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IEEE Std C57.19.100TM

-2012(Revision of

IEEE Std C57.19.100-1995)

IEEE Guide for Application of PowerApparatus Bushings

Sponsor

Transformers Committeeof theIEEE Power and Energy Society

Approved 5 December 2012

IEEE-SA Standards Board

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Abstract:Guidance on the use of outdoor power apparatus bushings is provided. The bushingsare limited to those built in accordance with IEEE Std C57.19.00TM-1991. General information andrecommendations for the application of power apparatus bushings when incorporated as part ofpower transformers, power circuit breakers, and isolated-phase bus are provided.

Keywords:circuit breakers, IEEE C57.19.100TM, isolated-phase bus, power apparatus bushings,transformers

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PDF: ISBN 978-0-7381-8132-5 STD98087Print: ISBN 978-0-7381-8133-2 STDPD98087

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vCopyright 2013 IEEE. All rights reserved.

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viCopyright 2013 IEEE. All rights reserved.

Participants

At the time this guide was submitted to the IEEE-SA Standards Board for approval, the Guide forApplication of Power Apparatus Bushings Working Group had the following membership:

Thomas Spitzer,Chair

Jesse Patton, Vice Chair

Carlo ArpinoRay BartnikasJeffrey Benach

Gene BlackburnJohn BrafaFlorian Costa

John CrouseLarry DavisArturo Del RioLonnie Elder

Fred ElliottKeith EllisMary Foster

Charles GarnerJoseph GarzaJohn Graham

Roger HayesChungduck KoReiner KrumpMario Locarno

Van Nhi Nguyen

Leslie RecksiedlerRandolph RensiDevki Sharma

Craig SteigemierJohn SteinJane Vermer

Eric WeatherbeeMichael WilliamsShibao ZhangPeter Zhao

The following members of the individual balloting committee voted on this guide. Balloters may have

voted for approval, disapproval, or abstention.

Mohamed Abdel KhalekStephen AntoszCarlo ArpinoPeter Balma

Robert BarnettBarry BeasterJeffrey BenachW. (Bill) J. Bergman

Wallace BinderThomas Blackburn

W. BoettgerJohn Brafa

William BushArvind K. Chaudhary

Bill ChiuRobert ChristmanKurt ClementeJerry Corkran

John CrouseWillaim DarovnyGary DonnerFred Elliott

Keith EllisGary EngmannJames FairrisJorge Fernandez Daher

Patrick Fitzgerald

Joseph FoldiMarcel FortinRobert Ganser

Charles GarnerSaurabh GhoshDavid Giegel

David GilmerJalal GohariEdwin GoodwinJames Graham

William Griesacker

Randall C. GrovesBal GuptaCharles HandDavid Harris

Roger HayesLee HerronGary HeustonGary Hoffman

Philip HopkinsonJohn Kay

Gael KennedySheldon Kennedy

Joseph L. KoepfingerJim Kulchisky

Saumen KunduJohn LackeyChung-Yiu LamHua Liu

Albert LivshitzThomas LundquistGreg LuriRichard Marek

J. Dennis MarlowLee MatthewsAndrew McNultyGeorges Montillet

Jerry Murphy

Ryan MusgroveK. R. M. NairArthur Neubauer

Michael S. NewmanJoe NimsTed Olsen

Lorraine PaddenBansi PatelShawn PattersonJesse Patton

Brian Penny

Paul PillitteriAlvaro PortilloLewis PowellIulian Profir

Reynaldo RamosJean-Christophe RiboudJohannes RickmannJohn Roach

Michael RobertsJohn Rossetti

Marnie RoussellThomas Rozek

Daniel SauerBartien Sayogo

Devki SharmaGil ShultzJames SmithJeremy Smith

Jerry SmithBrian SparlingThomas SpitzerGary Stoedter

John VergisJane VernerLoren WagenaarDavid Wallach

Barry Ward

Joe WatsonEric WeatherbeePeter Werelius

Kenneth WhiteWael YoussefJian Yu

Matthew ZeedykShibao ZhangPeter ZhaoXi Zhu

Waldemar Ziomek

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viiCopyright 2013 IEEE. All rights reserved.

When the IEEE-SA Standards Board approved this guide on 5 December 2012, it had the following

membership:

Richard H. Hulett,ChairJohn Kulick

,Vice ChairRobert Grow,Past Chair

Konstantinos Karachalios,Secretary

Satish AggarwalMasayuki AriyoshiPeter BalmaWilliam Bartley

Ted BurseClint ChaplinWael Diab

Jean-Philippe Faure

Alexander GelmanPaul HouzJim HughesYoung Kyun Kim

Joseph L. Koepfinger*David J. LawThomas Lee

Hung Ling

Oleg LogvinovTed OlsenGary RobinsonJon Walter Rosdahl

Mike SeaveyYatin TrivediPhil Winston

Yu Yuan

*Member Emeritus

Also included are the following nonvoting IEEE-SA Standards Board liaisons:

Richard DeBlasio,DOE RepresentativeMichael Janezic,NIST Representative

Patrick GibbonsIEEE Standards Program Manager, Document Development

Erin SpiewakIEEE Standards Program Manager, Technical Program Development

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ixCopyright 2013 IEEE. All rights reserved.

Contents

1. Overview .................................................................................................................................................... 11.1 Scope ................................................................................................................................................... 11.2 Purpose ................................................................................................................................................ 1

2. Normative references .................................................................................................................................. 23. Definitions .................................................................................................................................................. 24. Thermal loading above nameplate rating for bushings applied on power transformers ............................. 3

4.1 General ................................................................................................................................................ 34.2 Temperature calculations for short-time loads above bushing rating .................................................. 54.3 Test procedures for derivation of mathematical model ....................................................................... 8

5. Special considerations for application of bushings to power transformers ............................................... 115.1 General .............................................................................................................................................. 115.2 Loading of bushings with transformer top oil temperature rises between 55 C and 65 C .............. 115.3 Application of bushings in transformers with conservator oil preservation systems ......................... 125.4 Draw-lead application ........................................................................................................................ 125.5 Draw-rod application ......................................................................................................................... 13

6. Thermal loading for bushings applied on circuit breakers ........................................................................ 137. Thermal loading for bushings used with isolated-phase bus .................................................................... 13

7.1 Concerns for bushings used in isolated-phase bus ............................................................................. 137.2 Thermal coordination between the bushings and the isolated-phase bus ........................................... 14

8. Allowable line pull (cantilever loading) ................................................................................................... 148.1 General (transformers and circuit breakers) ...................................................................................... 148.2 Mounting angles greater than 20 ...................................................................................................... 148.3 Circuit breaker applications ............................................................................................................... 15

9. Application of bushings in unusual service conditions ............................................................................. 159.1 Contaminated environments .............................................................................................................. 159.2 High altitudes ..................................................................................................................................... 189.3 Application of outdoor bushings in indoor locations ......................................................................... 189.4 Bushing monitors ............................................................................................................................... 18

10. Bushing maintenance practices .............................................................................................................. 1810.1 Mechanical maintenance and inspection ......................................................................................... 1810.2 Bushing repair .................................................................................................................................. 2010.3 Routine and special tests .................................................................................................................. 2110.4 Bushing storage ............................................................................................................................... 2310.5 Bushing replacement ....................................................................................................................... 24

11. Checklist ................................................................................................................................................. 25Annex A (informative) Examples of calculation procedures to determine hot-spot temperatures for

bushings applied on transformers ................................................................................................................. 26Annex B (informative) Bibliography............................................................................................................ 30

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1Copyright 2013 IEEE. All rights reserved.

