Cigre 494

Reliability and Economic Analysis of Different Power Station Layouts

D. Braun (M)1, F. Granata1, M. Delfanti (M)2, M. Palazzo2, M. Caletti2

1ABB Switzerland Ltd. , Zurich, Switzerland, 2Politecnico di Milano, Milano, Italy

Abstract-- The liberalization of electric power systems puts a

strong pressure on the issues concerned with the reliability of power stations.

In the vertically integrated system there was no explicit penalization in case of unexpected outages of generators. In a market scenario, where power stations are held by different generating companies, outages are more critical and may have significant economic consequences.

Moreover, long unavailability periods (as in the case of severe failures of the main transformer) may affect the rate of return of investments related with power stations.

This work is based on a simulation of the behaviour of two Italian power stations carried out with the help of the Monte Carlo method. The analysis provides important insights in the optimization of power station layouts, evaluating the possible introduction of generator circuit-breakers in existing schemes, as well as the possibility of applying different power station topologies.

An economic evaluation of the different power station layouts is also provided, taking into account different scenarios in terms of energy selling price.

Index Terms-- Reliability, Availability, Power Station Layout, Generator Circuit-Breaker.

I. INTRODUCTION

HE question of operating power stations with the highest possible availability has become more and more important

in recent years; the layout of a power station obviously has a decisive influence in this respect. Furthermore the liberalization of the electric power systems (a process begun in Italy with the Decreto Bersani in 1999, and still in progress nowadays) pushes the operators of power stations in the direction of achieving the highest availability at the lowest costs.

This paper is specifically concerned with the reliability and security assessment of thermal power stations (a typical layout is shown in Figure 1). Failure and repair data obtained from the operating history of two Italian power stations have been used for the investigation.

The availability of the power stations has been assessed by means of Monte Carlo simulation. The simulations take into account the following options: different extra high-voltage substation schemes, air and gas insulation, the presence of a generator circuit-breaker and the number of station transformers.

Fig. 1. Layout of a 4 x 320 MW thermal power station (2 units out of 4 are shown)

T

132 kV

380 kV

3~ 3~

6 kV 6 kV 6 kV 6 kV 6 kV

MT 370 MVA

UT 16 MVA

UT 16 MVA

ST 20 MVA

6 kV

UT 16 MVA

UT 16 MVA

MT 370 MVA

GEN 20 kV

370 MVA

GEN 20 kV

370 MVA

Tesi0-7803-7967-5/03/$17.00 2003 IEEE

TesiPaper accepted for presentation at 2003 IEEE Bologna Power Tech Conference, June 23th-26th, Bologna, Italy

For the economic analysis, on one side the power delivered to the grid was taken into account, while, on the other side, the acquisition costs, the civil works costs, the maintenance costs and the costs of power losses of each layout of the power station have been considered. The optimal solutions, depending on the service time, the load profile and the energy selling price have been evaluated.

II. THE ITALIAN ELECTRICITY MARKET

Since 1999 the Italian electricity system has been experiencing a restructuring process in accordance with the general principles established in the EU Directive 96/92/EC. Up to 1999, the Italian electric sector was almost completely managed by ENEL (Ente Nazionale per lEnergia Elettrica), the state-owned vertically integrated utility, founded in 1962. In fact, ENEL had the control of about 75% of the generation (56 GW of installed power in 1999), and together with some municipal utilities had an actual monopoly in transmission and distribution. The other 25% of production was in the hands of industrial auto-producers, municipal and private utilities. After the approval of the decree No. 79/1999 (Transposition of the Directive 96/92/EC concerning common rules of the internal electricity market), the electricity sector has been restructured. Generation, import and export of electric energy are liberalized; by January 2003, no company is allowed to produce or to import more than 50% of the electric energy. The generation sector in Italy is expected to be radically transformed within a few years; this is also due to the increasing demand of electric energy. Transmission and dispatch functions are managed by an independent network operator (GRTN: Gestore della Rete di Trasmissione Nazionale). The Power Exchange (GME: Gestore del Mercato Elettrico) carries out an economic merit order dispatch based on producers bids to determine, hour-by-hour, the energy price for the transactions. The rules issued by the GME for the electric energy market lead all generating companies to the objective of achieving the highest possible plant availability at the lowest possible cost.

