IS 2026_3

© BIS 2003

B U R E A U O F I N D I A N S T A N D A R D SMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

IS : 2026 (Part III) - 1981(Reaffirmed 2001)

Edition 3.1(1994-03)

Price Group 9

Indian StandardSPECIFICATION FOR POWER TRANSFORMERS

PART III INSULATION LEVELS AND DIELECTRIC TESTS

( Second Revision )(Incorporating Amendment No. 1)

UDC 621.314.222.6 : 621.317.333

IS : 2026 (Part III) - 1981

© BIS 2003

BUREAU OF INDIAN STANDARDS

This publication is protected under the Indian Copyright Act (XIV of 1957) andreproduction in whole or in part by any means except with written permission of thepublisher shall be deemed to be an infringement of copyright under the said Act.



( Second Revision )

Transformers Sectional Committee, ETDC 16

Chairman Representing

SHRI D. V. NARKE Bharat Heavy Electricals Ltd, Bhopal

Members

SHRI PREM CHAND

SHRI D. P. GUPTA( Alternates to Shri D. V. Narke )

SHRI R. S. ARORASHRI D. R. CHANDRAN ( Alternate )

Directorate General of Supplies & Disposals(Inspection Wing), New Delhi

SHRI A. V. BHEEMARAOSHRI S. H. MAKHIJANI ( Alternate )

Gujarat Electricity Board, Vadodara

SHRI A. CHATTERJEESHRI T. K. GHOSE ( Alternate )

Calcutta Electric Supply Corporation Ltd,Calcutta

SHRI S. D. CHOTRANEYSHRI Y. K. PALVANKAR ( Alternate )

Bombay Electric Supply and TransportUndertaking, Bombay

SHRI D. DHARSHRI B. A. SUBRAMANYAM ( Alternate )

The General Electric Co of India Ltd,Allahabad

DIRECTOR (SUBSTATIONS)DEPUTY DIRECTOR (SUBSTATIONS)

( Alternate )

Central Electricity Authority, New Delhi

JOINT DIRECTOR TI (SUBSTATION)DEPUTY DIRECTOR STANDARDS

(ELEC) ( Alternate )

Research, Designs and StandardsOrganization, Lucknow

DR M. V. JOSHISHRI P. K. JOSHI ( Alternate )

Electrical Research and DevelopmentAssociation, Bombay

SHRI D. B. MEHTASHRI R. CHANDRAMOULI ( Alternate )

Tata Hydro-Electric Power Supply Co Ltd,Bombay

( Continued on page 2 )

IS : 2026 (Part III) - 1981

2

( Continued from page 1 )

Members Representing

SHRI V. R. NARASIMHANSHRI C. S. SARMA ( Alternate )

Central Power Research Institute, Bangalore

SHRI T. OMKUMARSHRI P. S. RAMAN ( Alternate )

NGEF Ltd, Bangalore

SHRI I. S. PATEL Hindustan Brown Boveri Ltd, BombaySHRI U. K. PATWARDHAN Prayog Electricals Pvt Ltd, BombayDR G. M. PHADKE

SHRI P. K. PHILIP ( Alternate )Indian Electrical Manufacturer’s Association,

BombaySHRI V. N. PRAHLAD

SHRI T. B. SEN ( Alternate )Voltas Ltd (Motor and Transformer Plant),

BombaySHRI CHANDRA K. ROHATGI Pradip Lamp Works, PatnaSHRI P. S. SAWHNEY

SHRI B. B. DASS ( Alternate )Delhi Electricity Supply Undertaking, New

DelhiSHRI P. K. SAXENA

SHRI G. L. DUA ( Alternate )Rural Electrification Corporation Ltd, New

DelhiSHRI R. K. SEHGAL

SHRI H. S. NATARAJAN ( Alternate )Bombay Suburban Electric Supply Ltd,

BombaySHRI V. T. D’SILVA

SHRI R. G. PARDHANANI ( Alternate )Siemens India Ltd, Bombay

SUPERINTENDING ENGINEER (TECHNICAL PROJECTS)

SUPERINTENDING ENGINEER (GRID OPERATION) ( Alternate )

Andhra Pradesh State Electricity Department(Electricity Projects and Board),Hyderabad

DR VAKIL AHMEDSHRI S. K. PALHAN ( Alternate )

Directorate General of TechnicalDevelopment, New Delhi

SHRI C. R. VARIERSHRI S. V. MANERIKAR ( Alternate )

Crompton Greaves Ltd, Bombay

SHRI T. V. VIDYARATNA RAJSHRI M. D. KALLIANPUR ( Alternate )

Kirloskar Electric Co Ltd, Bangalore

SHRI S. P. SACHDEV,Director (Elec tech)

Director General, BIS ( Ex-officio Member )

SecretarySHRI K. M. BHATIA

Deputy Director (Elec tech), BIS

Panel for Insulation Levels and Dielectric Tests for Power Transformers, ETDC 16/P9

Convener

SHRI D. V. NARKE Bharat Heavy Electricals Ltd, Bhopal

Members

SHRI S. C. NANDANKAR

SHRI PREM CHAND ( Alternates to Shri D. V. Narke )

SHRI A. K. CHOPRASHRI K. L. BHATIA ( Alternate )

Punjab State Electricity Board, Patiala

( Continued on page 42 )

IS : 2026 (Part III) - 1981

3



( Second Revision )0. F O R E W O R D

0.1 This Indian Standard (Part III) was adopted by the IndianStandards Institution on 26 May 1981, after the draft finalized by theTransformers Sectional Committee had been approved by theElectrotechnical Division Council.

0.2 This standard was first issued in 1962 and was revised in 1977with a view to align it with the revision of IEC Publication 76 ‘Powertransformers’ issued by the International ElectrotechnicalCommission and issued in the following four parts:

0.3 The second revision of this standard (Part III) has beenundertaken with a view to bring it in line with the latest thinking atthe IEC level. The most significant modification in this revision is thatthe line of demarcation to have lightning impulse test as a routine test,has been shifted from ≥ 145 kV to ≥ 300 kV.

0.4 This second revision also clarifies some anomalies noticed in thefirst revision with regard to induced overvoltages and impulsewithstand tests.

0.5 This standard (Part III) is to be read in conjunction with IS : 2026(Part I)-1977*, IS : 2026 (Part II)-1977† and IS : 2026 (Part IV)-1977‡.

0.6 This standard (Part III) is based on IEC Pub 76-3 (1980) ‘Powertransformers: Part III Insulation levels and dielectric tests’ and IECDocument 14 (Central Office) 51 Draft Amendment No. 1 to Pub 76-3,issued by the International Electrotechnical Commission.

Part I GeneralPart II Temperature-risePart III Insulation levels and dielectric testsPart IV Terminal marking, tappings and connections

*Specification for power transformers: Part I General ( first revision ).†Specification for power transformers: Part II Temperature-rise ( first revision ).‡Specification for power transformers: Part IV Terminal marking, tappings and

connections ( first revision ).

IS : 2026 (Part III) - 1981

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0.7 This edition 3.1 incorporates Amendment No. 1 (March 1994). Sidebar indicates modification of the text as the result of incorporation ofthe amendment.

0.8 For the purpose of deciding whether a particular requirement ofthis standard is complied with, the final value, observed or calculated,expressing the result of a test, shall be rounded off in accordance withIS : 2-1960*. The number of significant places retained in the roundedoff value should be the same as that of the specified value in thisstandard.

1. SCOPE

1.1 This standard (Part III) specifies the requirements relating toinsulation levels and dielectric tests for power transformers.

2. GENERAL

2.1 The dielectric tests ( see Table 1 ) shall generally be carried out atthe manufacturer’s works with the transformer approximately atambient temperature.

2.2 The transformers shall be completely assembled as in service,except that for liquid-filled transformers the fitting of external coolingand supervisory equipment shall not be necessary.

2.3 Transformers for cable box connections or direct connections tometal enclosed SF 6 installations shall be so designed that thetemporary connections can be made for dielectric tests, usingtemporary bushings, if necessary.

2.4 When the manufacturer proposes to use non-linear elements orsurge divertors (built into the transformer or fitted externally) for thelimitation of transferred overvoltage transients, this shall be broughtto the attention of the user.

