] give the following values for cables in distribution networks in Norway:

22 kV rural network ETTF = 20 years; repair time = 13.7 hours;



9.4. Ageing data

9.4.1. Bar et al. [1990) use a Delphi-method to determine the position of the knee in the bath-tub curve, i.e. the age where the wear-out phase starts to become-important.

For paper-insulated-Iead-covered low-voltage cable they find, in case of good-circumstances : 54 years average-circumstances : 44 years bad-circumstances : 28 years

For polymer insulated low-voltage cable they find, in case of good-circumstances : 48 years average-circumstances : 39 years bad-circumstances : 25 years

For paper-insulated lead-covered medium-voltage cable they find, in case of good-circumstances : 45 years average-circumstances : 42 years bad-circumstances : 34 years

For polymer-insulated medium-voltage cable they find, in case of good-circumstances : 34 years average-circumstances: 30 years bad-circumstances : 20 years

For "mass" terminations of low-voltage cables they find, in case of good-circumstances : 55 years average-circumstances : 48 years bad-circumstances : 35 years

For low-voltage synthetic terminations they find, in case of good-circumstances : 52 years average-circumstances : 46 years bad-circumstances : 31 years

For medium-voltage oil-filled cable terminations they find, in case of good-circumstances : 42 years average-circumstances : 40 years bad-circumstances : 31 years

For medium-voltage "dry" terminations they find, in case of good-circumstances : 31 years average-circumstances : 27 years bad-circumstances : 19 years

For medium-voltage "mass" terminations they find, in case of good-circumstances : 44 years average-circumstances : 36 years bad-circumstances : 22 years


9.4.2. Horton et al. [1990) provide estimates of the failure rates of the failure rates of underground distrbution system components. The estimates are based on information collected from a number of utility sources throughout the United States of America. For each component and each year they used the number of units installed or removed and the number of failures which have occured. A Weibull-distribution has been assumed for the time-to-failure with repair as-bas-as-old. Consequently the form of the failure rate is taken to be:

J.( t) = :;:. ·tm-1 ,

where m is the shape factor and 8 the characteristic time-to-failure.

From 764 failures in 98,797 km. years of 15 kV HMWPE unjacketed cable in operation by San Diego Gas and Electric they find:

m = 1.3; 8 = 85 years.

Assuming a constant failure rate would have led to ETTF = 129 years.

From 226 failures in 86,191 km.years of 15 kV XLPE 175 mm2 unjacketed cable in operation by Northwest Electric Light and Power Association they find a constant failure rate:

m = 1; 8 = 380 years.

An analysis of 18 years and 4700 km of 220 mm2 cable again gives a constant failure rate

m = 1; 8 = 1200 years.

The higher value for 220 mm2 cable might be attributed to lower average voltage stress in the larger cable.


9.5 Conclusions

If the results of surveys on cable reliability show one thing, it is the difficulty in obtaining usefull fail data. Braun [1986] shows a large yea r-to-yea r variation in failure data, for the same system. The year to year variation is considerably larger than the confidence intervals. The number of failures in a certain year is thus no good predictor for the number of failures in later years. Also Ferran et al. [1986]. Blechschmidt et al. [1991] and Dedman and Bowles [1990] show this large variation. The change in failure rate is due to, among others, ageing of cable types, introduction of new cable types, changes in the amount of excavations. Variations might even be due to improved registration of failures.

As an example, Figure 64 gives the prediction from 1980-1986 data and the value for 1987-1988, according to Blechschmidt [1989] and Blechschmidt et al. [1991]. For the prediction a 99.5% confidence interval has been chosen.

voltage type 1980·1986 1987-1988 10 kV XLPE 230-300 years 540 years

LOPE 95-135 years 270 years 20 kV XLPE 290-375 years 183_1ears

LOPE 31·37 y.ars 16 years 30 kV XLPE 30-50y.ars 33 years

LOPE 20·32 y.ars 17 years

Table 64: Predicted value (1980-1986) and actual value (1978-1988) for cable life time, according to Blechschmidt et al. [1989, 19911.

For 10 kV cables the prediction is too pessimistic, for 20 kV cables it is too optimistic; for 30 kV cables the prediction turned out to be fairly reasonable.

Despite these problems I will try to obtain some acceptable values for cable lifetimes, Tables 65 and 66 summarize therefore the available data. Table 65 gives data from suveys; Table 66 data used in reliability studies. The values hold for failures in an underground cable itself (i.e excluding failures in cable joints and in cable terminations). Between the strong variations, values between 40 and 75 years appear to be acceptable for reliability studies. These values are indeed often used in reliability studies.