IEEE Guide for Application of PowerApparatus Bushings

IMPORTANT NOTICE: IEEE Standards documents are not intended to ensure safety, health, or

environmental protection, or ensure against interference with or from other devices or networks.

Implementers of IEEE Standards documents are responsible for determining and complying with all

appropriate safety, security, environmental, health, and interference protection practices and allapplicable laws and regulations.

This IEEE document is made available for use subject to important notices and legal disclaimers.

These notices and disclaimers appear in all publications containing this document and may

be found under the heading Important Notice or Important Notices and Disclaimers

Concerning IEEE Documents. They can also be obtained on request from IEEE or viewed at

http://standards.ieee.org/IPR/disclaimers.html.

1. Overview

1.1 Scope

Guidance on the use of outdoor power apparatus bushings is provided in this document. The bushings are

limited to those built in accordance with IEEE Std C57.19.00TM-1991 (not the latest revision).1The latest

revision, IEEE Std C57.19.00TM-2004, does not address the use of condenser bushings in oil circuit breakers

and several voltage classes were dropped that are still in use.

1.2 Purpose

The purpose of this guide is to present general information and recommendations for the application of

power apparatus bushings when incorporated as part of power transformers, power circuit breakers, andisolated-phase bus. The loading model developed in this guide is based on oil-impregnated, paper-

insulated, capacitance-graded bushings. Similar loading models could be developed for other bushingconstructions.

1Information about references can be found in Clause2.

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IEEE Std C57.19.100-2012IEEE Guide for Application of Power Apparatus Bushings


2. Normative references

The following referenced document is indispensable for the application of this document (i.e., it must beunderstood and used, so each referenced document is cited in text and its relationship to this document is

explained). For dated references, only the edition cited applies. For undated references, the latest edition of

the referenced document (including any amendments or corrigenda) applies.

IEEE Std 4TM, IEEE Standard Techniques for High-Voltage Testing.2,3

IEEE Std C37.010TM, IEEE Application Guide for AC High-Voltage Circuit Breakers Rated on a

Symmetrical Current Basis (including Supplement IEEE Std C37.010d).

IEEE Std C37.017TM, IEEE Standard for Bushings for High Voltage [over 1000V(ac)] Circuit Breakers and

Gas-Insulated Switchgear.

IEEE Std C37.23TM, IEEE Standard for Metal-Enclosed Bus and Calculating Losses in Isolated-Phase Bus.

IEEE Std C57.12.00TM, IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power,

and Regulating Transformers.

IEEE Std C57.19.00TM-1991, IEEE General Requirements and Test Procedures for Outdoor Apparatus

Bushings.

IEEE Std C57.19.00TM-2004, IEEE General Requirements and Test Procedures for Power Apparatus

Bushings.

IEEE Std C57.19.01TM, IEEE Standard Performance Characteristics and Dimensions for Outdoor Apparatus

Bushings.

IEEE Std C57.91TM, IEEE Guide for Loading Mineral-Oil-Immersed Transformers.

3. Definitions

For the purposes of this document, the following terms and definitions apply. The IEEE StandardsDictionary Onlineshould be consulted for terms not defined in this clause. 4

For definitions of terms used in this standard, also see IEEE Std C57.19.00-1991.

draw-lead bushing:A bushing that will allow the use of a current-carrying draw-lead conductor drawn

through the hollow tube and enabling its connection to the top terminal.

draw-lead conductor:A cable or a solid conductor that has one end connected to the transformer or areactor winding lead and the other end drawn through the bushing hollow tube and connected to the top

terminal of the bushing. A solid conductor can be either one piece or multiple pieces connected together.

draw-rod:A non-current-carrying rod (current is carried by the bushing tube) that has one end connectedto the transformer/reactor lead end terminal and the other end drawn through the bushing hollow tube and

connected to the top end of the bushing. A draw-rod can be either one piece or multiple pieces connected.

2This publication is available from The Institute of Electrical and Electronics Engineers (http://standards.ieee.org/).3The IEEE standards or products referred to in this clause are trademarks of The Institute of Electrical and Electronics Engineers, Inc.4 IEEE Standards Dictionary Online subscription is available at: http://www.ieee.org/portal/innovate/products/standard/standards_dictionary.html.

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draw-rod bushing:A bushing that allows the use of a non-current-carrying rod drawn through the hollow

tube and enabling a connection between the bushings inboard end terminal and the transformer or reactor

winding lead.

4. Thermal loading above nameplate rating for bushings applied on powertransformers

4.1 General

The thermal loading capability of bushings varies with the way they are loaded, the way they are designed,and the ambient conditions in which they are applied.

4.1.1 Basis of rating and rationalization of thermal requirements rating

Capacitance-graded, paper-insulated bushings that, at rated current, meet the requirements of

IEEE Std C57.19.00-1991 and earlier versions of that standard may be applied in either 55 C or 65 C risetransformers. IEEE Std C57.19.00-1991 states that the temperature of the oil in which the lower end of the

bushing is immersed shall not exceed 95 C when averaged over a 24 h period. Refer to IEEE Std C57.91TMfor loading requirements and operating oil temperatures of power transformers.

4.1.1.1 Operation above normal temperature

When operating a bushing at rated current in conjunction with a 65 C average winding rise rated

transformer, the hottest-spot temperature of the bushing is limited to a 65 C rise over ambient or a 105 C

total temperature because of the use of temperature index 105 insulating paper for the bushing condenser. If

it should be determined that a transformer develops a top oil rise of 65 C at rated current when operatingin a 40 C ambient, then the hottest-spot temperature of the bushing can be expected to exceed 105 C. In

addition, transformers can be expected to have bushing temperatures above 105 C when loaded inaccordance with IEEE Std C57.91.

In each instance, the normal life expectancy of the bushing will be shortened by the higher operating

temperatures. The loss-of-life of a bushing will, like transformers, be a function of the actual temperatureand the time operating at that temperature.

The severity of loss-of-life in a bushing can be minimized by installing bushings that have nameplate

ratings greater than the transformer current ratings or by using bushings with special high-temperature

insulation. An alternative is to operate the bushing with the higher inherent temperatures and accept a

moderate degree of accelerated aging, as it is presently recognized for transformers.

4.1.1.2 Factors influencing bushing aging

There are several factors that tend to decrease the severity of bushing aging. These are as follows:

a) The top oil rise of many transformers is significantly below 65 C when the transformer is operatedat nameplate loading. This is most likely to occur on forced oil-cooled (OFAF or ODAF)transformers.

b) Bushings are totally sealed from the atmosphere at the time of manufacture, thus preserving theirdielectric and thermal integrity.

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c) Bushing insulation is generally processed to a greater degree of dryness than transformer insulation,thus providing a lower power factor, lower dielectric losses, and consequently prolonged life at any

temperature.

d) The end-of-life of cellulose insulation in transformers may be governed by its ability to withstandmechanical forces that are associated with fault currents through the transformers. Celluloseinsulation in bushings is not subjected to similar forces.

e) Although end-of-life of insulating materials is typically based on a given change in mechanical orchemical properties, no similar relationship for dielectric characteristics has been established.