NOTE — The insulating requirements for power transformers and the correspondinginsulation tests are given with reference to specific windings and their terminals. Forliquid-filled transformers the requirements apply to the internal insulation only, andare not related to the properties of external bushing insulation under differentweather conditions or contamination. Any additional requirement or tests regardingexternal insulation which are deemed necessary shall be subject to agreementbetween the purchaser and the supplier. When an oil-filled transformer is specifiedfor operation at an altitude higher than 1 000 m, it may then be necessary to selectbushings designed for higher insulation levels than those specified for the internalinsulation of the transformer windings. Bushings are subjected to separate type androutine tests in accordance with IS : 2099-1973† which also verify theirphase-to-earth insulation, external as well as internal. It is presupposed thatbushings and tap-changers are specified, designed and tested in accordance with

*Rules for rounding off numerical values ( revised ).†Specification for bushings for alternating voltages above 1 000 volts ( first revision ).

IS : 2026 (Part III) - 1981

5

IS : 2099-1973* and IS : 8468-1977†. The insulation tests on the completetransformer, however, check the correct application and installation of thesecomponents.

2.5 If a transformer fails to meet its test requirements due to a fault ina bushing, to facilitate continuation and completion of the test withoutdelay, the replacement of the faulty bushings shall be permissible. Aparticular case arises for tests with partial discharge measurements,where certain types of commonly used high voltage bushings createdifficulty because of their relatively high level of partial discharge inthe dielectric. When such bushings are mounted on the transformer itshall be permissible to exchange them for bushings of a partialdischarge free type during the testing of the transformer ( seeAppendix A ).

3. HIGHEST VOLTAGE FOR EQUIPMENT AND INSULATION LEVEL

3.1 Highest Voltage for Equipment — Each winding of atransformer shall be assigned a value of highest voltage for equipmentdenoted by Um which is the maximum value of the highest voltage of asystem to which the winding may be connected in respect of itsinsulation.

The rules for coordination of transformers insulation with respect totransient overvoltages are formulated differently depending on thevalue of Um. When rules about specific tests for different windings in atransformer are in conflict, the rule for winding with the highest Umvalue shall apply. Rules for a number of special cases are given in 4.3.1.1 The standard values of Um are listed in Tables 2 to 4. The valueto be assigned to a transformer winding shall be the one equal to ornearest above the rated voltage of the winding.

NOTE 1 — Single-phase transformers intended for connection in star to form athree-phase bank are designated by phase-to-earth rated voltage, for example,

kV. The phase-to-phase value determines the choice of Um (in this case,

consequently, Um = 420 kV).

NOTE 2 — It may happen, particularly for tapped windings, that for some reason therated voltage of a winding is chosen slightly higher than a standard value of Um butthat the system to which the winding will be connected has a system highest voltagewhich stays within the standard value. The insulation requirements are to becoordinated with actual system conditions, and therefore the standard value shall beaccepted as Um for the transformer, and not the nearest higher value.

3.1.2 The value Um assigned to each winding in the transformer ispart of the information to be supplied with an enquiry and order.

*Specification for bushings for alternating voltages above 1 000 volts ( first revision ).†Specification for on-load tap-changers.

4003

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IS:2026

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rtIII)

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TABLE 1 REQUIREMENTS AND TESTS FOR DIFFERENT CATEGORIES OF WINDINGS( Clauses 2.1, 3.2.1, 3.2.2 and 5.1 )

SL NO.

CATEGORY OFWINDINGS

WITHSTAND VOLTAGES CONSTITUTING INSULATION LEVEL, RELEVANT

CLAUSES AND TABLES

TESTS AND TEST CLAUSES

1. Um < 300 kVuniform insulation

a) Power frequency (5.2 and Table 2)b) Lightning impulse (5.2 and Table 2)

(optional for dry type transformers)c) Lightning impulse for neutral, if specified

(5.5.3)

a) Separate source AC (routine) (10)b) Lightning impulse (type) (12) on line

terminalsc) Modified impulse test on neutral (special)

(12.3.2)d) Induced overvoltage (routine) (11.2)

2. Um < 300 kV non-uniform insulation

a) Power frequency for line terminal (5.3 andTable 2)

b) Lightning impulse for line terminals (5.3 andTable 2)

c) Power frequency for neutral (5.5)

d) Lightning impulse for neutral, if specified(5.5.3)

a) Separate source AC (routine) (10)(corresponding to insulation level of neutral)

b) Lightning impulse on line terminals (type)(12)

c) Modified impulse test on neutral (special)(12.3.2)

d) Induced overvoltage (routine) (11.3)

3. Um ≥ 300 kV non-uniform insulation specified according to Method 1 (5.4.1)

a) Power frequency for line terminals (5.4.1 andTable 3)

b) Lightning impulse for line terminals (5.4.1and Table 3)




b) Lightning impulse on line terminals (routine)(12)


d) Induced overvoltage (routine) (11.3)

4. Um ≥ 300 kV non-uniform insulation specified according to Method 2 (5.4.2)

a) Lightning impulse for line terminals (5.4.2and Table 4)

b) Switching impulse for line terminals (5.4.2and Table 4)




b) Lightning impulse on line terminals (routine)(12)


d) Switching impulse on line terminals(routine) (14)

e) Induced overvoltage (routine) (11.4) (withpartial discharge indication)

IS : 2026 (Part III) - 1981

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3.2 Insulation Level — The rated withstand voltages for the windingwhich constitute its insulation level shall be verified by a set ofdielectric tests, and the set of tests is different depending on the valueof Um ( see 5 ).

3.2.1 Two alternative definitions are used to describe rated insulationlevel:

a) The rated lightning impulse and short duration power frequencywithstand voltages.NOTE — Definition (a) applies for all windings with highest voltage Um lower than300 kV, and for windings with Um equal to or greater than 300 kV that arespecified according to Method 1 ( see 5 and Table 1 ).

b) The rated lightning and switching impulse withstand voltages(phase-to-earth).NOTE — Definition (b) applies for windings with Um equal to or greater than 300kV that are specified according to Method 2 ( see 5 and Table 1).

3.2.2 If there is a winding with non-uniform insulation, the insulationlevel of the neutral terminal shall also be specified by the purchaser( see also 5.5.3 ). If there is a winding with non-uniform insulation andUm ≥ 300 kV, it shall be tested according to Method 1 or Method 2 ( see5, Table 1 ), and in the case of Method 2 further information shall begiven about the choice of certain alternative procedures in the inducedovervoltage withstand test ( see 11.4 ).

3.2.3 The insulation level assigned to each winding of a transformer ispart of the information to be supplied with an enquiry and order.

3.2.4 Abbreviated Notation for Insulation Levels — The ratedwithstand voltages for all windings rated 3.6 kV and above shallappear on the rating plate. The principles of the standard abbreviatednotation are shown by the following examples. The values of ratedlightning impulse (LI) switching impulse (SI) and power frequencywithstand voltage (AC) shall be taken from Tables 2, 3 or 4.

Example 1 : A transformer having windings with Um = 72.5 and 12kV, both uniformly insulated.

Data for different windings are separated by a stroke,and the impulse level is put first.

Insulation Levels : LI 325 AC 140/LI 60 AC 28

IS : 2026 (Part III) - 1981

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Example 2 : A transformer having a non-uniformly insulatedstar-connected high voltage winding with Um = 245 kVand neutral to be non-directly earthed. The nextwinding is also star-connected with uniform insulationand Um = 72.5 kV, and further a tertiary,delta-connected winding with Um = 24 kV.

For a non-uniformly insulated winding, line terminal data aregiven first, and then, after a separating dash, neutral terminal data.Example 3 : An autotransformer with Um = 420 and 145 kV

specified according to Method 2 ( see 5.4 ) and withneutral for direct connection to earth, and a tertiarywith Um = 24 kV.

In this example the specification of Method 2 determines the testingof the 145 kV winding as well, and this means that there is noseparately specified power frequency withstand voltage for the lineterminals of this winding. The induced overvoltage withstand test inaccordance with 11.4 applies to both autoconnected windings.

4. RULES FOR SOME SPECIAL CLASSES OF TRANSFORMERS

4.1 In transformers where uniformly insulated windings havingdifferent Um values are connected together within the transformer(usually autotransformers), the test voltage for separate-sourcepower-frequency withstand test shall be determined by the windingwith the highest Um value.