Data for cable terminations are summarized in Table 67 and Table 68. Values between 1000 and 3000 years seem to be reasonable.

Data for cable jOints are summarized in Table 69 and Table 70. Values between 500 and 2000 years seem to be reasonable.


Section Reference LV M'I Remarks 9.1.1 Green and Bourne 1972 190 26 9.1.2 Klo...,.1 et al. 1990 17 11-14 9.2.1 Dickinson 1962 22 armored

6 non-armored 9.2.2 COMer and Parkins 1966 26·82 depending on nom;nal voltage 9.2.3 IEEE 1974 75 50-90 9.2.4 Brall"l 1986 250 122 M'I strOM Iv time dependent 9.2.5 Ferran et at. 1986 36 DBDer insulation

44 polymer insulation 9.2.6 van Amelsfoort et al. 1986 100 9.2.7 lJaLlOans 1986 42-60 .1000 oolvmer armored

13-48 103 oolvmer non-armored 18·32 70 paper insulated lead covered

9.2.8 WEN 1988 35 79 9.2.9 pij Is 1988 90 9.2.10 Blechschmidt, 1989 7.3-260 depending on nominal voltage and on

tYPe of insulation Blechschmidt et at., 1991 16-540 depending on nominal voltage and on

tYPe of insulation 9.2.11 Dedman and Bowles 1990 44 strong year-to-Year variation 9.2.12 Franke 1990 45 9.2.14 Heiss and Schweer 1991 47 9.2.15 Morrison and Arhart 1991 30-35 9.2.16 BoyY et at. 1991 77 9.2.17 VerplBnke 1991 150

Table 65. Summary of cable life times: recommended values and data from surveys.

-, , 7- Cables and acessoiries

section Reference LV MY Remarks 9.3.1 Dickinson 1957 200 9.3.3 Capra et al. 1969 32-320 9.3.4 Boeker and Kaufmann 1977 20 9.3.5 Lonsdale and Hitchen 1977 42 9.3.6 Cheng. 1977 40 9.3.7 McNab 1977 42 9.3.8 Allan Dialynas and Hormer 1979 31 9.3.9 Allan et a1. 1980 200 9.3.10 Nelson and Johnson 1982 42-50 9.3.11 Ruoff and van Meeteren 1983 53 9.3.12 Will; oms end MudJje 1983 20-25 9.3.13 Billinton and Goel 1986 59 9.3.14 Dialynas and Allan 1986 31 9.3.15 Dialvnas and Allan 1987 30 9.3.16 Goldberg et al. 1987 32-530 9.3.17 Bil linton 1987 60 9.3.19 Brzozowska·Theil and Theil 1988 7 9.3.20 Olelynas end P!l"ld<>l1QUlos 1989 20 9.3.21 Fransen 1989 195 9.3.22 Roos 1989 31 9.3.23 Whiting, 1989 180 180 lead· covered

400 400 lool_r Insuleted 9.3.24 Horton et al. 198ge 32-530 9.3.25 Horton et al. 1989b 32-530 9.3.26 Allan and InQa·Rojas 1990 10 9.3.27 Sallern et at. 1990 7.8 9.3.28 Mohan Roo end Sekher 1990 230 50 9.3.29 Dialynas and Koskolos 1991 120 9.3.30 Volkmann et a1. 1991 32-535 9.3.31 Kjelle and Sand 1991 20-22

Table 66. Summary of cable lifetimes: data used in reliability studies.

Section Reference ETTF Remarks 9.2.1 Dickinson 1962 550 leaded

310 non· leaded 9.2.2 Connor and Parkins 1966 1000 11 kV

2500 33 kV 9.2.3 IEEE 1974 3300 9.2.5 Ferran et at. 1986 > 6000 9.2.9 Pi jls 1988 1850

Table 67. Summary of cable termination life times: recommended values and data from surveys.


Section Reference ETTF Remarks 9.3.3 Capra et al. 1969 670-1250 600 A

400-1250 200A 9.3.6 Chan •. 19n 1000 9.3.10 Nelson and Johnson 1982 3300 13.8 kV

2200 24.9 kV 1650 34.5 kV

9.3.27 Sallam et at. 1990 550 9.3.28 Mohan Rao and Sekhar 1990 550 13.8 kV

5000 480 Vol t

Table 68. Summary of cable termination life times: data used in reliability studies.

Section Reference ETTF Remarks 9.2.1 Dickinson 1962 550 leaded

310 non-leaded 9.2.3 IEEE 1974 1000 9.2.4 Braun 1986 470 15 kV

3000 25 kV 2200 35 kV

9.2.5 Ferran et al. 1986 4800 XLPE/XLPE 800 XLPE/paper

9.2.13 Horton et a l. 1990 325 15 kV 550 25 kV 400 35 kV

9.2.17 Verplanke 1991 180

Table 69. Summary of cable joint lifetimes: recommended values and data from surveys.