III. DATA ACQUISITION

The realization of the connection of a generator to the extra high-voltage grid and the way to supply power to the auxiliaries has a decisive impact on the availability of a power station. In order to assess the availability of power stations, reliability data has been collected from two Italian thermal power stations.

A. Layouts of the Power Stations The layout of one of the power stations from which the

data has been collected is shown in Figure 1. Both power stations consist of four 320 MW steam turbines. There are two unit transformers (UT) for each unit. The generator is directly connected to the main transformer (MT). The extra high-voltage substation (rated voltage 380 kV) consists of an air insulated double busbar with single circuit-breaker arrangement. The number of outgoing lines is different for the two power stations: one plant has two outgoing lines and the other one three. Moreover, one power station has two station transformers (ST), i.e. one per two units. This is the typical scheme used in Italy for thermal power stations. The other

power station has three station transformers: the first two units share the same station transformer and the other two units have one station transformer each. The reserve net (132 kV) is the backup source for the station auxiliaries; i. e. whenever a unit is shutdown, the reserve net supplies power to the auxiliary busbars through the station transformer. The layout of the high-voltage substation (rated voltage 132 kV) is again an air insulated double busbar with single circuit-breaker arrangement.

B. Data Collection For each component of the power station the following

data has been collected: number of failures; downtime; undelivered power; maintenance frequency; maintenance duration; energy production; operating hours per year.

Furthermore, an estimate of the number of close and open commands for the circuit-breakers has been made taking into account the layout and the operation of the plant. Data was available from the ten years period between 1991 and 2000.

IV. DATA ANALYSIS

From the data collected the reliability parameters to be used for the availability calculations have been evaluated. For each component the Mean Time To Failure (MTTF) and the Mean Time To Repair (MTTR) have been computed. Assuming that the data is exponentially distributed, the MTTF is given by

FOT

MTTF = (1)

where: OT is the total Operating Time of the component; F is the number of Failures that have occurred.

The MTTR has been calculated from

FDT

MTTR = (2)

where DT is the total Downtime after failure of the component. The results obtained are summarized in Table I.

TABLE I

RELIABILITY PARAMETERS COLLECTED FROM TWO ITALIAN THERMAL POWER STATIONS

Component MTTF [h] MTTR [h]

Unit 5859 69.75 Main transformer (MT) *) *) Unit transformer (UT) 265613 435.53

Station transformer (ST) 192720 39.3 Main net (380 kV) 58400 1.3

Reserve net (132 kV) 58400 1.3 380 kV busbar 350400 36.5

380 kV circuit-breaker *) *) 132 kV busbar *) *)

132 kV circuit-breaker *) *) MV busbar *) *)

MV circuit-breaker 730000 73.72 *) no failures were observed

In order to prove the assumption that the data is exponentially distributed, a more thorough approach has been used. In case of equipment failures the data is a sequence of times to failure. The first step to analyze the data was to compute a piecewise-continuous failure density function. A study of this function is then followed by the choice of a continuous model which fits the data satisfactorily.

The hazard rate z(t) is defined as the ratio of the number of failures occurring in the time interval to the population at the beginning of the time interval, divided by the length of the time interval [1]:

( ) ( ) ( )[ ] ( )i

iiii

dttndttntn

tz/+-

= (3)

where: n(ti) n(ti + dti) is the number of failures occurring in the

time interval; n(ti) is the size of the population at the beginning of the

time interval; dti is the length of the time interval.

The hazard rate is a measure of the instantaneous speed of failure. Figure 2 shows the hazard rate of the unit (turbine and generator).