4.2 For transformers with a high voltage winding having Um ≥ 300 kV,lightning impulse tests are routine tests for all windings.

4.3 In transformers which have one or more non-uniformly insulatedwindings the test voltage for the induced overvoltage withstand test,and for the switching impulse test, if used, are determined by thewinding with the highest Um value, and the windings with lower Umvalues may not receive their appropriate test voltages. Thisdiscrepancy should normally be accepted. If the ratio between the

Insulation Levels : LI 850 AC 360-LI 250 AC 95/ LI 325 AC 140/LI 125 AC 50

Insulation Levels : SI 1050 LI 1300 — AC 38/LI 550 — AC 38/LI 125 AC 50

IS : 2026 (Part III) - 1981

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windings is variably by tappings, this should be used to bring the testvoltage for the winding with lower Um voltage as close as possible tothe appropriate value.

4.4 During switching impulse tests, the voltages developed acrossdifferent windings are approximately proportional to the turns ratios.If rated switching impulse withstand voltages are assigned to severalwindings, the problem shall be solved as specified in 4.3. A tappedwinding of lower Um without assigned switching impulse withstandvoltage shall be connected on its principal tapping during theswitching impulse test.

4.5 Series windings in booster regulating transformers, phase shiftingtransformers, etc, where the rated voltage of the winding is only asmall fraction of the voltage of the system, shall have a value of Umcorresponding to the system voltage.

NOTE — It is often impracticable to test such transformers in formal compliance withthis specification and it should be agreed between manufacturer and the user whichtests have to be omitted or modified.

5. INSULATION REQUIREMENTS AND DIELECTRIC WITHSTAND TESTS

5.1 The requirements and tests for different categories of windingsshall be as given in Table 1.

NOTE — The extension of the lightning impulse test to include impulses chopped onthe tail is sometimes specified, particularly for cases where the transformer is notprotected by surge arresters. This modification is dealt with in 13.

5.2 Insulation requirements and dielectric withstand tests forwindings with Um < 300 kV, uniform insulation.

5.2.1 The rated withstand voltages of the winding shall be as follows:

a) A rated short-duration power-frequency withstand voltage inaccordance with Table 2.

b) A rated lightning impulse withstand voltage for the lineterminals in accordance with Table 2.

c) If specified, a rated impulse withstand voltage for the neutralterminal, with the same peak value as for the line materials.

5.2.1.1 For values of Um lower than 52 kV there are two lists ofalternatives impulse withstand voltages in Table 2.

For Um = 123, 145, 170, and 245 kV there are different alternativesof power frequency and impulse withstand voltages in Table 2.

IS : 2026 (Part III) - 1981

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The choice between List 1 and List 2 for Um < 52 kV and the choicebetween alternative rated withstand voltages for Um = 123, 145, 170and 245 kV depends on the severity of overvoltage conditions to beexpected in the system and on the importance of particularinstallation. Guidance may be obtained from IS : 2165-1977*. Thevalues chosen should be clearly stated in the enquiry.

TABLE 2 RATED WITHSTAND VOLTAGES FOR TRANSFORMER WINDINGS WITH HIGHEST VOLTAGE FOR EQUIPMENT Um < 300 kV

( Clauses 3.1.1, 5.2.1, 5.2.1.1, 5.3.1, 5.5.3.1, 7.1 and 11.2 )

HIGHEST VOLTAGEFOR EQUIPMENT

Um

RATED SHORT DURATION POWER FREQUENCY

WITHSTAND VOLTAGE

RATED LIGHTNINGIMPULSE WITHSTAND

VOLTAGE

(1) (2) (3)

kV rms kV rms kV peak

List 1 List 2

1.13.67.2

31020

—2040

—4060

1217.524

283850

607595

7595

125

36 70 145 1705272.5

95140

250325

123 185230

450550

145 230275

550650

170230275325

550650750

245325360395

750850950

NOTE — The underlined values are preferred in IS : 585-1962 Specification forvoltages and frequency for ac transmission and distribution systems ( revised ).

*Specification for insulation coordination ( second revision ).

IS : 2026 (Part III) - 1981

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5.2.2 The rated withstand voltages shall be verified by the followingdielectric tests:

a) A separate-source power frequency voltage withstand test( see 10 ) (routine test). This test is intended to verify thepower-frequency withstand strength of the winding under test toearth and other windings.

b) An inducedo vervoltage withstand test ( see 11.2 ) (routine test).This test is intended to verify the power frequency withstandstrength along the winding under test, between its phases, and toearth and other windings.

c) A full-wave lightning impulse test for the line terminals ( see 12 )(type test). This test is intended to verify the impulse withstandstrength of each line terminal to earth and other windings, andalong the winding under test.NOTE — This test becomes a routine test when the winding considered forms partof a transformer of which at least one winding has the highest voltage forequipment Um ≥ 300 kV.

d) An impulse test for the neutral terminal ( see 12.3.2 ) (specialtest), if a rated impulse withstand voltage for the neutralterminal has been specified. This test is intended to verify theimpulse withstand strength of the neutral terminal to earth andother windings.NOTE — Distribution transformers for suburban or rural installations are in somecases severely exposed to overvoltages. In such cases, higher test voltages oradditional tests, which are not mentioned here, may be agreed to between themanufacturer and the user.

5.3 Insulation Requirements and Dielectric Withstand Tests for Windings with Um < 300 kV, Non-uniform Insulation

5.3.1 The rated withstand voltages of the winding shall be as follows:

a) A rated short-duration power-frequency withstand voltage for theline terminals in accordance with Table 2,

b) A rated lightning impulse withstand voltage for the lineterminals in accordance with Table 2,

c) A rated short-duration power-frequency withstand voltage for theneutral terminal in accordance with 5.5, and

d) If specified, a rated impulse withstand voltage for the neutralterminal in accordance with 5.5.3.NOTE — Concerning List 1 and List 2 in Table 2, and alternative values forUm ≥ 123 kV in Table 2, see 5.2.

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5.3.2 The rated withstand voltages shall be verified by the followingdielectric tests:

a) An induced overvoltage withstand test ( see 11 ) (routine test).This test is intended to verify the power frequency voltagewithstand strength of the line terminals to earth and otherwindings and the withstand strength between the phases andalong the winding under test. The test is carried at accordingto 11.3.

b) A full-wave lightning impulse test for the line terminals ( see 12 )(type test). The purpose of the test is as specified under 5.2.2 (c).NOTE — This test becomes a routine test when the winding considered forms partof a transformer of which at least one winding has the highest voltage forequipment Um ≥ 300 kV.

c) A separate-source power-frequency voltage withstand test for theneutral terminal ( see 10 ) (routine test). This test is intended toverify the power-frequency voltage withstand strength of theneutral terminal to earth.

d) An impulse test for the neutral terminal ( see 12.3.2 ) (specialtest), if a rated impulse withstand voltage for the neutralterminal has been specified. The purpose of the test is as specifiedunder 5.2.2 (d).

5.4 Insulation Requirements and Dielectric Withstand Testsfor Windings with Um ≥≥≥≥ 300 kV, Non-uniform Insulation —There are two alternative methods, Method 1 ( see 5.4.1 ) and Method2 ( see 5.4.2 ) for the specification and testing of transformers whichhave winding belonging to this category. Which method has beenselected is part of the information to be supplied with an enquiry andwith an order, and if Method 2 has been selected it is also necessary toindicate the choice between alternative procedures in the inducedovervoltage withstand test ( see 11.4 ).Method 1 — For specifying and testing this method makes use of ratedlightning impulse withstand voltage and a rated short-durationpower-frequency withstand voltage. The latter is also intended torepresent a sufficient withstand strength against switching impulsevoltages ( see 5.4.1 ).Method 2 — For specifying and testing this method makes use of arated switching impulse withstand voltage and a rated lightningimpulse withstand voltage. The induced power-frequency overvoltagetest is related only to stresses under normal operating conditions andtemporary overvoltages. The induced voltage test procedure specifieddiffers from that of Method 1 in that the duration is longer, the testvoltage phase-to-earth is lower, and the test criterion is based on themeasurement of partial discharges in the transformer ( see 5.4.2 ).