However, considering increased insulation power factor and capacitance as important criteria, well-

dried bushing cellulose material is probably equal in life expectancy to thermally upgraded (65 C)

transformer insulation.

f) The use of bushings with current ratings greater than the transformer current ratings as described in4.1.1.1 reduces the temperature rise inside the bushing at rated transformer current.

4.1.2 Overload concerns

When a bushing is loaded above nameplate, it is exposed to the risks described in4.1.2.1 through4.1.2.5.

4.1.2.1 Pressure buildup

When load current through a bushing exceeds the nameplate rating, internal pressures can develop that

could cause one or more of the sealing gaskets to leak or fail. This pressure increase is caused by the

expansion of the insulating oil within the bushing. The rate of oil expansion is normally considered to beapproximately 0.0725% to 0.0755% per C temperature increase for temperatures ranging from 0 C to

100 C.

4.1.2.2 Gasket seals

Gasket materials will age according to the temperature adjacent to the gasket surface and the duration at

that temperature. Usually gaskets will perform well at elevated temperatures; however, progressive changesin physical properties will occur when excessive temperatures are maintained for long durations. Thesechanges could result in loss of seal and consequent loss of dielectric strength. Therefore, repeated

occurrences at high temperature will require inspection for oil leaks and corrective actions where necessary.

4.1.2.3 Power factor and capacitance

There are many reasons why insulation power factor and/or capacitance may increase over the life of a

bushing. In fact, some slight increase of power factor can be tolerated. However, degradation of that

portion of the insulation that operates at greatly elevated temperature could result in a substantial increasein power factor. An unusual increase in power factor may become an indicator of the detrimental

mechanical and electrical effects of loading beyond nameplate rating. Bushings that have been loaded

beyond nameplate rating should be tested more frequently.

4.1.2.4 Dielectric performance at elevated temperatures

When bushing insulation is subjected to high electrical stress at above its normal operating temperatures,the insulation power factor increases due to increased dielectric loss. When the increase in dielectric loss

exceeds the ability of the insulation to dissipate this increased loss, the temperature of the dielectric is

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further increased. Under some extreme conditions, thermal runaway may occur. This risk should be

considered when the guide is applied.

Special capacitance-graded bushings built with insulation systems such as thermally upgraded paper or

resin impregnated paper, rated higher than temperature index 105 insulation class, are sometimes used.

These insulation systems may have higher power factors particularly at higher temperatures and may

experience thermal runaway if loaded significantly beyond the nameplate rating. For specific information,the manufacturer should be contacted.

4.1.2.5 Stray magnetic flux

Additional heating may occur in bushings placed in the magnetic field of the windings and leads. The

heating can result from the eddy current flowing in the metallic portions of the bushing below the mounting

flange. The magnetic flux will increase with the load current.

Magnetic fields can create high eddy current losses in tanks, flanges, and bus enclosures during overload

conditions, causing them to reach high temperatures. High temperatures of the part itself may not be ofconcern, but the heat may transfer to the bushing causing high-temperature bushing concerns.

4.2 Temperature calculations for short-time loads above bushing rating

The hottest-spot temperature of a bushing is of importance when it is loaded under various conditions. The

five key elements that affect the bushing hottest spot are the bushing current, the ambient air temperature,the surrounding oil temperature, the air-end-connection temperature, and the oil-end-connection

temperature. Easley and McNutt [B3]5gave an expression that contains each of these elements.

Accurate information about the end-connection temperatures and coefficients is usually not available.

Therefore, this guide uses a more conservative method that requires information only about the bushing

current, the ambient air temperature, and the surrounding oil temperature to calculate the bushing hottest-spot temperature.

This method was developed from experimental data for bushings with no appreciable dielectric losses and

no cooling ducts. Three constants are determined as described in4.3.3.These constants are then used to

estimate the steady-state and transient bushing hottest-spot temperatures. Mathematical models for

bushings with appreciable dielectric losses and/or with cooling ducts may be developed in the future and

could be used in the same manner.

4.2.1 Steady-state hottest-spot temperature calculations

The steady-state temperature rise at the hottest spot of the conductor for bottom connected bushings with

no appreciable dielectric losses and no cooling ducts is estimated withEquation (1):

HS 1 2 0

nK I K = + (1)

where

HS

is the steady-state bushing hottest-spot rise over ambient (C)

0 is the steady-state immersion oil rise over ambient (C) (transformer top oil rise)

5The numbers in brackets correspond to those of the bibliography inAnnex B.

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I is the per unit load current based on bushing rating

n,K1, andK2 are constants that can be determined as described in4.3

Typical values of K1 range from 15 to 32. Typical values of K2 range from 0.6 to 0.8. The exponent n

generally ranges between 1.6 and 2.0, with 1.8 being the most common value.

When a bushing is operated in the draw-lead mode, the thermal performance is dominated by an integralpart of the transformer that is inserted through the tube of the bushing. This lead is not an integral part of

the bushing, so the thermal performance cannot be directly related to a specific design of bushing that may

also be operated in other transformers with different size draw leads.

The temperature of the hottest spot of the conductor, when operated in the draw-lead mode, may be

determined in the same manner, withIbeing the per unit load current of the draw lead.

4.2.2 Transient hottest-spot temperature calculations

After changes in load current or ambient temperature occur, both the immersion oil temperature and

bushing hottest-spot temperature will change with time from the initial to the final value in an exponential

manner. Therefore, it is necessary to determine the initial and final transformer top oil temperature and therate of change by the procedures established in IEEE Std C57.91. After the changed per unit current I, the

transformer top oil rise 0, and the transformer top oil time constant ot have been established, the

transient response of the bushing may be determined usingK1,K2, n, and the bushing time constant t.

K1,K2, and nare the same constants and exponent used for the steady-state bushing calculations.

The bushing time constant tois the length of time required for the temperature change to reach 63.2% of the

final temperature change.

4.2.2.1 Iterative method

One method is to simulate the exponential rise by making a series of repeated calculations of the bushing

hottest-spot temperature rise in successive time increments following steps AF.

where

T is the elapsed time of the transient load (minutes)

t is the an arbitrary time increment to divide the elapsed time of the transient load

into steps for calculation (minutes)

t1 is the initial time at start of an increment (minutes)

t2 is the time when transformer oil 0reaches practical equilibrium (minutes)

is the bushing time constant (minutes)

o is the oil time constant of transformer (minutes)

HS(t1) is the bushing hottest-spot temperature rise at time t1(C)HS(t2) is the ultimate bushing hottest-spot temperature rise as calculated from the steady-

stateEquation (1)

For the new load (C)

HS(T) is the bushing hottest-spot temperature rise at the end of the transient load period

or

HS(T) = HS(t1+t) (C)

O(t1) is the immersion oil temperature rise as determined for time t1(C)

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O(t2) is the ultimate immersion oil temperature rise as determined from

IEEE Std C57.91 for the new load conditions that apply during the transient load

(C)

O(t1 + t) is the new immersion oil temperature rise at end of time increment t1+ t, (C),

as calculated in Equation (2):

( ) ( ) ( ) ( ) ( )0/0 1 0 1 0 2 0 1 1 t

t t t t t e + = + (2)

HS

(t1+t) is the new bushing hottest-spot temperature rise at end of

time increment t1+ t, (C), as calculated in Equation (3):

( ) ( ) ( ) ( ) 0( / )HS 1 HS 1 HS 2 HS 1 1t

t t t t t e + = + (3)

Step A: Determine initial bushing hottest-spot temperature rise at start of first increment, HS(t1) fromEquation (1) for prior per unit loadIand O(t1).