Section Reference ETTF Remarks 9.3.1 Dickinson 1957 200 9.3.2 Dickinson 1960 1000 straigt joint

500 leaded tee joint 1000 rubber tee joint

9.3.10 Nelson and Johnson 1982 3300 13.8 kV 2200 24.9 kV 1650 34.5 kV

Table 70. Summary of cable joint life times: data used in reliability studies.


10. BUSBARS


10.1.1. Kloeppel et al. [19901 give recommended values of component data for reliability studies. The values were based on data from several industries in Eastern Germany, as well as from the public supply. For busbars they give a relation between the failure rate of the circuit-breakers, AC.b.' and the failure rate of the busbar, Abusbar:

open busbars:

Abusbar = 0.1 * Ac.b. * # c.b.

closed, air isolated busbars:

Abusbar = 0.05 * Ac.b. * # c.b.

closed, solid isolated busbars:

Abusbar = 0.01 * Ac.b. * # c.b.

Where # c.b. is the number of circuit breakers connected to the bus.

10.1,2. The IEEE gold book IIEEE,19911 recommends for switchgear buses (indoor and outdoor):

insulated switchgear with rated voltages between 600 and 15,000 volts ETTF = 885 yrs; repair time = 28 - 261 hours;

bare switchgear with rated voltages below 600 volts ETTF = 1250 yrs; repair time = 27 - 550 hours;

bare switchgear with rated voltages above 600 volts ETTF = 521 yrs; repair time = 17.3 - 36 hours.

All these values are for one "unit". The number of units is the number of connected circuit breakers and connected switches.

The gold book recommends for bus ducts:

ETTF = 2450 years; repair time = 9.5 - 128 hours.

The unit is 1 circuit meter.

-120- Busbars


10.2.1. The 1972 IEEE sponsored survey [lEEE,1974] resulted in the following values for switchgear busses: (the unit was the number of compartements).

All switchgear busses insulated 600 - 15,000 Volt bare 0 - 600 Volt bare 600 - 15,000 Volt

: ETTF = 1 500 year : ETTF = 600 year : ETTF = 3000 year : ETTF = 1 600 year

; c.i. = [1000,2300] ; c.i. = [400,1300] ; c.i. = [1500,12000] ; c.i. = [900,5000)

The survey resulted in the following values for bus ducts (1 unit = 1 circuit meter):

ETTF = 2500 years; c.i. = [1500,5500)

10.2.2. O'Donnell [1979) presents the results of an IEEE survey after switchgear bus reliability of industrial plants and commercial buildings, performed in 1977. This survey was held because a major controversy emerged in the results of the 1972 survey [IEEE, 1974). Insulated bus showed a higher failure rate than bare bus, but data were heavily influenced by chemical industry. The new survey shows the opposite of this, with less chemical industry influence. The main results are given below:

All buses : ETTF = 950 years ; c.i. = <750, 1300>

Insulated buses above 600 V : ETTF = 890 years ; c.i. = <650, 1450>

Bare buses all : ETTF = 1 000 years ; c.L = <700, 1700> below 600 V : ETTF = 1250 years ; c.L = <850,2350> above 600 V : ETTF = 500 years ; c.L = <300,1800>

The number of units is the number of switchgear connected circuit breakers and connected switches.

The following data have been found for the repair time:

Insulated busses repair "round clock" average repair time

median repair time repair "normal hour" average repair time

median repair time

Bare busses repair "round clock" average repair time

median repair time repair "normal hour" average repair time

median repair time

-121-



Busbars

10.2.3. The PNEM survey [van Amelsfoort et al.,1986) resulted for busbars (the authors refer to ·substations") in:

ETTF = 1800 years, c.i. = [1200,3200).

10.2.4. Pijls [1988] finds from the failure data of an industrial power system collected during 8 years, for 10 kV busbar systems.

ETTF = 90 years, c.i. <40, ..... >

Most busbars in the system being studied consist of three rails.

10.2.5. Wahlstrom et al. [1988] describe the experience with new 84 kV-420 kV GIS in Sweden, between 1974 and 1986. By observing a population of 27 GIS with a total number of 102 bays they find:

ETTF = 40 years

The repair time is between 2 and 4 days.

This low value is due to the problems in manufacturing, installation and maintenance of this new type of switchgear. This value is therefore not representive for GIS switchgear.