IS : 2026 (Part III) - 1981

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5.4.1 Method 1

5.4.1.1 The rated withstand voltages of the winding shall be as follows:a) A rated short-duration power-frequency withstand voltage for

line terminals in accordance with Table 3;b) A rated lightning impulse withstand voltage for line terminals, in

accordance with Table 3;c) A rated short-duration power-frequency withstand voltage for

neutral terminal, in accordance with 5.5; andd) If specified, a rated lightning impulse withstand voltage for

neutral terminal, in accordance with 5.5.3.

5.4.1.2 The withstand voltages shall be verified by the followingdielectric tests:

a) An induced overvoltage withstand test ( see 11 ) (routine test).The test is carried out in accordance with 11.3. The purpose ofthis test is as specified under 5.3.2 (a).

b) A full-wave lightning impulse test for the line terminals ( see 12 )(routine test). The purpose of this test is as specifiedunder 5.2.2(c).

TABLE 3 TEST VOLTAGES FOR LINE TERMINALS OF WINDINGS WITHUm ≥≥≥≥ 300 kV, SPECIFIED IN ACCORDANCE WITH METHOD 1

( Clauses 3.1.1, 3.2.4, and 5.4.1.1 )


Um

RATED SHORT DURATION POWER-FREQUENCY

WITHSTAND VOLTAGE

RATED LIGHTNINGIMPULSE WITHSTAND

VOLTAGE

(1) (2) (3)

kV rms kV rms kV peak

300 395460

9501 050

362 460510

1 0501 175

420 570630

1 3001 425

NOTE — The underlined values are preferred value in IS : 585-1962 Specification forvoltages and frequency for ac transmission and distribution systems ( revised ).

NOTE 2 — Guidance for the choice between alternative rated withstand voltagecombinations may be obtained from IS : 2165-1977 Specification for insulationco-ordination ( second revision ).

IS : 2026 (Part III) - 1981

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c) A separate-source power-frequency voltage withstand test for theneutral terminal ( see 10 ) (routine test). The purpose of this testis as specified under 5.3.2(c).

d) An impulse test for the neutral terminal ( see 12.3.2 ) (specialtest) if a rated impulse withstand voltage for the neutral terminalhas been specified. The test is carried out on all units of lot. Thepurpose of the test is as specified under 5.2.2(d).

5.4.2 Method 2

5.4.2.1 The rated withstand voltages of the winding shall be as follows:

a) A rated switching impulse withstand voltage for line terminals,in accordance with Table 4.

b) A rated lightning impulse withstand voltage for line terminals, inaccordance with Table 4.

c) A rated short-duration power-frequency withstand voltage forneutral terminal, in accordance with 5.5.

d) If specified a rated lightning impulse withstand voltage forneutral terminal, in accordance with 5.5.3.

5.4.2.2 The rated withstand voltages shall be verified by the followingdielectric tests:

a) A switching impulse test for the line terminals ( see 14 ) (routinetest). This test is intended to verify the switching impulsewithstand strength of the line terminals to earth, and betweenline terminals on three-phase transformers.

b) A full wave lightning impulse voltage withstand test for lineterminals ( see 12 ) (routine test). The purpose of this test is asspecified under 5.2.2(c).

c) A separate-source power-frequency voltage withstand test for theneutral terminal ( see 10 ) (routine test). The purpose of this testis as specified under 5.3.2(c).

d) A lightning impulse test for the neutral terminal ( see 12.3.2 )(special test) if a rated impulse withstand voltage for the neutralterminal has been specified. The test is carried out on all units ofthe lot. The purpose of the test is as specified under 5.2.2(d).

e) An induced power frequency overvoltage test with partialdischarge measurement in accordance with 11.4 (routine test).There are alternative procedures specified in this clause, and thechoice between these should be decided at the time of the order.

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The test procedure applies to all windings of the transformerhaving non-uniform insulation.

This test shall be carried out after completion of the other dielectrictests. This test is carried out from the point of view of stresses undernormal operating conditions and temporary overvoltages.

5.5 Insulation Requirements and Tests for the Neutral Terminal of a Winding with Non-uniform Insulation

5.5.1 General — The necessary insulation level depends on whetherthe neutral terminal is intended to be directly earthed or not. In thelatter case an overvoltage protective device should be installed on the

TABLE 4 TEST VOLTAGES FOR LINE TERMINALS OF WINDINGS WITHUm ≥≥≥≥ 300 kV SPECIFIED IN ACCORDANCE WITH METHOD 2

( Clauses 3.1.1, 3.2.4 and 5.4.2.1 )


Um

RATED SWITCHING IMPULSE WITHSTAND VOLTAGE (PHASE-TO-NEUTRAL)

RATED LIGHTNING IMPULSE WITHSTAND VOLTAGE

(1) (2) (3)

kV rms kV peak kV peak

300 750850

850 and 950950 and 1 050

362 850950

950 and 1 0501 050 and 1 175

420 9501 050

1 050 and 1 1751 175, 1 300 and 1 425

525 1 0501 175

1 175, 1 300 and 1 4251 425 and 1 550

765 1 4251 550

1 550 and 1 8001 800 and 1 950

NOTE 1 — During the switching impulse withstand test on a three-phase transformerthe line-to-line test voltage shall be approximately 1.5 times the phase-to-neutralvoltage ( see 14.3 ).

NOTE 2 — The underlined value is the preferred value in IS : 585-1962 Specificationfor voltages and frequency for ac transmission and distribution systems ( revised ).

NOTE 3 — Guidance for the choice between alternative rated withstand voltagecombinations may be obtained from IS : 2165-1977 Specification for insulationcoordination ( second revision ).

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neutral terminal in order to limit transient over voltages otherwisenon-uniform insulation of the winding is not recommended.

NOTE — 5.5.2 and 5.5.3 deal with determination of the necessary minimumwithstand voltage for the neutral terminal. An increase of the value may sometimeseasily be arranged and can improve the interchangeability of the transformer in thesystem. It may also be necessary to design the winding with higher neutralinsulation level because of the test connection to be used for the inducedpower-frequency test of the transformer ( see 11.3 ).

5.5.2 Neutral Terminal Intended to be Directly Earthed — This isneutral terminal which is permanently connected to earth directly orthrough a current transformer but without any intentionally addedimpedance in the connection.

5.5.2.1 In this case the short-time power-frequency withstand voltageshall be at least 38 kV.

5.5.2.2 No impulse test on the neutral terminal is recommended.During impulse tests on a line terminal the neutral terminal shall beconnected directly to earth.

5.5.3 Neutral Terminal not Intended to be Directly Earthed —This is neutral terminal which is not permanently in direct connectionto earth. This may be connected to earth through a considerableimpedance (for example, arc-suppression coil earthing). Separatephase-winding neutral terminal may be connected to a regulatingtransformer. The rated voltage of the surge arrester which is to beinstalled for neutral protection shall be selected at least equal to themaximum power-frequency voltage under such conditions of systemfaults as are considered.

5.5.3.1 It is the responsibility of the user to select the overvoltageprotective device, to determine its impulse protection level, and tospecify the corresponding impulse withstand voltage for the neutralterminal of the transformer. A suitable standard value shouldpreferably be selected from Table 2. The corresponding ratedpower-frequency withstand voltage from the table shall also apply. Itshould be checked that the power-frequency withstand voltage isgreater than the above mentioned system-fault voltage.

5.5.3.2 The rated impulse withstand voltage of the neutral terminal isverified by either of the two tests described under 12.3.2. Achopped-wave impulse test on the neutral is not recommended.

6. TESTS ON A TRANSFORMER WITH A TAPPED WINDING

6.1 If the tapping range is ± 5 percent or less, the dielectric test shallbe done with the transformer connected on the principal tapping.

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6.2 If the tapping range is larger than ± 5 percent, the choice oftapping cannot be prescribed universally. Testing conditionsdetermine the choice of tapping required for induced power-frequencytest and for switching impulse test ( see 4 ).6.3 Under lightning impulse test the dielectric stresses are distributeddifferently depending on the tapping connection and general design ofthe transformer. Unless impulse testing on a particular tapping hasbeen agreed, the two extreme tappings and the principal tapping shallbe used, one tapping for each of the three individual phases of athree-phase transformer or the three single-phase transformersdesigned to form a three-phase bank.