Step B: Determine new transformer immersion oil temperature rise at end of first increment,O(t1+ t) fromEquation (2).

Step C: Determine the new ultimate bushing hottest-spot rise HS (t2) for the conditions that

apply from Equation (1) using O(t1+ t) from step B.

Step D: Calculate the new transient bushing hottest-spot rise HS(t1+t) at the end of the timeincrement fromEquation (3) using HS(t1) and HS (t2) from steps A and C.

Step E: Use this new transient bushing hottest-spot rise HS (t1+t) as the new HS (t1) forinput to the subsequent incremental step.

Step F: Repeat the incremental procedure of Steps AE until the end of the transient load period(t = T).

See the example inFigure 1.

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Figure 1 Bushing hottest-spot transient response

4.2.2.2 Single step method

A simpler but less precise method is to make a single step calculation using Equation (4).This method

yields a higher bushing hottest-spot temperature and therefore can be considered more conservative thanthe method in4.2.2.1.

( ) ( ) ( ) ( ) ( )( )( ) ( ){ }{ }0( / ) ( / )HS HS 1 1 2 0 1 0 2 0 1 HS 1 1 1n T T t T t K I K t t t e t e = + + + (4)

4.3 Test procedures for derivation of mathematical model

When performance is to be determined by test, it is highly desirable that a uniform procedure be followed

so that data may be accumulated on a consistent basis. These procedures are in no way to be construed as a

mandatory design test for all bushings.

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4.3.1 Procedure for performance testing of bottom-connected bushings

This procedure applies to bushings that comply with Table 3 through Table 6 of IEEE Std C57.19.01TM.

a) Prepare the test unit by installing thermocouples on each terminal and in at least four locations

not more than 762 mm (30 in) apart on the center conductor. The thermocouples may beattached directly to the outside of the conductor by removing portions of the insulation, or the

thermocouples may make contact with the inside of a hollow conductor by means of a phosphor

bronze thermocouple brush. The thermocouple leads may be threaded through the bottom end ofa hollow center conductor, through a small hole in the top terminal, or brought out at some

convenient location above the internal oil level.

b) Install a pressure gage in such a way that the additional gas space of the gage and connectionswill not exceed 0.5% of the normal gas space. Seal the test unit with the gas chamber chargedwith the proper gas at the sealing pressure. If the thermocouple connections of item a) have

disturbed the sealing characteristics of the test unit, then a duplicate unit may be prepared for

pressure monitoring. If the test tank is of sufficient size to avoid proximity effects, then the

pressure unit may be mounted adjacent to and connected in series with the test unit. As analternative, the pressure unit may be tested separately.

c) Mount the bushing on a suitable nonmagnetic metal plate that complies with the minimum sizetab calculated as follows:

Bushing mounting plate

bolt circle

Cover plate size

(square or round)

Thickness

(in) (mm) (in) (mm) (in) (mm)6 to 9 1/4 152 to 235 18 457 6.4

13 to 15 337 to 400 24 610 1321 to 25 530 to 635 36 914 5/8 16

d) Attach oil-end terminal connectors suitable for the rated current.

e) Attach air-end terminal connectors suitable for the rated current.

f) Attach air-end bus at least 1 m (3 ft) long, projecting from the terminal connector in a horizontalplane. The cross section of the bus should be such that at rated current the temperature rise at a

location 1 m (3 ft) from the bushing should be at least 30 C above ambient.

g) Attach thermocouples to the bus work connectors, mounting plate, and exterior of the bushing.

h) Mount the bushing so that the oil level complies with either 5.4.1 of IEEE Std C57.19.00-1991or the level required in the actual bushing application after the steady-state test tank oil

temperature has been achieved.

i) Heat and circulate the oil to maintain a minimum vertical temperature gradient over the bushingimmersion depth without oil flow being directed at the test bushing.

j) The ambient environment should be indoor air between 10 C and 40 C.

k) Make load tests, as required, for obtaining the data necessary for a good statistical basis for a

bushing mathematical model. Some suggested conditions are as follows:

I o

Current (pu) Oil rise (C)

0.0 55

0.7 55

1.0 55

1.25 55

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I o

Current (pu) Oil rise (C)

0.0 70

1.0 70

1.5 70

2.0 70

l) Record temperatures at appropriate intervals until the thermal conditions become constant or untilthe measured temperatures do not increase by more than 1 C for 2 h for bushings up through

900 kV basic insulation level (BIL) and not more than 1 C for 4 h for bushings 1050 kV BIL and

above.

m) Report initial and final values of conductor hottest-spot rise, top and bottom terminal connectorrises, and increase in pressure. Also report the bushing time and temperature readings.

4.3.2 Tests on draw-lead bushings

When the thermal performance of a bushing with aspecific transformer lead is to be determined by test, a

procedure similar to the applicable portions of 4.3.1 may be followed.

4.3.3 Derivation of model constants

Nominal values of theK1,K2, and nconstants can be determined as follows:

a) Obtain a steady-state temperature profile at rated current with the bottom end immersed in hotoil by the procedure discussed in 4.3.1. This establishes

o(I= 1 pu) and

HS(I= 1 pu).

b) Reduce the current to zero and determine the steady-state temperature of the location that wasthe hottest spot at rated current. This establishes o(I= 1 pu) and HS(I= 0 pu).

c) The constantsK1andK2can be calculated usingEquation (5) and Equation (6):

( ) ( )2 HS o 0 pu / 0 puK I I= = = (5)

( ) ( )1 HS 2 o 1 pu [ 1 pu ]K I K I= = = (6)

d) The exponent ncan be calculated from additional tests usingEquation (7):

( )] [ ( ) ( )HS 2 o 1[1/ ln pu ln{ pu pu ] / }n I X I X K I X K = = = = (7)

whereXis the per unit value of the current.

e) The bushing time constant can be determined by analysis of the timetemperature curves fromthe tests.

Additional tests as recommended in item k) in 4.3.1 will confirm the nominal values of constants K1,K2,and nor give additional data to refine the estimates by graphical or statistical means.

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5. Special considerations for application of bushings to powertransformers

5.1 General

The temperature limits of bushings applied to power transformers can be exceeded by the transfer of heatfrom transformer components and accessories. If the thermal coordination of these sources is not correct,

then the bushing hottest-spot temperature may exceed 105 C. The result may be accelerated aging. An

additional concern is that the higher temperatures may deteriorate sealing gaskets. Potential sources of heattransferred to the bushing include the following:

a) Operation of bushings in transformers with top oil temperature rise greater than 55 C

b) Increased transfer of heat into the bushing from top oil in transformers with conservator oilpreservation systems

c) Improper thermal coordination of isolated-phase bus equipment (see Clause7)

d) Stray flux heating in the flange and other metallic bushing parts

5.2 Loading of bushings with transformer top oil temperature rises between 55 Cand 65 C

If a transformer has a top oil temperature rise greater than 55 C but less than or equal to 65 C, then a

bushing with a higher nameplate current rating than the transformer current rating may be applied by using

an appropriate derating factor. If the bushing thermal constants are known, then the derated currentIdmay

be determined from the following:

Id= dIr (8)

where

Id is the derated current at new transformer top oil temperature rise o

d is the [(65 K2 o) /K1]1/n

Ir is the bushing current rating

K1,K2, and n are as defined in4.2 and4.3

If the bushing thermal constants are not known, then the curve in Figure 2, derived by setting K1= 21,

K2= 0.8, and n= 1.6 inEquation (8),may be used to determine itemd) in5.1.