0

0,0001

0,0002

0,0003

0,0004

0,0005

1 2 3 4 5 6 7

Time intervals

Haz

ard

rat

e [1

/h]

Trend line

Fig. 2. Hazard rate of the unit

The reliability of a piece of equipment is the probability that the item will perform a specified function under specified operational and environmental conditions, at and throughout a specified time. The reliability function R(t) is given by the equation [1], [2]:

( ) ( )Ntn

tR = (4)

where: n(t) is the size of the population at the time t; N is the size of the original population.

The reliability function of the unit (turbine and generator) is depicted in Figure 3.

The hazard function z(t) graphed in Figure 2 fluctuates somewhat, but the overall trend seems to be constant, increasing with a small slope.

0

0,1

0,20,3

0,4

0,5

0,6

0,70,8

0,9

1

0 5000 10000 15000 20000 25000 30000 35000 40000

Time [h]

R(t

)

Fig. 3. Reliability function of the unit

A constant hazard rate implies an exponential density function and, as shown in Figure 3, an exponential reliability function. Similar results were also obtained for the other components.

V. POWER STATION LAYOUTS

The following power station layouts have been considered for the investigations.

A. Layout of Extra High-Voltage Substation The secure operation of extra high-voltage substations is

greatly influenced by their layout. In order to assure the continuity of the supply, the links between incoming and outgoing feeders of a substation have to remain intact, even in spite of a number of connecting elements not being available. Obviously every effort is made to attain this goal with a minimum capital outlay.

The following substation schemes have been investigated: single-busbar (Figure 4a); double busbar with single circuit-breaker (Figure 4b); one and half circuit-breaker (Figure 4c); double busbar with double circuit-breaker (Figure 4d); ring (Figure 4e); crossed-ring (Figure 4f).

The single busbar arrangement (Figure 4a) is suitable for smaller installations. Its costs are low but maintenance is difficult to carry out and the availability is lower than that of most other schemes. A circuit-breaker failure leads to the loss of all feeders connected to the busbar and the busbar protection may cause the loss of the whole substation.

For larger installations the double busbar with single circuit-breaker arrangement (Figure 4b) is preferred. The presence of two busbars makes maintenance possible without interrupting the supply. On the other hand a circuit-breaker failure again leads to the loss of all feeders connected to that busbar and the busbar protection may cause the loss of the substation if all feeders are connected to the same busbar.

a) b)

Fig. 4. Schemes of extra high-voltage substations

A scheme representing a mixture of equipment and structural redundancy is the one and half circuit-breaker arrangement (Figure 4c). It is often used for very important substations because of its high availability and good operational flexibility. In this case three circuit-breakers are employed for two outgoing feeders. All circuit-breakers are normally closed. Uninterrupted supply is thus maintained even if one busbar fails.

A feature of the double busbar with double circuit-breaker arrangement (Figure 4d) is that each outgoing feeder is connected to the rest of the installation by two parallel circuit-breakers, i.e. this scheme uses circuit-breaker redundancy to secure operation under disturbed conditions. Pure equipment redundancy is employed, resulting in high costs. Since each line has two circuit-breakers, one circuit-breaker can be taken out of service at any time without interrupting the operation.

c) d)


A more economic kind of redundancy is achieved with the ring arrangement (Figure 4e) which is considered as an appropriate solution for substations with only a few feeders. Each feeder requires only one circuit-breaker and each circuit-breaker can be isolated without interrupting the supply.

e) Fig. 4. Schemes of extra high-voltage substations

Starting from this scheme, new concepts were developed to increase structural redundancy. Among these schemes, the crossed-ring substation arrangement (Figure 4f) is the easiest to put into practice [3]. In the normal state of such an arrangement the circuit-breakers of the basic ring (BR) are closed while those of the cross-links (CL) are open. If one circuit-breaker in the basic ring fails, another ring can be formed so that the original availability is maintained. It can be seen that even in the case of non-availability of two adjacent circuit-breakers, the respective node can be fed via the remaining circuit-breaker. With any of the other topologies introduced above, this situation would automatically lead to the loss of the node.

f)


The extra high-voltage substations from which the data has been collected are of air insulated design. The impact of the use of Gas Insulated Switchgear (GIS) instead of Air Insulated Switchgear (AIS) has also been investigated. The GIS solution leads to a lower failure rate and to a higher MTTR and, even though it is more expensive, it is to be preferred when problems of space or pollution are present.