7. INSULATION REQUIREMENTS AND TEST CONDITIONS FOR DRY TYPE TRANSFORMERS

7.0 Pending preparation of a separate standard for dry typetransformers the provisions of 7.1 shall apply.7.1 Dry type transformers are not a uniform category with respect toinsulation requirements and tests. The clauses of this standard areapplicable when dry type transformers are intended for general powerdistribution in public or industrial systems. They are then designed inaccordance with 5.2 and Table 2 (List 1 or 2).

However, for application in particular systems where the insulationrequirements are lower than in general, and where this has beenproven by experience, dry type transformers not designed for impulsetype tests and with even lower power frequency test voltage may beapplied. No definite figures are recommended here.

8. REPEATED DIELECTRIC TESTS

8.1 If a transformer has already withstood complete dielectricacceptance tests according to this standard, in accordance with 5.2, 5.3or 5.4.1, and subsequently acceptance tests are to be repeated, the testvoltage levels shall be reduced to 75 percent of the original values,unless otherwise agreed, and provided that the internal insulation hasnot been modified in the meantime.

NOTE — The rule does not apply to the induced power frequency overvoltage test( see 11.4 ) on transformers specified in accordance with 5.4.2.

9. INSULATION OF AUXILIARY WIRING

9.1 Unless otherwise specified, the wiring for auxiliary power andcontrol circuitry shall be subjected to a one-minute power-frequencywithstand test with 2.0 kV rms to earth. Motors and other apparatusfor auxiliary equipment shall fulfil insulation requirements accordingto the relevant Indian Standards (which are generally lower than the

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value specified for the wiring alone and which may sometimes make itnecessary to disconnect them in order to test the circuits).

NOTE — Auxiliary equipment for large transformers is usually dismantled forshipment. After completion of erection on site a 1 000 V megohm meter test isrecommended.

10. SEPARATE-SOURCE VOLTAGE WITHSTAND TEST

10.1 The separate-source voltage test shall be made with single-phasealternating voltage as nearly as possible to the sine-wave form and ofany convenient frequency not less than 80 percent of the ratedfrequency.10.2 The peak value of voltage shall be measured. The peak valuedivided by shall be equal to the test value.10.3 The test shall be commenced at a voltage not greater thanone-third of the specified test value and shall be increased to this valueas rapidly as is consistent with measurements. At the end of the test,the voltage shall be reduced rapidly to less than one-third of the testvalue before switching off.10.4 The full test voltage shall be applied for 60s between the windingunder test and all terminals of the remaining windings, core, frameand tank or casing of the transformer, connected together to earth.10.5 The test shall be successful if no collapse of the test voltageoccurs.

NOTE — On windings with non-uniform insulation the test shall be carried out withonly the test voltage specified for the neutral terminal. The line terminals thereforereceive a modified induced overvoltage test in accordance with 11.3 or 11.4.

11. INDUCED OVERVOLTAGE WITHSTAND TEST

11.1 General — The test shall be carried out in three alternativeways in accordance with 11.2, 11.3, or 11.4 for different categories ofwindings.11.1.1 An alternating voltage shall be applied to the terminals of onewinding of the transformer. The voltage shall be, as nearly as possible,to the sine-wave form and of a frequency suitably increased above therated frequency to avoid excessive excitation current during the test.11.1.2 The peak value of the induced test voltage shall be measured.The peak value divided by shall be equal to the test value.11.1.3 The test shall be commenced at a voltage not greater thanone-third of the test value and shall be increased to the test value asrapidly as is consistent with measurement. At the end of the test, thevoltage shall be reduced rapidly to less than one-third of the test valuebefore switching off.

2

2

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11.1.4 Unless otherwise specified in the subsequent clauses, theduration of the test at full test voltage shall be 60s for any test frequencyup to and including twice the rated frequency. When the test frequencyexceeds twice the rated frequency, the duration of the test shall be

but not less than 15 seconds.11.2 Induced Overvoltage Withstand Test for Transformers withUniformly Insulated High-Voltage Winding — The test voltageacross an untapped winding of the transformer shall be equal to twice therated voltage, but the line-to-line test voltage of any three-phase windingshall not exceed the rated withstand voltage as given in Table 2, col 2.11.2.1 A three-phase winding shall preferably be tested withsymmetrical three-phase voltages induced in the three windingphases. If the winding has a neutral terminal, this may be earthedduring the test.11.2.2 The test shall be successful if no collapse of the test voltage occurs.11.3 Induced Phase-to-Earth Overvoltage Withstand Test forTransformer with Non-uniformly Insulated High-VoltageWindings — Um < 300 kV ( see 5.3 ), or Um ≥ 300 kV, specifiedaccording to Method 1 ( see 5.4.1 ).11.3.1 The line terminals shall receive the test voltage value specifiedin the appropriate table.11.3.2 On single-phase transformers the test is normally carried outwith the neutral terminal earthed. If the ratio between the windings isvariable by tappings, this should be used to satisfy test voltageconditions on different windings simultaneously as far as possible. Inexceptional cases ( see 4 ) the voltage on the neutral terminal may beraised by connection to an auxiliary booster transformer. Anotherwinding of the transformer under test may also be connected in serieswith the high voltage winding.11.3.3 The test sequence for a three-phase transformer consists ofthree single-phase applications of test voltage with different points ofthe winding connected to earth at each time. Recommended testconnections which avoid excessive overvoltage between line terminalsare shown in Fig. 1. There are also other possible methods.

Other separate windings shall generally be earthed at the neutral ifthey are star-connected and at one of the terminals if they aredelta-connected.11.3.4 The voltage per turn during the test reaches different valuesdepending on the test connection. The choice of a suitable testconnection is determined by the characteristics of the transformer andof the test plant.

120 rated frequency×test frequency

------------------------------------------------------------ seconds,

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NOTE — In the case of transformers with complicated windings arrangements it isrecommended that the complete connection of all windings during the test should bereviewed between manufacturer and user at the contract stage so that the testrepresents a realistic service stress combination as far as possible.

11.3.5 The test is successful if no collapse of the test voltage occurs.

FIG. 1 CONNECTIONS FOR SINGLE-PHASE INDUCED OVERVOLTAGEWITHSTAND TESTS ON TRANSFORMERS WITH

NON-UNIFORM INSULATION

NOTE 1 — Connection (a) may be used when the neutral is designed to withstand atleast one-third of the voltage U. Three different generator connections to the lowvoltage winding are shown. Only (a1) is possible, if the transformer has unwoundmagnetic return paths (shell form or five-limb core form).

NOTE 2 — Connection (b) is possible and recommended for three-phase transformershaving unwound magnetic return paths for the flux in the tested limb. If there is adelta-connected winding, it has to be open during the test.

NOTE 3 — Connection (c) shows an auxiliary booster transformer, which gives a biasvoltage Ut at the neutral terminal of an auto-transformer under test. Rated voltagesof the two auto-connected windings are Un1, Un2, and the corresponding testvoltages, U1, U2. This connection may also be used for a three-phase transformerwithout unwound magnetic return paths having the neutral insulation designed forless than one-third of the voltage U.

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11.4 Induced Overvoltage Withstand Test for Transformerswith Non-uniformly Insulated High-Voltage Windings, Um ≥≥≥≥300 kV, Specified According to Method 2 (5.4.2)

11.4.1 The test applies to all non-uniformly insulated windings of thetransformer, regardless of whether they are auto-connected orseparate.

11.4.2 The neutral terminal of the winding under test shall be earthed.For other separate windings, if they are star-connected they shall beearthed at the neutral, and if they are delta-connected they shall beearthed at one of the terminals.

A three-phase transformer shall be tested either phase by phase ina single-phase connection that gives voltages on the line terminals asshown in Fig. 2, or in symmetrical three-phase connection. The choiceshall be agreed between the parties at the time of placing the order.

FIG. 2 CONNECTIONS FOR INDUCED OVERVOLTAGE WITHSTAND TEST ON NON-UNIFORMLY INSULATED HIGH VOLTAGE WINDING

ACCORDING TO METHOD 2

11.4.3 The time sequence for the application of test voltage shall be asshown in Fig. 3. The voltage shall be switched on at a level not higherthan one-third of U2, raised to U2, held there for a duration of 5 min,raised to U1, held there for a duration of 5 seconds, immediatelyreduced again without interruption to U2, held there for a duration of30 min, and reduced to a value below one-third of U2 before switchingoff.