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Figure 2 Bushing current derating factor for transformer top oil temperature risesbetween 55

C and 65

C

5.3 Application of bushings in transformers with conservator oil preservationsystems

IEEE Std C57.19.00-1991 establishes bushing current ratings based on thermal tests run with the lower endof the bushing immersed to the minimum oil level, normally the bottom of the ground sleeve. When

bushings are applied to transformers with conservator oil preservation systems, the bushing lower end is

totally immersed in oil. If the transformer top oil temperature is higher than the bushing internal

temperature, then additional heat from the transformer oil will transfer into the bushing reducing its

current-carrying capability. Consult the bushing manufacturer for appropriate derating factors for these

applications.

5.4 Draw-lead application

The following guidelines can be used when bushings are used in a draw-lead application mode.

5.4.1 Bushing current rating

The maximum continuous current rating of the bushing in the draw-lead mode is limited to the rating

specified on the bushing nameplate. The maximum continuous current rating of the draw-lead cable is

determined by the size and type of the cable supplied by the transformer manufacturer. Since the draw-lead

cable is an integral part of the transformer, the current rating specified on the transformer nameplate

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determines the rating of the draw-lead cable. When the bushing manufacturer supplies the draw-lead

conductor as part of the bushing, the current is limited to the rating specified on the bushing nameplate.

5.4.2 Draw-lead cable/bushing loading

The current-carrying capacity of the draw lead cable is determined by the size and type of the cable used bythe transformer manufacturer. The current is limited to the rating specified on the transformer nameplate.

When applying bushings in the draw-lead mode, the bushing manufacturer should be consulted for

guidelines on draw-lead sizes, ratings, and loading. To minimize cable insulation loss of life during

overloads, it would be preferable to choose a cable with current rating of at least 20% above the ratedcurrent of the transformer. If higher overloads are anticipated, then cables with even greater margin should

be considered.

Although specific guidelines should be obtained from bushing or transformer manufacturers, following

general guidelines can be used for rating draw-lead cables with thermally upgraded 65 C insulation.

Maximum ambient 40 C

Maximum cable hottest-spot temperature rise above ambient air at rated current 80 C

Maximum cable hottest-spot temperature rise above ambient air under overload condition100 C

The 80 C rise limit agrees with the maximum winding hottest-spot temperature rise for 65 C rise

transformers as per IEEE Std C57.12.00.

The 100 C hottest-spot temperature rise limit is based on maximum oil temperature of 140 C withmaximum ambient air temperature of 40 C. Operation at hottest-spot temperature above 140 C may result

in gassing of oil/paper insulation.

Because it is not possible to detect the absolute hottest spot in the cable, it is prudent to keep temperatures

below these maximums.

5.5 Draw-rod application

In a draw-rod bushing, the load current is carried mainly by the bushing central tube. The thermal

characteristics of such a bushing are essentially the same as a conventional bushing.

6. Thermal loading for bushings applied on circuit breakers

Bushings applied on power circuit breakers will be subject to the requirements in IEEE Std C37.010TMand

IEEE Std C37.017TM.

7. Thermal loading for bushings used with isolated-phase bus

7.1 Concerns for bushings used in isolated-phase bus

Bushings used with isolated-phase bus meeting the requirements of IEEE Std C37.23TMmay be subjected

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Standard bushings may not be acceptable for replacements on mobile transformers or substations. Always

consult with the bushing manufacturer regarding the specific application before selecting a replacement.

8.3 Circuit breaker applications

Condenser bushings applied on oil circuit breakers should be capable of withstanding the forces specified inTable 1 of IEEE Std C37.017-2010.

If the bushing is not identified as being transformer breaker interchangeable (TBI), the manufacturer needs

to be contacted to ensure that the bushing will withstand the forces of the breaker operation.

9. Application of bushings in unusual service conditions

Usual service conditions are described in subclause 4.1 of IEEE Std. C57.19.00-1991.

9.1 Contaminated environments

Standard bushing characteristics are specified for a standard clean environment. This promotes a common

understanding between manufacturers and users of what bushing ratings mean.

Proper application of bushings in environments different from the standard requires knowledge of how

bushing performance changes from one environment to another.

The purpose of this subclause is to highlight those issues that need to be considered in applying bushings invaried environments.

9.1.1 Types of environments

Contaminated environments can be divided into the general types summarized inTable 1.

9.1.2 Types of contaminants

9.1.2.1 Natural deposits

Natural deposits on bushings include such things as salts, dust, sand, and so on, left on the bushings as the

result of natural action. They may be airborne, waterborne, or left behind after the melting of snow and ice.

9.1.2.2 Automotive/industrial effluents

These are by-products put into the air as a result of industrial/commercial activity. They include

particulates and gaseous materials that condense on bushing surfaces.

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Table 1 General types of contaminated environments

Contamination level Typical environments

Light Areas without industries and with a low density of emission-producing residential

heating systems. Areas with some industrial or residential density but subject tofrequent winds and/or precipitation. Agricultural areas (exposure to wind-bornefertilizer spray or crop-burning residues can lead to higher contamination levels).

Mountainous areas. These areas are not exposed to sea winds or located near the sea.Typical measured equivalent salt deposit density (ESDD) levels are 0.03 mg/cm 2 to0.08 mg/cm2.

Medium Areas with industries not producing highly polluting smoke and/or with an averagedensity of emission-producing residential heating systems. Areas with high industrialand/or residential density but subject to frequent winds and/or precipitation. Areas

exposed to sea winds but not located directly on the coast. Typical measured ESDDlevels are 0.08 mg/cm2to 0.25 mg/cm2.

Heavy Areas with high industrial density and large city suburbs with a high density ofemission-producing residential heating systems. Areas close to the sea or exposed tostrong sea winds. Typical measured ESDD levels are 0.25 mg/cm2to 0.6 mg/cm2.

Extra heavy Small areas subject to industrial smoke-producing thick conductive deposits. Smallcoastal areas exposed to very strong and polluting sea winds. Typical measured

ESDD levels are above 0.6 mg/cm2.

9.1.2.3 Other deposits

Other types of deposits such as agricultural residues can also occur as a result of specific types of activitiesin the vicinity of a bushing location.

9.1.3 Artificial contamination testing

A design or production test method that fully duplicates an actual environment where a bushing will be

applied is usually not practical. Therefore, artificial test methods have been developed that are intended to

provide a realistic assessment of the characteristic being tested (see reports by the General ElectricCompany [B4] and the IEEE Working Group on Insulator Contamination [B11] for discussions of test

methods). The three major categories of testing are discussed in9.1.3.1 through9.1.3.3.