B. Layout of High-Voltage Substation The layout of the high-voltage substation used for the

investigation is a double busbar with single circuit-breaker arrangement (Figure 4b) and uses air insulated switchgear.

C. Generator Circuit-Breaker For each layout the possible use of a generator circuit-

breaker has been investigated. The presence of a generator circuit-breaker located between the generator and the main transformer improves the availability of the power station [4]. Firstly it allows the plant auxiliaries to be fed directly from the extra high-voltage transmission system (main net). Supply from this source is considered more reliable than the connection to a local sub-transmission system (reserve net) and results in an improved plant auxiliary equipment availability. Secondly, the rapid interruption of generator-fed short-circuit currents reduces the extent of fault damage and the related downtime, contributing to an increased power station availability. Specifically, investigations have shown that a generator circuit-breaker can prevent transformer tank rupture in about 80% of all cases [5]. According to experience available in Italy, the time to repair of a main transformer is about one month when the failure does not result in explosion and about one year when the transformer tank bursts. The presence of a generator circuit-breaker can thus lead to a reduction of the MTTR of the transformer.

D. Station Transformer The power station layout used for the investigation has two

station transformers, i.e. one per two units. In the cases with a generator circuit-breaker, only one station transformer (one station transformer per four units) rated as an emergency shut-down transformer has been considered. The influence of using two station transformers or no station transformer has also been investigated for the cases with a generator circuit-breaker installed. When no station transformer is available, the backup source for the auxiliaries are the auxiliary busbars of another unit.

BR BR BR CL

BR CL CL BR

BR BR BR CL

VI. SIMULATIONS

The simulations have been carried out with the help of a computer program based on the Monte Carlo method [6], [7]. This is a very powerful technique to quantitatively estimate the reliability of complex systems like power stations; furthermore it allows to quantify the impact of the connection scheme of a generator to the extra high-voltage network on the availability of the plant.

Monte Carlo methods estimate the reliability of a system by simulating the process and its random behaviour. The simulation consists in a repeated process of generating deterministic solutions to a given problem with each solution corresponding to a set of deterministic values of the underlying random variables. The main element of Monte Carlo simulation is therefore the generation of random numbers from probability distributions describing the random variables of interest, e. g. the failure and repair rates of different items of power station equipment.

During a simulation run, when a failure occurs it is treated by tripping the circuit-breakers forming the protection group of the failed component immediately after the occurrence of the failure. After the time necessary to isolate the failed component (i. e. the switching time) the circuit-breakers are closed again. When the repair of the component is completed (or a spare part has become available), the above procedure is repeated. Also the transfer of the auxiliaries between different sources during the starting-up and the shutting-down of the unit (or when a failure occurs) is modelled. The operational state of a unit further depends on the state of its auxiliaries, as the number of auxiliaries available influences the level of possible power production.

One of the results obtained from the simulations is the throughput power of the power station, i. e. the power delivered to the grid.

Reliability parameters evaluated from the data collected have been used for the simulations (see Tables I and II).

TABLE II

SWITCHING TIMES, MAINTENANCE FREQUENCY AND DURATION COLLECTED FROM TWO ITALIAN THERMAL POWER STATIONS

Component Switching time [h] Maintenance frequency [h]

Maintenance duration [h]

Unit N/A 15768 1939.5 Main transformer (MT) 16 8760 *) Unit transformer (UT) 16 8760 *)

Station transformer (ST) 16 8760 *) Main net (380 kV) N/A N/A N/A

Reserve net (132 kV) N/A N/A N/A 380 kV busbar 2 *) *)

380 kV circuit-breaker 8 87600 *) 132 kV busbar 2 *) *)

132 kV circuit-breaker 8 87600 *) MV busbar 2 *) *)

MV circuit-breaker 0.5 *) *) *) no data available N/A data not applicable

For the main transformer, the 380 kV circuit-breaker, the 132 kV busbar, the 132 kV circuit-breaker and the medium voltage busbar where no failures have been observed, reliability parameters have been taken from published literature [8], [9], [10], [11]. The circuit-breaker fail-to-close

and fail-to-open probabilities have also been taken into account [9].