11.4.3.1 The duration of the test shall be independent of the testfrequency.

11.4.4 During the whole application of test voltage partial dischargeshall be monitored as described below. The apparent charge shall notbe higher than a specified value q.

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FIG. 3 TIME SEQUENCE FOR THE APPLICATION OF TEST VOLTAGE

11.4.4.1 The test voltages between line and neutral terminals shall beexpressed in per unit (PU) of Um/ as follows:

U1 shall be . Um/ = UmU2 shall be either 1.5 Um/ with q = 500 pC

or 1.3 Um/ with q = 300 pC

11.4.4.2 The choice shall be as agreed to between the manufacturerand the user at the time of placing the order.

NOTE — The values of q are provisional and subject to review in the light ofexperience.

11.4.4.3 The partial discharges shall be observed and evaluated asfollows. Further information may be obtained from Appendix A, which,in turn, refers to IS : 6209-1971*.

a) Measurements shall be carried out at the line terminals of allnon-uniformly insulated windings, which means that the higherand lower voltage line terminals of an autoconnected pair ofwindings will be used simultaneously.

b) The measuring channel from each terminal used shall becalibrated with repetitive impulses between the terminal andearth, and this calibration is used for the evaluation of readingsduring the test.

c) The apparent charge measured at a specific terminal of thetransformer, using the appropriate calibration as described in (b)shall refer to the highest steady-state repetitive impulses.Occasional higher kicks should be disregarded.

d) Before and after the application of test voltage, the backgroundnoise level shall be recorded on all measuring channels.

*Methods for partial discharge measurements.

3

3 333

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12.3.3 Transferred Surge Method12.3.3.1 When the low voltage winding cannot in service be subjectedto lightning overvoltages from the low voltage system, this windingmay by an agreement between the manufacturer and the user, beimpulse-tested with surges transferred from the high voltage winding.A guidance for this purpose is provided in Appendix B.12.3.3.2 This method is justified when the design is such that animpulse directly applied to the low voltage winding could result inunrealistic stressing of higher voltage windings, particularly whenthere is a large tapping winding physically adjacent to the low voltagewinding.12.3.3.3 In applying the transferred surge method, the tests on the lowvoltage winding are carried out simultaneously with the impulse testson the adjacent higher voltage winding. The line terminals of the lowvoltage winding are connected to earth through resistances of suchvalue that the amplitude of transferred impulse voltage between lineterminal and earth or between different line terminals or across aphase winding is as high as possible but not exceeding the ratedimpulse withstand voltage. The resistance shall not exceed 5 000ohms. The wave at the low voltage winding terminals may have anyshape and shall be acceptable.12.3.3.4 The details of the procedure shall be agreed before the test.12.4 Records of Test

12.4.1 The oscillographic recordes obtained during calibrations andtests shall clearly show the applied voltage impulse shape (front time,time to half value).12.4.2 At least one more measurement channel shall be used. In mostcases an oscillogram of the current flowing to earth from the testedwinding will present the best sensitivity for fault indication. Thecurrent flowing from tank to earth, or the transferred voltage in anon-tested winding are examples of alternative suitable measuringquantities.12.5 Test Criteria — The absence of significant differences betweenvoltage and current transients recorded at reduced voltage and thoserecorded at full test voltage constitute evidence that the insulation haswithstood the test.

NOTE 1 — The detailed interpretation of the oscillographic test records anddiscrimination of marginal disturbances from true records of failure require a greatdeal of skill and experience.NOTE 2 — If there is doubt about the interpretation of possible discrepanciesbetween oscillograms, three subsequent impulses at full voltage shall be applied, orthe whole impulse test on the terminal shall be repeated.

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NOTE 3 — Additional observations during the test (sound effect, etc) may be used toconfirm the oscillographic records, but they do not constitute evidence in themselves.

13. TEST WITH LIGHTNING IMPULSE, CHOPPED ON THE TAIL

13.1 General — This test is a special test to be carried out on lineterminals of a winding. When it has been agreed to carry out this testit shall be combined with the full lightning impulse test in the mannerdescribed below test as per clause 12. The peak value of the choppedimpulse shall be the same as for the full impulse.

Usually, the same settings of the impulse generator and measuringequipment are used, and only the chopping gap equipment is added.The standard lightning impulse shall have a time to chopping between2 to 6 microseconds.

13.2 Chopping Gap and Characteristics of the Chopping — Theuse of a triggered-type chopping gap with adjustable timing isrecommended although a plain rod-rod gap is allowed. The choppingcircuit shall be so arranged that the amount of overswing to oppositepolarity of the recorded impulse will be limited to not more than 30percent of the amplitude of the chopped impulse.

13.3 Test Sequence and Test Criteria — As indicated under 13.1,this test is combined with full impulse test in a single sequence. Therecommended order of the different pulse applications is:

a) one reduced full impulse,

b) one 100 percent full impulse,

c) one or more reduced chopped impulses,

d) two 100 percent chopped impulses, and

e) two 100 percent full impulses.

13.3.1 The same types of measuring channels and oscillograms as forthe full impulse test ( see 12 ) shall be used.

13.3.2 In principle, the detection of faults during a chopped impulsetest depends essentially on a comparison of the oscillographic recordsof 100 percent and reduced chopped impulses. The neutral currentrecord (or any other supplementary recording) presents asuperposition of transient phenomena due to the front of the originalimpulse and from the chopping. Account should therefore be taken ofthe possible variations, even slight, of the chopping time delay. Thelatter part of the oscillation pattern is then modified, and this effect isdifficult to separate from the record of a fault.

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13.3.3 The recordings of successive 100 percent full impulse testsconstitute a supplementary criterion of a fault, but they do notconstitute in themselves a quality criterion for the chopped impulsetest.

14. SWITCHING IMPULSE TEST

14.1 General

14.1.1 Measuring equipment and calibration methods shall be inaccordance with IS : 2071 (Part I)-1974*. The test is a routine test forwindings with Um ≥ 300 kV specified according to Method 2( see 5.4.2 ).

14.1.2 The impulses shall be applied either directly from the impulsevoltage source to a line terminal of the winding under test, or to alower voltage winding so that the test voltage is inductivelytransferred to the winding under test. The specified test voltage shallappear between line and neutral terminals and the neutral shall beearthed. In a three-phase transformer the voltage developed betweenline terminals during the test shall be approximately 1.5 times thevoltage between line and neutral terminals ( see 14.3 ).

14.1.3 The test voltage should normally be of negative polarity becausethis reduces the risk of erratic external flashover in the test circuit.

14.1.4 The voltage developed across different windings of thetransformer are approximately proportional to their effective numbersof turns, and the test voltage shall be determined by the winding withthe highest Um value ( see 4 ).

14.1.5 The voltage impulse shall have a virtual front time of at least 20microseconds, a time above 90 percent of the specified amplitude of atleast 200 microseconds, and a total duration from the virtual origin tothe first zero passage of at least 500 microseconds.

NOTE 1 — The impulse form is purposely different from the standard waveshape of250/2 500 microseconds.

NOTE 2 — The front time shall be selected by the manufacturer so that the voltagedistribution along the winding under test will be essentially uniform. Its value isusually less than 250 microseconds. During the test considerable flux is developed inthe magnetic circuit. The impulse voltage can be sustained up to the instant whenthe core reaches saturation and the magnetizing impedance of the possible impulseduration can be increased by introducing remanence of opposite polarity before eachfull voltage test impulse. This is accomplished by lower voltage impulses of similarshape but opposite polarity or by temporary connection to a dc voltage source.

*Methods of high voltage testing: Part I General definitions and test requirements( first revision ).

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14.2 Test Sequence and Records — The test sequence shall consistof one impulse (calibration impulse) of a voltage between 50 percentand 75 percent of the full test voltage and three subsequent impulsesat full voltage. If the oscillographic recording should fail, thatapplication shall be disregarded and a further application made.Oscillographic records shall be obtained of at least the impulsewave-shape on the line terminal under test.

14.3 Terminal Connections

14.3.1 During the test the transformer shall be in a no-load conditionin order to present sufficient impedance. Windings not used for thetest shall be suitably earthed at one point but not short-circuited. For asingle-phase transformer the neutral of the tested winding shall beearthed.