9.1.3.1 Salt fog

A bushing is energized at a constant test voltage and subjected to a salt fog of controlled salinity. Typicalsalinity values range from 2.5 g to 160 g of salt per cubic meter of fog solution. The fog is sprayed on the

bushing through an array of nozzles with compressed air. The withstand salinity is the salinity at which

there is a withstand in at least three of four 1 h test periods.

9.1.3.2 Wet-contamination

Artificial contamination is applied to a bushing by a spray or flow-coating method. Three to five minuteslater, before the contaminant has time to dry, a test voltage is applied to the bushing. The voltage is either

raised until the bushing flashes over or raised to a test value and held constant until the bushing flashes over

or the contaminant dries out and all scintillation activity stops. The contaminant is a mixture of water and

kaolin or other nonconductive material with a controlled amount of salt added.

A withstand value is sometimes determined by either three successful withstands without a flashover at a

given test voltage or by statistical analysis of a number of trials.

This method has an advantage over the other methods in simplicity, ease of use, and low test cost.

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9.1.3.3 Clean fog

A dry, artificially contaminated bushing is subjected to clean fog and test voltage. In one variation, the fog

is applied to the bushing and then it is energized. In the other variation, the bushing is energized and then

the fog is applied to it.

9.1.4 Natural contamination testing

The primary way to identify the types of natural contaminant on a bushing is through chemical analysis and

testing. This is especially important for cases of industrial pollutants when the identity of the polluting

agent is not immediately known. In addition, special tests can be used to quantify the effect of the

contaminants on the electrical bushing characteristics.

The primary test for this purpose is the ESDD. This test is used to establish the conductivity of the water-soluble deposits on a bushing surface in terms of the density of a standard soluble salt deposited on a

surface that would produce the same conductivity.

A measured surface area on a bushing is washed in a known amount of water of very low conductivity. The

resistivity of the wash water is then measured and the amount of sodium chloride (NaCl) needed to producethe same conductivity in the known quantity of wash water is calculated. The result is expressed as

milligrams of NaCl per square centimeter of washed bushing surface area (mg/cm2).

Additional information on this method is contained in Appendix 1C of IEEE Std 4TM-1995.

9.1.5 Countermeasures

The user will need to evaluate the following and any other options available to determine their suitability to

the situation:

a) Install extra creep distance bushings. The following minimum creep values based on thebushing nominal line-to-ground kV rating are recommended. These values may need to be

adjusted for factors such as shape, number of sheds, and bushing inclination.

Contamination Creep distance

Light 28 mm/kV

Medium 35 mm/kV

Heavy 44 mm/kV

Extra heavy 54 mm/kV or greater

b) Apply protective coatings. Protective coatings can be applied to the surface of the bushings toimprove their dielectric performance. There are temporary coatings, such as silicone grease, thatrequire periodic replacement and permanent coatings that are nonremovable. These coatings

may cause power factor testing issues and they may become contaminated.

c) Install conductive glaze bushings. Consult the manufacturer for specific application information.

d) Install composite insulated bushings with nonceramic, contamination-resistant externalinsulation. Consult the manufacturer for specific application information.

e) Periodic cleaning of bushing surfaces. Bushings with known contamination cycles can becleaned periodically as part of a maintenance program.

f) Eliminate air bushings. Installations can be designed to minimize the number of bushingsexposed to atmospheric contamination.

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9.2 High altitudes

Refer to IEEE Std C57.19.00-2004 for altitude correction factors.

9.3 Application of outdoor bushings in indoor locations

Bushings designed for outdoor application can also be applied indoors; however, the following should be

considered:

a) The rating of bushings is assigned at an ambient air temperature not exceeding 40 C, and theaverage temperature of the ambient air is not exceeding 30C in any 24 h period. These limits

may be exceeded in the indoor locations. If it should be determined that the ambient air

temperatures will exceed the allowable temperature, then the user may either select a bushing

with higher rating or accept a moderate degree of accelerated aging.

b) The environmental contamination should be reviewed. If the ventilation system allows the airfrom the outdoors to circulate freely indoors without any filtration, contaminants present

outdoors will be indoors and will be deposited on the bushing. However, unlike outdoor

bushings where the contaminants are washed away by rain, the contamination level on indoorbushings will keep increasing. The user may select bushings with higher creepage distance and

institute maintenance programs to include bushing insulator cleaning.

9.4 Bushing monitors

Bushing monitors are installed on bushing voltage/test taps to allow for on-line testing and monitoring of

the bushing power factor. These allow for continuous monitoring or periodic testing of bushings without

removing the bushing from service. Since the monitors replace the existing voltage/test tap cover, the user

should ensure proper fit of the monitor to prevent moisture ingress into the test tap. A voltage is thenbrought from the monitor to an accessible area of the transformer to perform the monitoring. The user

should consult with the manufacturer of the monitor to determine the voltage levels that are normallyexpected and the maximums that could be reached should the device fail. Operational guidelines can then

be developed by the user to determine the level of safety requirements and experience of personnel whoshould have access to the devices.

10. Bushing maintenance practices

The in-service maintenance frequency of bushings will normally vary according to circumstances and is

generally combined with the inspection and maintenance of the associated equipment.

10.1 Mechanical maintenance and inspection

10.1.1 External porcelain

Inspect the porcelain for damage and pollution deposits. At least the following guidelines should be

considered during the examination.

Small chips or breaks in the sheds are generally of no concern. The exposed unglazed surface may be

painted with a suitable paint to improve the appearance. Large breaks or chips may reduce the creep

distance and may require bushing replacement.

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Small cracks in the sheds may be ground off to prevent further propagation. Large cracks may require

bushing replacement.

Any damage to the main porcelain body would be a cause for concern and may require bushing

replacement.

Bushings may be periodically cleaned by either hand-washing (deenergized installation) or by a suitablespray or jet method using low conductivity water.

Silicone-based greases and coatings can be applied to increase the time interval between cleanings.

However, this treatment prevents normal rainfall from cleaning the porcelain surfaces.

10.1.2 Terminals

Inspect bushings for overheated connections when the unit is energized and loaded. Infrared cameras are

sometimes used to detect overheated terminal connections. Loose connections should be tightenedaccording to the bushing manufacturers recommendations.

10.1.3 Mounting hardware

Inspect the mounting hardware for tightness.

10.1.4 Gaskets

Gaskets that are part of the bushing normally do not require replacement. Be sure that replacement gasketsbetween the bushing flange and the associated equipment are the right thickness and suitable material.

Gasket stop rings, if used, should be in place.

Gaskets that are sensitive to ultraviolet radiation may deteriorate rapidly when exposed to combinedsunlight, high humidity, and contamination. These materials should be avoided in these conditions. As an

added precaution, gaskets in these conditions should be protected from exposure to sunlight.

10.1.5 Oil level

Loss of oil threatens the integrity of a bushing; therefore, any bushing that shows an abnormal oil levelshould be investigated as soon as possible. Follow the manufacturers recommendations in correcting the

cause of the abnormal oil level and in refilling the bushing.

The associated apparatus should be checked to ensure that the lower end of the bushing is immersed in oil

to the proper level.

Special measures may be required to keep oil over internal insulation in bushings mounted at angles greater

than 20 from vertical.