10.2.6. Bovy et al. [1991] analysed the outage occurances in the 10 kV networks of a Dutch utility for a timeperiod of 6 years. For one section of a 10kV busbar they find:

ETTF = 10.000 years repair time = 3.2 hours ETTF = (phase-to-earth faults) = 5000 years probability of mUltiple fault following phase-to-earth fault: 41 %

10.2.7. Verplanke [1991] studied failures that occured in the power system of a chemical industry in The Netherlands between 1970 and 1991. He gives the following data:

Busbars 50 kV and 11 kV

ETTF = 125 years; c.i. <75,300>

-122- Busbars


10.3.1. Snaith [1977) uses the following values for the electricity supply of a nuclear power station:

3.3 kV busbar, per outer section ETIF = 200 years; maintenance interval = 1 year; repair time = 24 hours.

415 Volt busbar, per outer section ETIF = 200 years; maintenance interval = 0.5 years; repair time = 2 hours.

10.3.2. Bocker and Kaufmann (1977) use the following values for the 10 kVequipment in a system substation 10/0.4 kV in a public distribution network.

ETIF = 300 years; repair time = 26 hours.

10.3.3. Allan et al. [1977) use the following values for 11 kV busbars in the electrical auxiliary systems of power stations:

ETIF = 200 years; repair time = 10 hours; time to maintenance = 1 year; duration of maintenance = 20 hours.

10.3.4. Allan et al. [1980) use, in a reliability study of electrical auxiliary systems of a power station, the busbar reliability data shown in Table 71.

ETTF repair time time to I duration of maintenance

(vr) (h) (vr) (h)

11 tv 220 120 2 8 3.3. tv 220 48 2 8 415 V 220 24 2 4

Table 71.' Busbar data used by Allan et al. [1980J.

10.3.5. Adams and Jasmob [1981) use the following values for busbars in a distribution system:

ETIF = 42 years; repair time = 3 hours.

10.3.6. Nelson and Johnson [1982) compare three distribution voltages for the power supply to a gas centrifuge uranium enrichment plant. They use the following reliability data (based on 1974 IEEE survey and other available datal for one section of a bus:

13.8 kV : ETIF = 1580 years; 24.9 kV : ETIF = 1580 years;

-123- Busbars

34.5 kV : ETTF = 1580 years.

For the bus duct between the power transformer and the switchgear they use:

13.8 kV : ETTF = 100 years; 24.9 kV : ETTF = 100 years; 34.5 kV : ETTF = 100 years.

10.3.7. Ruoff and van Meeteren [1983] use a value of ETTF = 1000 years for busbar faults in distribution systems.

10.3.8. William and Mudge [1983] use a value of ETTF = 5000 year, with a repair time of 60 hours, for an 11 kV busbar. These values have been drawn from distribution system statistics.

10.3.9. Koval [1983] uses the following values for secondary busses in an industrial power system:

ETTF = 500 years; replacement ime = 10 hours; restoration time = 4.0 hours.

10.3.10. Dialynas and Allan [1987] describe a reliability model for a power distribution network with local generation. For busbars they use:


10.3.11 . Dialynas [1988] uses a value of ETTF = 45 years for 1 km of busbar (i.e ETTF = 45,000 year for 1 meter of busbar).

10.3.12. Whiting [1989] determines the reliability of power supplies to broadcast transmitting stations. Component data are taken from various sources. He uses the following data:

11 kV bus bars ETTF = 200 years repair time = 120 hours

415 V bus bars ETTF = 500 years repair time = 24 hours

10.3.13. Duke et al. [1989] assess the reliability of an industrial distribution system. For an 11 kV bus they use the following data:


10.3.14. Allan and Inga-Rojas [1990] describe a method for distribution system reliability. In an example they use for busbars:

-124- Busbars

ETTF = 200 years.

10.3.15. Sallam et al. [1990] calculate, as an example, reliability indices for the MVnetwork of Port Fouad, Egypt. They use the following data.

Switchgear bus, connected to 2 circuit breakers ETTF = 150 years repair time = 26,8 hours

Switchgear bus, connected to 4 circuit breakers ETTF = 75 years repair time = 26.8 hours

10.3.16. Mohan Rao and Sekhar [1990] compare the relaibility of a number of distribution systems. They use the following data:

Switchgear bus, 480 V, bare, connected to 7 breakers ETTF = 420 years repair time = 24 hours

Switchgear bus, 13.8 kV, insulated, connected to 1 breaker ETTF = 300 years repair time = 26.8 hours

10.3.17. Allan at al [1991] present a reliability test system for distribution networks. For bus bars they use the data given in Table 72.

permanent active ten.,orary fai lures fai lures fai lures

33 kV 1000_yrs 1000 vrs 100 vrs

11 kV 1000 yr. 1000 yrs 100 yrs

Table 72: Busbar data used by Allan et al. [T99T}.