14.3.2 A three-phase winding shall be tested phase by phase with theneutral terminal earthed and with the transformer so connected that avoltage of opposite polarity and about half amplitude appears on thetwo remaining line terminals ( see Fig. 2 ).

14.3.3 Bushing spark gaps and additional means for limitation ofovervoltages shall be as specified for the lightning impulse test( see 12.1 ).

14.4 Test Criteria — The test is successful if there is no suddencollapse of voltage indicated on the oscillograms.

NOTE — However, the successive oscillograms may be different because of theinfluence of magnetic saturation on impulse duration.

14.4.1 Additional observations during the test (abnormal soundeffects, etc) may be used to confirm the oscillographic records, but theydo not constitute evidence in themselves.

15. INFORMATION REQUIRED WITH ENQUIRY AND ORDER

15.1 The technical information on insulation and dielectric tests to besupplied with the enquiry and order is given in Appendix C.

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A P P E N D I X A

( Clauses 2.5, 11.4.4.3 and 11.4.4.4 )

APPLICATION GUIDE FOR PARTIAL DISCHARGE MEASUREMENTS DURING INDUCED OVERVOLTAGE

WITHSTAND TEST ON TRANSFORMERS

A-1. INTRODUCTION

A-1.1 A partial discharge (PD) is an electric discharge that onlypartially bridges the insulation between conductors.

In a transformer such a partial discharge causes a transient changeof voltage to earth, at every externally available winding terminal.

A-1.2 Measuring impedances are connected effectively between theearthed tank and the terminals, usually through a bushing tap orthrough a separate coupling capacitor as described under A-2.

A-1.2.1 The actual charge transferred at the site of a partial dischargecannot be measured directly. The preferred measure of the intensity ofa partial discharge is the apparent charge, q, as defined in 23.1 of IS :6209-1971*.

A-1.2.2 The apparent charge, q, related to any measuring terminal isdetermined by a suitable calibration ( see A-2 ).

A-1.2.3 A particular partial discharge gives rise to different values ofapparent charge at different terminals of the transformer. Thecomparison of simultaneously collected indications at differentterminals may give information about the location of the partialdischarge source within the transformer ( see A-5 ).

A-1.2.4 The acceptance test procedure specified in the standard callsfor measurement of apparent charge at the winding line terminals.

This is considered to give sufficiently good sensitivity to arbitrarydischarge sources irrespective of location, provided that therecommendations below are observed. The specified, tentativeacceptance values of apparent charge are based on practical experiencefrom partial discharge measurements on transformers which have inaddition passed traditional ac dielectric tests.


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A-2. CONNECTION OF MEASURING AND CALIBRATION CIRCUITS — CALIBRATION PROCEDURE

A-2.1 The measuring equipment is connected to the terminals bymatched coaxial cables. The measuring impedance in its simplest formis the matching impedance of the cable which may, in turn, be theinput impedance of the measuring instrument.

A-2.2 In order to improve the signal-to-noise ratio of the completemeasuring system, it may be convenient to make use of tuned circuits,pulse transformers, and amplifiers between the test object terminalsand the cable. The circuit shall represent a reasonably constantresistance, when viewed from the test object terminals, throughout thefrequency range used for the partial discharge measurement.

A-2.3 During the measurement of partial discharge between a lineterminal of a winding and the earthed tank, the preferred arrangementis to instal the measuring impedance effectively between the condenserbushing capacitance tap and the earthed flange (Fig. 4). If acapacitance tap is not provided it is also possible to insulate thebushing flange from the tank and use it as the measuring terminal. The

FIG. 4 CIRCUIT FOR PARTIAL DISCHARGE MEASUREMENT WHEN CONDENSER BUSHING CAPACITANCE TAP IS AVAILABLE

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use it as the measuring terminal. The equivalent capacitancesbetween the central conductor, the measuring terminal and earthwork as an attenuator for the partial discharge signal. This is,however, covered by the calibration which takes place between the topterminal of the bushing and earth.

A-2.4 If measurements have to be taken at a live terminal without anyavailable condenser bushing tap (or insulated flange), the method witha high-voltage coupling capacitor shall be used. A partialdischarge-free capacitor shall be used, and its capacitance value shallbe suitably large in comparison with the calibration generatorcapacitance Co. The measuring impedance (with a protective gap) shallbe connected between the low-tension terminal of the capacitor andearth ( see Fig. 5 ).

FIG. 5 CIRCUIT FOR PARTIAL DISCHARGE MEASUREMENT USING A HIGH-VOLTAGE COUPLING CAPACITOR

A-2.5 The calibration of the complete measuring system is made byinjecting known charges between the calibration terminals. Acalibration generator in accordance with IS : 6209-1971* consists of astep voltage pulse generator with short rise time and a small seriescapacitor of known capacitance Co. The rise time should be not morethan 0.1 microsecond and Co should be around 50 pF. When this


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generator is connected between two calibration terminals presenting acapacitance much greater than Co, the injected charge from the pulsegenerator will be

qo = U. Cowhere U is the voltage step (usually 2 to 50 V).

A-2.5.1 It is convenient if the calibration generator has a repetitionfrequency of the order of one impulse per half cycle of the powerfrequency used for the test on the transformer.A-2.5.2 If the calibration terminals are spaced far apart, there is a riskthat stray capacitances from connecting leads may cause errors. Onemethod for calibration between earth and another terminal is shown inFig. 4. Capacitor Co shall then be placed at the high-voltage terminaland a coaxial cable with a matching resistor shall be used from thestep voltage generator.A-2.5.3 If neither of the calibration terminals is earthed, capacitancefrom the pulse generator itself will also be a source of error. Thegenerator shall preferably be battery-operated and have small physicaldimensions.A-3. INSTRUMENTS AND FREQUENCY RANGEA-3.1 The instruments used shall have response characteristics inconformance with IS : 6209-1971*.A-3.2 Oscillographic monitoring of the test is generally useful,particularly because it offers possibility of discriminating between truepartial discharge in the transformer and certain forms of externaldisturbances. This is based on rate of repetition, point on the wave,polarity differences, etc.A-3.3 The indications shall be observed continuously or at frequentintervals throughout the test period. Continuous recording byoscillograph or tape recorder is not obligatory.A-3.4 Measuring systems for partial discharges are classified asnarrow-band or wide-band systems.A-3.4.1 A narrow-band system operates with a band width of about 10kHz or less at a certain tuning frequency (for example radio noisemeters). A wide-band system utilizes a relatively large ratio betweenupper and lower limit of the frequency band, for example, 150 to 50kHz or even 400 to 50 kHz.A-3.4.2 By using a narrow-band system, interference from localbroadcasting stations may be avoided by suitably adjusting themid-band frequency, but a check has to be made to show that winding


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resonances near the measuring frequency do not greatly affect themeasurement. The narrowrband instrument should be operated at afrequency not higher than 500 kHz, and preferably less than 300 kHz.There are two reasons for this — first the transmission of thedischarge pulse entails a high attenuation of the higher frequencycomponents and second when applying a calibration pulse to the lineterminal, the pulse is likely to excite local oscillations at and near theterminal, and this will complicate the calibration when mid-bandfrequencies greater than 500 kHz are used.A-3.4.3 A wide-band measuring system is less critical to attenuationand response to different pulse shapes and more receptive todisturbances in test locations without electromagnetic shielding.Band-stop filters may be used against radio transmitters.Identification of partial discharge sources by comparison of shape andpolarity of individual pulses may be possible.

A-4. TEST CRITERIA-PROCEDURE AFTER AN UNSUCCESSFUL TESTA-4.1 At the end of 11.4 acceptance criteria are given. The steady statepartial discharge level, expressed as apparent charge measuredbetween the prescribed measuring terminals shall not be above thespecified limit, and there shall not be a significant, rising trend in thevicinity of this limit.

If there has been no voltage collapse; but the test has beenunsuccessful because of too high but still moderate partial dischargereading (within a few thousand PC or less) then the test is regarded asnon-destructive. The test object shall not be rejected immediately uponsuch a result but further investigations shall be undertaken. Thetesting environment should first be investigated to find any obvioussign of irrelevant sources of partial discharges. This should be followedby consultations between the manufacturer and purchaser to agree tofurther supplementary tests or other action to show either thepresence of serious partial discharge, or that the transformer issatisfactory for service operation.