10.1.6 Bushing taps

Inspect the bushing voltage and test taps for proper gaskets and grounding. The voltage tap compartments

shouldbe filled with insulating oil or compound when recommended by the bushing manufacturer.

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10.2 Bushing repair

Maintenance and repair recommendations vary widely from one manufacturer to another. In all cases, the

user is advised to consult the manufacturer prior to attempting any bushing repairs.

Extreme care should be taken when working on or around damaged bushings. Many bushings are

constructed with high compressive forces and can be dangerous when components are damaged. Iftransporting the bushing is required, check with local regulations prior to shipment to determine oil

polychlorinated biphenyl (PCB) level limitations.

The following are common repair needs that may be encountered by bushing users and generalrecommendations for each:

a) Damaged porcelain: Small chips or breaks in porcelain are not detrimental to bushingperformance, as modern electrical porcelain is a nonporous material. Larger chips or breaks

generally will not hinder performance either but may have improved appearance by painting

over the area with a suitable paint (bright-colored paint is often used for safety purposes).

Cracks in the main porcelain body are cause for immediate replacement. Damages to sheds thatsignificantly reduce creepage distances are also cause for bushing replacement or professional

repair. Care should be taken when working with damaged porcelain, as broken edges are oftenextremely sharp.

b) Damaged oil sight glasses: A cracked, chipped, or shattered oil sight glass often allows moistureto enter into the bushing and is cause for bushing replacement or professional repair. To

minimize damage to the bushing core, it is recommended to wrap the affected area as soon aspossible. Field repairs are not recommended.

c) Malfunctioning oil-level gages: An oil-level gage that does not correctly indicate the oil level inthe bushing can lead to future problems and is reason for repair. Many oil level gages can be

replaced without disassembling the bushing. The mechanism that measures oil level, however, isinternal to the bushing and cannot usually be replaced in the field. The manufacturer or a

professional repair service should be contacted to determine if the gage design can be replaced

in the field.

d) Leaking seals: Loss of oil threatens the integrity of a bushing and may be cause for bushingreplacement or professional repair. Gaskets that are bulging out between bushing sections,

unevenly seated, cracked, overheated, have lost their elasticity, or appear in any way damaged,

have most likely permanently ceased to protect the bushing and should be replaced, even if no

leaks are in evidence, or at least the bushing should be closely monitored. Bushings that have

been subjected to overload may have damaged gaskets. These should be examined closely and

replaced if any signs of damage are present even if they are not currently leaking. Some leaksmay be temporarily sealed using an oil-compatible sealant, but it is important to repair the seal

as soon as possible. Professional repair services can often eliminate the leak by disassembling

the bushing and replacing the gaskets. The manufacturer or a professional repair service shouldbe contacted if a significant reduction in oil level has occurred.

e) Leaking taps: Loss of oil into the bushing tap space can often be temporarily sealed with theprovided tap cover. Some bushing taps can be replaced in the field by positioning the bushing

horizontally and installing a suitable replacement. The manufacturer or a professional repairservice should be contacted to determine if tap replacement is possible in the field.

f) Damaged taps: Damage due to electrical activity at the bushing tap can cause high- or low-power factor readings and is reason for repair. Some bushing taps can be replaced in the field by

positioning the bushing horizontally and installing a suitable replacement. The manufacturer or

a professional repair service should be contacted to determine if tap replacement is possible in

the field.

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g) Power factor or capacitance abnormalities Significant or rapid changes to power factor orcapacitance are often precursors to dielectric failure and may be cause for bushing replacement

or professional repair. Investigation should follow manufacturer recommendations and the

guidelines in10.3.1.Baking the bushing core in an oven, a popular remedy in the past, will not

result in a long-term improvement in power factor. Field repairs are not recommended.

h) Overheated terminals: Excessive heat at bushing terminals is most often caused by loose

connections or damaged threads. This situation can be corrected by tightening connections orrepairing damaged threads, where needed. In these cases, consideration should be given to the

duration the bushing was overheated and the impact on seals in proximity to the heat source.

10.3 Routine and special tests

10.3.1 Power factor and capacitance

The bushing power factor and capacitance should be measured when a bushing is first installed and again

one year after installation. After these initial measurements, the bushing power factor and capacitance

should be measured at regular intervals (3 years to 5 years typically). If the bushing is installed in a

transformer, then it is important to remember that the transformer winding can also influence the testresults.

Since the power factor can vary with temperature, test results should be converted to a commontemperature base (usually 20 C) using temperature-correction data that are available from manufacturers

and various industry sources. The following procedure should be used:

a) Measure the bushings power factor

b) Determine the bushings temperature

c) Obtain the appropriate correction factor corresponding to the bushings temperature

d) Multiply itema) and itemc)see example

Example

A 115 kV oil-impregnated bushing:

Calculated power factor = 0.42%

Bushing temperature = 30 C

Multiplier from the temperature-correction table at 30 C = 1.11

Corrected to 20 C power factor = 0.42% 1.11 = 0.47%

These correction factors are average at best and therefore are subject to some error. The magnitude of the

error is minimized if the tests are performed at temperatures near the reference temperature of 20 C. Ifquestionable power factors are recorded at a relatively high temperature, then the bushing should not be

condemned until it has been allowed to cool down to near 20 C and repeat tests have been performed. This

also applies to bushings tested near freezing where a large (greater than 1.00) correction may cause the

result to be unacceptably high; in this case, the equipment should be retested at a higher temperature.

Bushings should not be tested when their temperatures are below freezing because moisture may havechanged to ice, which has a significantly higher resistivity and therefore are undetected. In the case of

bushings mounted in transformers, the bushing temperature is approximated by taking the average between

the ambient and transformer top-oil temperatures.

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Any bushing that exhibits a history of continued power factor increase should be further investigated and

considered for removal from service. The bushing manufacturer should be consulted for guidance. If any

bushing exhibits an increase in power factor over a period of time, then the rate of change of this increase

should be monitored by more frequent tests. The decision on when to increase test frequency or remove

from service should consider manufacturer recommendations, user and industry experience with serviceaged equipment, and bushing design, among other factors. Some bushing manufacturers consider an

increase between 1.5 and 2 times the initial reading to be significant. Bushing capacitance should be

measured with each power factor test and compared carefully with both nameplate and previous tests inassessing bushing condition. This is especially important for capacitance-graded bushings where an

increase in capacitance of 5% or more over the initial/nameplate value is cause to investigate the suitability

of the bushing for continued service. The manufacturer should be consulted for guidance on specific

bushings.

It is usually impossible to make absolute ungrounded specimen test (UST) measurements of the bushing

core capacitance and power factor of resistance-graded bushings because of the influence of the resistiveglaze on the surface of the bushing porcelain. Differences in the glaze can cause significant variations in

measurements between different bushings of the same voltage class and type. In some instances, the

measured UST power factor may even be negative.

Standard practice during diagnostic testing of resistance-graded bushings is to record the measured UST

values of capacitance and power factor for comparison with other measurements made on the samebushing. When there is evidence of a permanent increasing or decreasing trend in the measured values, thebushing manufacturer should be consulted for assistance in evaluation of the condition of the bushing.