10.3.18. Dialynas and Koskolos [1991] use a value of ETTF = 45 years for permanent failures on busbars. The repair time is 19.0 hours. Maintenance is performed every two years during 4.0 hours. The busbars in the system under study are operated at voltages of 150 kV, 20 kV, 6.6 kV and 380 V.

10.3.19. Prescott et al. [1991] use a value of ETTF = 200 years for the failure of an HV distribution switchboard, with and average repair time of 120 hours. For the LV distribution switchboard they use ETTF = 330 years and an average repair time of 24 hours.

10.3.20. Dortolino et al. [1991] use in a substation reliability avaluation study a value of ETTF = 100 years for busbars, with a repair time of 24 hours. Maintenance is performed once every 3 years during 8 hours.

-125- Busbars

10.4. Ageing data

10.4.1. Bar et al. [1990] use a Delphi method is determine the position of the knee in the bath-tub curve, i.e. the age where the wear-out phase-starts to become important. For the busbar in a low-voltage substation they find, in case of

good circumstances : 53 years; average circumstances : 49 years; bad circumstances : 29 years.

For a low-voltage busbar in an MV/LV-substation they find, in case of good circumstances : 43 years; average circumstances : 37 years; bad circumstances : 25 years;

For a closed busbar in and MV substation they find, in case of good circumstances : 26 years; average circumstances : 26 years;

For an open bus bar in an MV-substation they find, in case of good circumstances : 14 years; average circumstances : 14 years;

-126- Busbars

10.5. Conclusions

The failure rate of a busbar is dependent on the number of sections connected to the busbar, as well as on the actual length of the bus duct. Some authors therefore give a failure rate per section. The 1972 IEEE survey [IEEE, 1974] also gives a failure rate per meter. Unfortunately not authors refer their data to the dimensions of the bus, making a comparison with others difficult.

The results for busbars are summarized in Table 73 and Table 74. From this survey a value between 500 and 2000 years for the ETTF of one section might be concluded. The influence of failures per meter of bus duct appears to be neglectable.

Most reliability studies use a value of some hundreds of years for the whole busbar. This is consistent with the results from surveys.

Section Reference ETTF cer Remarks

section meter busbar 10.1.2 IEEE 1991 1250 < 600 Volt- bare

885 600-15 000 Volt" insulated 521 > 600 Volt- bare

2450 All types 10.2.1 IEEE 1974 3000 < 600 Volt- bare

600 600-15 000 Volt" insulated 1600 600-15 000 Volt· bare

2500 All types 10.2.2 O'Donnell 1979 1250 < 600 Volt- bare

890 > 600 Volt- insuLated 500 > 600 Volt- bare

10.2.3 van Amelsfoort et at. 1986 1800 10.2.4 Pi jls 1988 90 10.2.6 Bovy et al. 1991 10 000 10.2.7 Verplanke 1991 125

Table 73. Summary of busbar lifetimes: recommended values and data from surveys.

-127- Busbars

Section Reference ETTF ""r Remarks section meter busbar

10.3.1 snatth 1977 200 10.3.2 BOeker and Kaufmam 1977 300 10.3.3 A.lIan et at. 1977 200 10.3.4 Allan et at. 1980 220 10.3.5 Adams and Jasmb 1981 42 10.3.6 He 1 son ard Johnson 1982 1580 10.3.7 Ruoff and van Meeteren, 1000

1983 10.3.8 Will i ams and Muclge 1983 5000 10.3.9 Koval 1983 500 10.3.10 Dialynas and Allan 1987 1000 10.3.11 Diahnas 1988 45 000 10.3.12 WhithiM. 1989 200 11 tv

500 415 Volt 10.3.13 Duke et at. 1989 750 10.3.14 Allan and InRa-Ro"ss 1990 200 10.3.15 Sallam et al. 1990 300 10.3.16 Mohan Rao and Sekhar 1990 3000 480 Volt

300 13.8 tv

10.3.17 Allan et al. 1991 1000 10.3.1S Dialvnas and Koskolos 1991 15 10.3.19 Prescott et al. 1991 200 HV

330 LV 10.3.20 Dortolino et at. 1991 100

Table 74: Summary of busbar life times: data used in reliability studies.