The reliability data for gas insulated substations (GIS) has been collected from literature as well [12]. Moreover, information about switching times, maintenance frequency and maintenance duration was obtained from the power stations (see Table II).

VII. RESULTS

The simulations have been carried out assuming that the power station supplies base load. The availability of the unit (turbine and generator) has been set to 86.67%. This value takes into account forced and scheduled outages of the unit. The results of the simulations are summarized in Table III and Table IV.

TABLE III RESULTS OF SIMULATIONS: INFLUENCE OF THE PRESENCE OF

A GENERATOR CIRCUIT-BREAKER

EHV substation (refer to Figure 4)

HV substation (refer to Figure 4)

Gen circuit-breaker

Station transformer

Throughput power

SCHEME AIS GIS SCHEME AIS GIS YES/NO No. MW

a) x b) x no 2 1098.8 b) x b) x no 2 1098.9 c) x b) x no 2 1099.1 d) x b) x no 2 1099.3 e) x b) x no 2 1087.9 f) x b) x no 2 1099.2 a) x b) x no 2 1099.0 b) x b) x no 2 1099.0 c) x b) x no 2 1099.2 d) x b) x no 2 1099.4 e) x b) x no 2 1088.2 f) x b) x no 2 1099.2 a) x b) x yes 1 1104.6 b) x b) x yes 1 1104.8 c) x b) x yes 1 1105.1 d) x b) x yes 1 1105.1 e) x b) x yes 1 1105.1 f) x b) x yes 1 1105.1 a) x b) x yes 1 1104.8 b) x b) x yes 1 1104.9 c) x b) x yes 1 1105.2 d) x b) x yes 1 1105.2 e) x b) x yes 1 1105.1 f) x b) x yes 1 1105.2

The difference in the throughput power directly reflects the contribution of the different schemes used to connect the generators to the extra high-voltage transmission network on the availability of the power station. The results show that the use of a layout with a generator circuit-breaker positively affects the availability. Figure 5 depicts the possible availability improvements when a layout with a generator circuit-breaker is used. This improvement is in the order of 0.5%. The ring scheme seems to be very interesting: in this case the availability improvement is in the order of 1.6%. The results clearly indicate that, from a point of view of power station availability, a layout with a generator circuit-breaker offers a distinct advantage over the unit connection.

0,00%

0,20%

0,40%

0,60%

0,80%

1,00%

1,20%

1,40%

1,60%

1,80%

a) b) c) d) e) f)

Layout of EHV substations (refer to Figure 4)

AISGIS

Fig. 5. Relative availability improvement for a layout with generator circuit-breaker (compared to the same layout without generator circuit-breaker)

With respect to the design of the extra high-voltage

substation, it can be seen that the difference in the throughput power between a gas insulated substation and an air insulated substation is generally very small.

On the other side, in case of a layout with generator circuit-breaker, the number of station transformers has a negligible influence on the power station availability (see Table IV).

TABLE IV RESULTS OF SIMULATIONS: INFLUENCE OF THE NUMBER

OF STATION TRANSFORMERS

EHV substation (refer to Figure 4)

HV substation (refer to Figure 4)

Gen circuit-breaker

Station transformer

Throughput power

SCHEME AIS GIS SCHEME AIS GIS YES/NO No. MW

b) x b) x yes 2 1104.8 b) x b) x yes 1 1104.8 b) x - - - yes 0 1104.8

VIII. ECONOMIC EVALUATION

In order to make a more thorough comparison between the different options, an economic analysis has also been carried out. For the economic evaluation the following issues have been considered: life cycle costs for selectable equipment (extra high-

voltage and high-voltage switchgear, station transformers, generator circuit-breakers, medium voltage switchgear);

costs of load and no-load power losses when a unit is shut down;

power delivered to the grid; energy selling price.