Most modern high-voltage bushings are equipped with voltage or test taps. Voltage (or capacitance) tapsare used generally on bushings rated above 69 kV, and test (or power factor) taps are used generally on

bushings rated at 69 kV and below. Routine bushing field tests should include power factor tests on the tap

insulation. These measurements are effective for detecting problems such as deteriorated or contaminated

oil, defective tap compartment seals, mechanical damage to the tap insulators, and the failure of the topterminal gaskets. Abnormal readings can alert the user to a potential failure hazard before the C1 insulation

is affected. While the test tap (C2) insulation is not controlled during the manufacturing process, significant

changes in either the C2 capacitance and/or power factor from an initial value may be indicative of a

problem. The manufacturer should be contacted.

10.3.2 Gas in oil

This test is not recommended as a routine test because it requires that the bushing be opened up and

exposed to the outside atmosphere. This introduces the possibility of moisture entering the bushing while

the bushing is open or after improper sealing of the opening. While this test is performed by some

companies, the degree of expertise to perform and interpret it makes it impractical for most users. There are

no IEEE values for DGAs in bushings. Transformer values are not applicable due to the differences in oilvolumes and paper/oil ratios. IEC/TS 61464 [B7] may be used for a reference.

The gas-in-oil test should only be used for diagnostic purposes on bushings that are suspect due to high-

power-factor measurements or other reasons. Gas-in-oil results should be compared with test results from

other bushings and not with power transformer test results. The different mixture of materials in bushingsand in transformers will give different results. Experts with experience in interpreting bushing gas-in-oil

tests should be consulted if help is needed.

A good noninvasive alternative to taking an oil sample is to perform the C2 test. This test energizes the oil

in the vicinity of the energized tap lead near the bushing flange and thus includes any watts loss caused by

contamination of that oil in the results. This contamination would be apparent as long as there was a

sufficient amount in the tap lead area and if the tap housing area were clean and dry so as not to mask the

contamination in the oil.

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The bushing manufacturer should be consulted for assistance in taking samples and interpreting results. The

bushing oil level should be checked and adjusted if needed after oil samples are taken.

10.3.3 Dielectric tests

Bushing dielectric tests are sometimes performed in the field. Insulation dielectric strength generallydepends on the level of insulation degradation. When dielectric tests are performed on service aged

bushings, the following guidelines can help in determining the appropriate test levels:

a) Transformer bushings that will be removed from the transformer for testing should undergo60 Hz tests at the 100% voltage test levels specified in IEEE Std C57.19.01. This will minimize

any problems that may develop during the testing of the transformer after the bushing is

reinstalled.

b) Transformer bushings that will be tested while mounted in the transformer can only be tested atthe lower of either the applicable bushing or transformer test levels. Test levels of 60 Hz should

be limited to 1.5 times rated line-to-ground voltage or 85% of the withstand voltage level,

whichever is lower. The voltage application should be limited to 1 min.

Partial discharges should be monitored during these tests to provide data for evaluating the condition of thebushing. IEEE Std C57.19.00-1991 gives additional information on partial discharge testing.

10.4 Bushing storage

10.4.1 New bushings

It is recommended that the bushing manufacturer be consulted concerning bushing storage. They will

provide instructions as to the proper storage of their bushing. Improper storage can result in damaging not

only the bushing but also the equipment in which it is installed.

New bushings should arrive from the manufacturer properly crated for short-term storage. If the bushingsare to be used in a short time (6 months maximum), storage in the original crate should be adequate. If the

bushings are kept at a construction location, then they should be positioned to be where they are protected

from accidental breakage or vandalism before they can be installed.

New bushings for stores or spares are possibly going to remain unused for long periods. Since most crates

are not designed for long-term storage, arrangements should be made to protect them from weather. This

can be done by applying a waterproof covering, plastic sheeting, exterior rated plywood, roofing material,

etc., or by storing them indoors. They can also be uncrated and stored in metal racks either indoors oroutdoors. Storage in this manner has the advantage that the bushings can be periodically tested. The

disadvantage is the possibility of breakage as they are uncrated and then recrated for shipping. Since they

are designed to be used in oil-filled equipment, storing them out of oil exposes them to air, moisture, and

possibly ultraviolet rays. To ensure they are usable when needed, the manufacturer should be consulted for

storage information. This is for both short- and long-term storage.

10.4.2 Used bushings

Bushings manufactured before 1980 may be PCB contaminated. Consult with your environmental expert

concerning the local and Environmental Protection Agency laws before attempting to store these older

bushings of unknown contents. Newer bushings should be tagged to be non-PCB. If the tag is missing, then

the manufacturer may be able to assist you in determining the PCB content.

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The type of bushing and expected storage time should both be considered when determining storage

options. Knowledge of bushing construction is helpful in developing storage plans. In all cases, the goal

should be to cause no damage to the bushing and to keep it in good operating condition. If the bushings are

subject to periodic testing, then the storage facility should be arranged such that this can be accomplished.

10.5 Bushing replacement

Bushing replacement is a common practice, taking place thousands of times every year. The most critical

item to remember when replacing existing bushings is that you do not change the electrical geometry insidethe transformer. Any change to the electrical clearances or insulation structure inside the transformer may

lead to a transformer failure. When specifying bushings, future replaceability should be a serious

consideration.

When replacing an existing bushing, a number of key items need to be addressed before a replacement

bushing is selected. The following is a list of items to be considered:

Bushing mounting flange configuration; bolt circle diameter and number of bolt holes.

Bushing length below the flange, this is normally noted on the nameplate as the L dimension.

How is the bushing connected to the transformer winding? Via draw lead cable or conductor orconnected to the bottom of the bushing.

Bottom terminal; threaded stud, two-hole spade, or bottom plate.

Height of bushing above the mounting flange.

Bushing current rating.

A complete list is included in Clause11when bushings must be ordered.

10.5.1 Replacing bottom-connected bushings

If the replacement bottom connected bushing meets all the dimensional requirements below the mounting

flange as the existing bushing, then the only other item to consider is the shielding. The key item to confirm

is the thickness and diameter of the bottom plate. It must not be thicker than the existing bushings bottomplate nor larger in diameter.

Bushing shields are related to both transformer and bushing design. Determination of shielding

requirements will need input from both parties or from knowledgeable consultants.

10.5.2 Replacing draw lead connected bushings

If the replacement draw lead bushing meets all the dimensional requirements below the mounting flange asthe existing bushing, then the only other item to consider is the shielding.

Bushing shields are related to both transformer and bushing design. Determination of shielding

requirements will need input from both parties or from knowledgeable consultants.

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11. Checklist

The following is a checklist for ordering replacement bushings.

Checklist for ordering replacement bushings

Bushing manufacturer

Year built

Bushing style

Outline drawing (to be attached with order)

Insulation class kV

BIL kV peak

V max L-g kV

V withstand/1 min kV

Rated current Amp

Conductor connection Bottom connected or draw lead

Air side dimension inch

Oil side dimension inch

CT pocket lengtha

inchFlange BCD/#Holes/ODa

Minimum creep distance inch

Arcing distance inch

Insulator color

Top terminal

Bottom terminal

Additional notes:aBCD = bolt circle diameter; CT = current transformer; OD = outside diameter.

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Annex A

(informative)

Examples of calculation procedures to determine hot-spot temperatures for

bushings applied on transformers

A.1 General information

This annex contains examples showing the use of calculation procedures given in this guide. In general, the

conditions to be evaluated will include a period