-128- Busbars

11. LARGE MOTORS

11 .1. Recommended values

11.1.1. Green and Bourne [1972) give average component failure-rates for electrical components. For motors they give;

Motors in general induction motors above 200 kW

below, 200 kW synchronous motors small, general motors stepper motors

: ETTF = 11 years : ETTF = 11 years : ETTF = 23 years : ETTF = 16 years : ETTF = 29 years : ETTF = 23 years

11.1.2. IEEE standard 500 [lEEE,1983) gives reliability data for components of nuclear-power stations. The data has been derived by a Delphi-method combined with the results form several surveys and databases. For motors the following values are recommended (catastrophic failures only)

Induction squirrel cage, 150 kW and smaller Fails to run once started : ETTF = 105 years Fails to start : ETTF = 98 years

Induction squirrel cage, above 150 kW Fails to run once started : ETTF = 185 years Fails to start : ETTF = 185 years

Wound rotor induction Fails to run once started Fails to start

Single phase induction Fails to once started Fails to start 000>

Synchronous single phase Fails to run once started Fails to start


: ETTF = 175 years : ETTF = 40,000 years


; range = < 150, 1000 > ; range = <50, 1000>

; range = < 30, 350 > ; range <45, 500>

; range = < 35,475 > ; range = < 30, 400 >

; range = < 50, 4000 > ; range = < 15 000,250 -

; range = < 135, 200> ; range = < 170, 260>

11.1.3. Kloeppel et al. [1990) give recommended values of component data for reliability studies. The values were based on data from several industries in Eastern Germany, as well as from the public supply. For motors they recommend the following values:

below 1 kV

6 kV

: ETTF = 23 years : repair time = 6 hours : ETTF = 12 years : repair time = 20 hours

; range = <14,50> ; range = <1,10> ; range = <3,65> ; range = < 10, 50 >

-129- Large motors.


11.2.1. Dickinson [19621 gives the results of an AlEE survey held in 1959 after the reliability of electrical equipment in industrial plants. For electric motors the results are presented below.

Synchronous motors. 180 kW and larger: ETTF = 15 year ; c.i. = <10,20>

Induction motors. 180 kW and larger : ETTF = 9.2 year ; c.i. = <8, 11> 150 kW and below: ETTF = 19.5 year; c.i. = <19,20>

The repair times reported are: synchronous motors : 209 hours large induction motors : 162 hours small induction motors : 41 hours

11.2.2. A 1981 survey [lEEE,19821 in the electric utility industry reported, for motors 750 kW and up and not over 15 years of age, for insulation-related failures:

ETTF = 30 years; c.i. = < 25, 40>

11.2.3. O'Donell et al. [1983,19841 performed a survey after the reliability of motors of rated power 200 hp (150 kW) and higher. The population under study consisted of 1141 motors in 75 plants of 33 compagnies. A total of 360 failures occured in 5085 unit years i.e.

ETTF = 14years,c.i. = <12,16yr>.

A subdivision to type of motor is given in Table 75. DC motors appear to be somewhat more reliable than AC motors, but the difference is not significant.

ETTF c. i. " DOD

induction motors: 14 yr. <12, 16 yr.> 79" synchronous motors: 13 yr. <9, 19 yr.> 10 " "wollld rotor" 19 yr. <13, 34 yr.> 8" D.C. Motors 2S yr. <14 140 yr.>

3 "

Table 75: Subdivision to type of motor.

Table 76 gives a subdivision to rated voltage. The rated voltage has no influence on the reliability of a motor.

ETJF c. i. " pop 0-1 kV 13 yr. <10, 16 yr.> 27 " 1-5 kV 15 yr. <13, 17 yr.> 70" 5-15 kV 16 yr. <9 50 yr.> 3"

Table 76: Subdivision to rated voltage_

Table 77 gives a subdivision to rated power. High-power motors appear to be less reliable than low-power motors.

-130- Large motors.

ETTF c. i.

150-375 kW 15 yr <13. 17 yr.> 375-3750 kW 14 yr <11. 17 yr> 3750-7500 kW 5 yr <3 12 yr>

Table 77: Subdivision to rated power_

Table 78 gives a subdivision to motor speed_ The higher the speed of the motor, the higher its reliability_

ETTF c. i . 0-720 RPM 10 yr <8. 13 yr.> 721-1800 RPM 14 yr <12. 16 yr> 1801-3600 RPM 19 yr <15 26 Yr>

Table 78: Subdivision to motor speed.

The repair time found from this survey is:

97.7 hours in case of repair during normal working hours, 81.4 hours in case of repair round the clock, 18.2 hours in case of replacement by a spare motor.

11.2.4. Albrecht et a!. [1987] describe a survey after the reliability of large motors in power stations. The survey was limited to generating units above 150 MW, lowvoltage motors above 75 kW and all medium-voltage motors, generator units that went into service between January 1969 and December 1979.