A. Life Cycle Costs The life cycle costs consist of the following components:

acquisition costs; civil work costs; installation costs; maintenance costs.

The corresponding data was obtained from Italian power station operators. Figure 6 shows the difference in the

acquisition, civil work and installation costs of the selectable equipment for different layouts.

-40%

-20%

0%

20%

40%

60%

80%

100%

120%

a) b) c) d) e) f)


1234

Fig. 6. Difference in acquisition, civil work and installation costs of selectable equipment (referred to layout of Figure 1)

# EHV substation Generator circuit-breaker No. of station transformers 1 AIS no 2 2 GIS no 2 3 AIS yes 1 4 GIS yes 1

B. Costs of Load and No-Load Losses An important aspect in this conjunction is given by the

load and no-load losses occurring while a unit is shut down. The costs of these losses have been computed for three different cases, namely: L1: layout without generator circuit-breaker and with

station transformer (auxiliaries are supplied by station transformer);

L2: layout with generator circuit-breaker and with station transformer (depending on downtime, auxiliaries are supplied either by main and unit transformers or by station transformer);

L3: layout with generator circuit-breaker and without station transformer (auxiliaries are supplied by main and unit transformers).

In the first case (L1) the auxiliaries are supplied by the reserve net via the station transformer and hence losses occur in the station transformer. As the extra high-voltage circuit-breaker is open there are no losses in the main and unit transformers.

In the last case (L3) the auxiliaries are supplied by the main net via the main and the unit transformers and losses occur both in the main and in the unit transformer.

In a layout with generator circuit-breaker and station transformer (L2) it is convenient to open the extra high-voltage circuit-breaker if the downtime of the unit is longer than two days, i. e. the auxiliaries are then supplied by the reserve net via the station transformer. If the downtime is shorter than two days only the generator circuit-breaker is opened and the auxiliaries remain supplied by the main net via the main and the unit transformers. This corresponds to the practice in Italian power stations.

Taking into account the technical data of the transformers as well as the number and duration of scheduled and forced outages in the power station considered, costs of losses as shown in Figure 7 were determined. The results are referred to the case L1.

0%

100%

200%

300%

400%

500%

600%

700%

800%

1168 h

Downtime

Co

sts

of

loss

es ST MT UT

a) Case L1

0%

100%

200%

300%

400%

500%

600%

700%

800%

200 h 968 h

Downtime

Co

sts

of

loss

es

b) Case L2

0%

100%

200%

300%

400%

500%

600%

700%

800%

1168 h

Downtime

Co

sts

of

loss

es

c) Case L3

Fig. 7. Costs of load and no-load losses when the unit is shut down (referred to layout of Figure 1)

These costs have been included in the economic

assessment of the layout.

C. Power Delivered to the Grid The power delivered to the grid is given by the throughput

power minus the power consumed by the auxiliaries.

D. Economic Analysis A figure of merit has been calculated for each layout

according to the following formula:

( ) ICCWCACPWLCMCIMFPV -----=_ (5)

where: PV_MF is the Present Value of the Merit Figure of the

layout; I is the Income per year; MC are the Maintenance Costs of selectable equipment

per year; LC are the Losses Costs when the unit is shut down per

year; AC are the Acquisition Costs of selectable equipment; CWC are the Civil Work Costs of selectable equipment; IC are the Installation Costs of selectable equipment; PW is the Present Worth factor (based on a discount rate

of 5%).

The differences in the figure of merit of different power station layouts are depicted in Figure 8. It can be noticed that layouts with a generator circuit-breaker generally have a higher figure of merit than layouts without a generator circuit-breaker.