The report covers information on 5797 motors in service at 132 generating units owned by 56 utilities. From the total population they find:

ETTF = 29.4 years; c.i. = < 28,31 >

There is a large spread in reliability among different units, utilities and manufactures. Figure 22 shows the distribution of estimated failure rate by unit. A statistical analysis by the authors shows that there is little spread among units of one utility. This leads them to the conclusion that "a major factor in the reliability of the motors is a function of who is using them".

40

FREOUENCY 30 (UNITS)

20

10

2 • 6 8 10 >10

ESTIMATED fAILURE RATE, ./. YEAR

Figure 22. Distribution of estimated failure per unit, according to Albrecht et a!. [1987J.

-131- Large motors_

Also among different manufacturers there is a large spread in reliability:

Manufacturer 1 : ETTF = 39 years ; c.i. = <34,45> Manufacturer 2 : ETTF = 33 years ; c.L = <28,38> Manufacturer 3 : ETTF = 22 years ; c.i. = <19,25> Manufacturer 4 : ETTF = 42 years ; c.L = <33,57> Manufacturer 5 : ETTF = 32 years ; c.i. = <32, 54> Manufacturer 6 : ETTF = 6 years ; c.L = <5,8> Manufacturer 7 : ETTF = 120 years ; sample too small Manufacturer 8 : ETTF = 19 years ; c.i. = <28,31 >

None of them shows a confidence interval overlapping the overall confidence interval <28,31 >.

A subdivision to failure mode yields for this survey:

bearing related stator related rotor related

: 41 % : 37 % : 10 %

11.2.5. Franke [1990] observed, during 12 years, failures in a chemical plant in Eastern Germany. Early failures in the wear-in phase (1.5 ... 2 years) have been removed from the data. He present the following data:

6 kV motors motors < 1000 V

: ETTF = 14 years; c.i. = <10,25> : ETTF = 17 years ; c.i. = < 12,25>

-132- Large motors.


11.3.1. Snaith [1977) uses the following value for the electricity supply of a nuclear power station:

probability of failure to start of 415 V motor: 0.1 %.

11.3.2. Allan et al. [1980) use the following data in a reliability study of electrical auxiliary systems of a power station:

11 kV motors 3.3 kV motors 415 V motors

: ETIF = 75 years : ETIF = 100 years : ETIF = 250 years

-133- Large motors.

11,4. Ageing data

No ageing data on large motors has been found.

-134- Large motors.

11.5. Conclusions

Table 79 summarizes the available data on large motors. From the surveys follows an ETTF value between 15 and 30 years.

Section Reference ETTF 11.1.1 Green and Bourne 19n 11-29 11.1.2 IEEE 1983 40-175 11.1.3 Kloeppel et ale 1990 12-23 11.2 _1 Dickinson 1962 9.2-19.5 11.2.2 IEEE 1982 30 11.2.3 O'DonneL l 1983 1984 14 11.2.4 Albrecht et ale 1987 30 11.2.5 frenke 1990 14-\7 11.3.2 Allan et ale 1980 75-200

Table 79. Summary of motor life times.

-135- large motors.

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iOin<:ihoven Universitv of Technology Research Reports ISSN 0167-9708

[acul!.L of El8ctrica I Enqineerinq

12511 EIJndhovell. J.T.J. van and G.6. de Jonq. L. Stok THE ASCIS DATA FLOW GRAPH: ~emantlc'--';;d textual format. EUT Report 91-E-251. 1991. ISBN 90-614H5H

(2521 Chen. J. ,nd PJ.I de Maag!. M.H.U. Herben

Coden: TElTEDE

WIDE-ANGLE RADIATION PATTERN mCULATION OF PARABOLOIDAL REFLECTOR ANTENNAS A compmtm study. EUT Report 91-E-252. 1991. ISBN 90-6144-252-4

(25J) Haan. S.W.H. de A PWM CURRENT -SOURCE INVERTER FOR INTERCONNECTION BETWEEN A PHOTOVOLTAIC ARRAY AND THE UTILiTY LI~E.