Additional calculations have shown that layouts with a generator circuit-breaker and without a station transformer may even have somewhat higher figures of merit, especially in cases with low downtimes (e. g. power stations which supply base load) where the losses during the time when the unit is shut down do not matter very much.

-1,20%

-1,00%

-0,80%

-0,60%

-0,40%

-0,20%

0,00%

0,20%

0,40%

0,60%

0,80%

a) b) c) d) e) f)


1234

Fig. 8. Differences in figure of merit of different power station layouts (referred to layout of Figure 1)

# EHV substation Generator circuit-breaker No. of station transformers 1 AIS no 2 2 GIS no 2 3 AIS yes 1 4 GIS yes 1

Moreover, the use of a generator circuit-breaker makes the

ring scheme (Figure 4e) without station transformer one of the best options; such a conclusion is due to the fact that this scheme is very cheap (low number of components) and shows a similar reliability as the other schemes with a generator circuit-breaker. Figure 9 shows the single line diagram of a recent pumped storage power station project where exactly this scheme will be applied.

IX. CONCLUSIONS

An analysis of different power station layouts from the point of view of reliability and economy has been carried out. Failure and repair rates collected from two Italian thermal power stations have been used for this purpose. The investigations show that the use of generator circuit-breakers results in a higher power station availability for every kind of extra high-voltage substation scheme, but especially in cases not commonly considered till today. For example the ring scheme shows the highest availability improvement when a

generator circuit-breaker is installed. The use of a generator circuit-breaker thus frees the choice of the scheme for the extra high-voltage and high-voltage substations, leading to a higher number of options for power station layouts.

X. ACKNOWLEDGEMENTS

The authors are extremely thankful to prof. Andrea Silvestri, who tutored the thesis on which the paper is based and promoted the cooperation between ABB Zurich and Politecnico di Milano researchers.

Fig. 9. Single line diagram of a pumped storage power station

XI. REFERENCES [1] M. L. SHOOMAN, Probabilistic Reliability: an Engineering

Approach, Krieger, 1990. [2] P. D. T. OCONNOR, Practical Reliability Engineering, John Wiley

& Sons Ltd, 1991. [3] BBC Brown, Boveri & Company, Ltd., New Concepts for High-

Voltage Switchyards, Publication No. CH-A 035780E, Baden, Switzerland, 1985.

[4] D. BRAUN, L. WIDENHORN AND J. ISCHI, Impact of the Electrical Layout on the Availability of a Power Plant, Conference Proceedings of 11th CEPSI, Kuala Lumpur, 1996.

[5] B. CULVER, K. FROELICH AND L. WIDENHORN, Prevention of Tank Rupture of Faulted Power Transformers by Generator Circuit Breakers, ETEP, Vol 6, pp 39-45, January/February 1996.

[6] DUBI, Monte Carlo Applications in Systems Engineering, John Wiley & Sons Ltd, 2000.

[7] CLOCKWORK GROUP, A Users Guide to Thermal Power Plant Workbench Version 1.1, Austin , 1999-2000.

[8] IEEE POWER ENGINEERING SOCIETY, Survey of Generator Step-Up (GSU) Transformer Failures, a Special Publication of the IEEE Power Engineering Society Transformers Committee, 1998.

[9] CIGRE WORKING GROUP 13.06, Final Report of the Second International Enquiry on High Voltage Circuit-Breaker Failures and Defects in Service, Cigre Publication No. 83, 1994.

[10] M. H. J. BOLLEN, Literature Search for Reliability Data of Components in Electric Distribution Networks, Eindhoven University of Technology, 1993.

[11] CIGRE WORKING GROUP 12.05, An International Survey on Failures in Large Power Transformers in Service, Electra, No. 88, pp 21-42, 1983.

[12] CIGRE WORKING GROUP 23.02, Report on the Second International Survey on High Voltage Gas Insulated Substations (GIS) Service Experience, Cigre Publication No. 150, 2000.

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Cigre 494