EIlT Report 9H-253. 1991. ISBN 90-6144-253-2

(254) Velde. M. van de and U.K. Clultmans EEG ANALYSIS FOR MONITORING OF ANESTHETIC DEPTH. EUT Report 9H-254. 1991. ISBN 90-6144-254-0

(255) Smolders, 1.8, AN EFFICIENT METHOD FOR ANALYZING MICROSTRIP ANTENNAS WITH A DIELECTRIC COVER USING A SPECTRAL DOMAIN MOMENT METHOD. EUT Renort 9\-E-255. 1991. iSBN 90-6144-255-9

(256) Backx, A,e,p,M. and A.A.H. Damen IDENTIFICATION FOR THE CONTROL Of MIMO INDUSTRIAL PROCESSES, EUT Report 9H-256. 1991. ISBN 90-6144-25(,-7

1257) Maaqt, P J! 1\, and H,G. ter Morsche, J.L.M. van den Broek A SPXmL RECONSTRUCTION TECHNIQUE APPl.ICABLE TO MICROWAVE RADIOMETRY EUT Report 92-E-257. 1992. ISBN 90-6144-257-5

!25GI Vieeshoums. J.M 0ERIV!TION OF A MODEL OF THE EXCITER OF A BRUSH LESS SYNCHRONOUS MACHINE. EUT Report 92-£-250 1992 ISBN 90-6144-258-3

(259) Orlov, V.B. DEfECT MOTION AS THE ORIGIN Of THE IIF CONDUCTlNCF. NOISE iN SOLIDS. WT Report 92-E-259. 1992. ISBN 90-6144-259-1

(2601 RODI!"kers.· J.E. ALGDRITIIMS FOR SPEECH CODING SYSTEMS BASED ON LINEAR PREDICTION. EUT Report 92-E-260. 1992. ISRN 90-6144-260-5

(261) Boom, LJ.J. van den and A.A.H D,men, Martin Klompstra IDENTIFICATION FOR ROBUST CONTROL USING AN H-infinlty NORM. EUT Report 91-E-261. 1992. ISBN 90-6144-261-)

12621 Grot,n. M and W. van Etten LASER LINEWIDTH MEASUREMENT IN THE PRESENCE OF RIN AND USING THE RECIRCULATING SELF HETERODYNE METHOD. WT Report 92-£-262. 1992. ISBN 90-6l44-262-1

(2631 Smoiders. t ~ RIGOROliS ANALYSIS Of THICK MICROSTRIP ANTENNAS AND WIRE ANTENNAS EMBEDDED IN A ,lISmATE EUT Heport 92-E-263. 1991. Isr,N 90-6144-161-j

EHQhoven UniversitY of Tpchn~ R-es8arch_R-eoorts IS~JN 01fi7--9708 Coden, TEl JEDE

!264\ Frenks, L.~ MId U ~. Clult"'n~,. M,j von Gils THE ADAPTIVE RESONINCE THEORY NETWORK IClusterillg-1 behdVIour In re'dilon with hrmst,,, audltory evoked D"tent"l pdtterllS, WT Report n-E-164, 1992. ISBN 90-6144-1648

1165i Wellen, J,S. and F. ~"out" M.r.e. Schem"ann, E. Sm,l\lruqqe, L N,F. KeulIDdfln MANUFACTURING AND CHARACTERIZATION OF GA!5mGlAS MULTIPLE QUINTliMWELl. RIDGE mEGUIDE LASERS WT Report 92+20j 1m. ISBN 90-6\44-165-6

12661 Cluitruans, U.M USING GENETIC ALGORITHMS fOR SCHEDULING DATA FLOW GRAPHS. Wi Report n+M \99, IS~N 9H14H66-'l

12671 J6zmk, L, dnd A,P.H. van DiJk

12681

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( 273)

( 274J

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Boom. H. Vdo den and W VdO Etten, W,H.C, de Krom. P VdO Benneko •. f. HUl)skens, L. NJessen, F de LelJe~ = ~ AN OPTICAL ASK AND fSK PHASE DIVERSITY TRANSMISSION SYSTEM, RUT Report 92+266, 1992. ISBN 90-6144-168-0

Putten, P H,i, Vdn der MUlTlOISCIPLlNAIR SPEClFICEREN EN ONTWERPEN VAN MICROELEKTRONICA IN PROaUKTEN 110 Dutch I EUT Report 93-F.-26~. 1993. ISBN 90-6144-269-9

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Bollen, M,H.J, and M I van Houten ON INSULAR POWER SYSTEMS· DraWlng up an inventory of phenomena dOd research POSSibIlities. EUT Report 9J-f,-274. 1993. ISBN 90-6144-274-5

i27SI O(,I1[:;,n I.P.J van rIl:CTROMIGNETlC COMPATTBILITY' Part 5, lust,ll,ilon and mitIgatIOn '!llldelJOes, scellOn 3, ,a\llilla dnd w'rllO EliT Report 9H-175. 1993, iSBN 90-614075-3

12761 Bollen. M.H .1. LITERATURE SEARCH fOR RELIABILITY om OF COMPONENTS IN ELECTRIC OISTPI8IITlON NFTWORKS, EUT Report 93+276, 1993, ISBN 90-6144-276-1

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Literature Search for Reliability Data of Components in Electric Distribution Networks