Below are some suggestions which may be useful during the abovecourses of action:

a) Investigation whether the indications are truly correlated to thetest sequence or just represent coincident, irrelevant sources.This is often facilitated by oscillographic monitoring of thetest-disturbances may, for example, be identified by their beingasynchronous with the test voltage.

b) Investigation whether the partial discharge may be transmittedfrom the supply source. Low-pass filters on the supply leads tothe transformer under test can help in such cases.

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c) Investigation to determine whether the partial discharge sourceis within the transformer or outside (spitting from objects atfloating potential in the hall, from live parts in air, or from sharpedges on earthed parts of the transformer). As the test concernsthe internal insulation, provisional electrostatic shielding on theoutside is permitted and recommended.

d) Investigation of the probable location of the source (or sources) interms of the electrical circuit diagram of the transformer. Thereare several known and published methods. One is based oncorrelation of readings and calibrations at different pairs ofterminals (in addition to the obligatory readings between liveterminal and earth). It is described in A-5. It is also possible toidentify individual pulse shapes during the test withcorresponding calibration wave forms, if records from wide-bandcircuits are used. A particular case is the identification of partialdischarge in the dielectric of the condenser bushings ( see A-5 ).

e) Investigation by acoustic or ultrasonic detection of thegeographical location of the source or sources within the tank.

f ) Determination of the probable physical nature of the source byconclusions drawn from variation with test voltage level,hysteresis effect, pulse pattern along the test voltage wave, etc.

g) Partial discharge in the insulation system may be caused byinsufficient drying or oil impregnation. Re-processing of thetransformer, or a period of rest, and subsequent repetition of thetest may, therefore, be tried. It is also well known that a limitedexposure to a relatively high partial discharge may lead to localcracking of oil and temporarily reduced extinction andreinception voltages, but that the original conditions may beself-restored in a matter of hours.

h) If the partial discharge indications are above the acceptance limitbut are not considered as very important, it may be agreed torepeat the test, possibly with extended duration, and even withincreased voltage level. Relatively limited variation of the partialdischarge level with voltage increase, and absence of increasewith time may be accepted as evidence that the transformer issuitable for service.

j ) Traces of partial discharges, visible after untanking are usuallynot found unless the transformer has been exposed for aconsiderable duration of time to levels which are very high incomparison with the acceptance limit. Such a procedure may bethe last resort if other means of improving the behaviour of thetransformer or identifying the source have failed.

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A-5. LOCATION OF PARTIAL DISCHARGE SOURCES BY MEANS OF ‘MULTI-TERMINAL MEASUREMENT’ AND ‘PROFILE COMPARISON’

A-5.1 An arbitrary partial discharge source will deliver signals at allaccessible measuring terminal pairs of the transformer, and thepattern of these signals is a unique ‘fingerprint’. If calibration pulsesare fed in at alternative calibration terminal pairs, these pulses alsodeliver characteristic combinations of signals at the measuring pairs.

If there is an evident correlation between the profile of the testreadings at different measuring terminal pairs and the profileobtained at the same measuring terminals for pulses fed in atparticular pair of calibration terminals, then it is assumed that theactual partial discharge source is closely associated with thiscalibration pair.

This means that it is possible to draw a conclusion as to the locationof the partial discharge source in terms of the electric circuit diagramof the transformer. The ‘physical location’ is different concept — apartial discharge source which is ‘electrically’ located in the vicinity ofa particular terminal may be physically located at any place along theterminal conductors associated with this terminal or at thecorresponding end of the winding structure.

A-5.2 The procedure for obtaining the profile comparison is as follows:

While the calibration generator is connected to a specific pair of acalibration terminals, the indications at all pairs of measuringterminals are observed. The procedure is then repeated for other pairsof calibration terminals. Calibrations are made between windingterminals and earth, but may also be applied between the liveterminals of the high voltage bushings and their capacitance taps(simulating partial discharge in the bushing dielectric) between highvoltage and neutral terminals, and between high-voltage andlow-voltage winding terminals.

All combinations of calibration and measuring pairs form a‘calibration matrix’ which gives the interpretation reference for thereadings in the actual test.

The example, Fig. 6, shows an extra high-voltage single-phaseauto-connected transformer with a low voltage tertiary winding.Calibrations and tests are made with reference to the terminals asindicated in the informal table in Fig. 6. The line with results at 1.5Um is compared with different calibrations and it is easy to see, in this

IS : 2026 (Part III) - 1981

37

case, that it corresponds best to calibration ‘2.1 earth’. This suggeststhat there are partial discharges with apparent charge of the order of1 500 pC, associated with terminal 2.1, and probably from live parts toearth. The physical location may be at any place along the connectingleads between the series winding and the common winding, or at theadjacent winding ends.

The method as described is successful mainly in those cases whereone distinct source of partial discharge is dominant, and thebackground noise is low. This is certainly not always the case.

Channel

1.1 2.1 2.2 3.1arbitrary units

Calibration1.1-earth 2 000 pC2.1-earth 2 000 pC2.2-earth 2 000 pC3.1-earth 2 000 pC

50523

205010

2

530

35035

1084

25Test

U = 0U = UmU = 1.5 Um

< 0.5< 0.5

6

< 0.5< 0.540

< 0.50.5

25

< 0.50.58

FIG. 6 IDENTIFICATION OF PD SOURCES

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A particular case of interest is to determine whether observedpartial discharges may originate in the high-voltage bushing dielectric.This is investigated by a calibration between bushing line terminaland bushing capacitance tap. This calibration gives the closestcorrelation to the profile of partial discharges in the bushing.

A P P E N D I X B( Clause 12.3.3.1 )

OVERVOLTAGE TRANSFERRED FROM THE HIGH-VOLTAGE WINDING TO A LOW-VOLTAGE WINDING

B-1. GENERAL

B-1.1 The problem of transferred overvoltage is treated from a systemviewpoint in IS : 2165-1977*. The information given below concernsonly problems associated with the transformer itself under particularconditions of service. The transferred overvoltages to be considered areeither transient surges or power frequency overvoltages.

B-2. TRANSFER OF SURGE VOLTAGE

B-2.1 General — A study of a particular transformer installation withregard to transferred surge overvoltages is in general justified only forlarge generator transformers — which have a high turns ratio and forlarge high-voltage system transformers with a low voltage tertiarywinding.

It is convenient to distinguish between two mechanisms of surgetransfer, namely, ‘capacitive transfer’ and ‘inductive transfer’.

B-2.2 Capacitive Transfer

B-2.2.1 The capacitive transfer of overvoltage to a low voltage windingmay in the first approximation be described as a capacitive voltagedivision. The simplest equivalent circuit as seen from the low voltagewinding consists of an emf source in series with a transfer capacitanceCt ( see Fig. 7 ).

B-2.2.2 The equivalent emf is a fraction of the incoming surge on thehigh voltage side. Ct is of the order of 10–9 F. Cs and Ct are notwell-defined quantities but dependent on the shape of the surge front.They can be determined together by oscillographic measurements.Pre-calculation is uncertain.

*Specification for insulation coordination ( second revision ).

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FIG. 7 EQUIVALENT CIRCUIT FOR CAPACITIVE TRANSFER OF OVERVOLTAGE

B-2.2.3 A loading of the secondary terminals with switchgear, shortcables or added capacitors (a few nF), which act as lumped capacitanceCs directly on the terminals (even during the first microsecond), willreduce the transferred overvoltage peak. Longer cables or busbars arerepresented by their wave impedance. The resulting shape ofsecondary overvoltage will normally have the character of a short(microsecond) peak, corresponding to the front of the incoming surge.

B-2.3 Inductive Transfer — The inductive transfer of surge voltagedepends on the flow of surge current in the high-voltage winding. If noexternal loading is applied to the secondary winding, the voltagetransient usually has a super-imposed damped oscillation with afrequency determined by leakage inductance and windingcapacitances. A reduction of the inductively transferred overvoltagecomponent can be effected either by resistive damping through a surgediverter or by modification of the oscillation with capacitive loading. Ifcapacitors are used, the capacitance value has usually to be of theorder of tenths of microfarads. (They will therefore automaticallyeliminate the capacitively transferred component so long as the circuitinductance is low.)

The transformer parameters which are involved in inductive surgetransfer are better defined and less dependent on rate of rise (orfrequency).

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40

B-3. POWER FREQUENCY TRANSFERRED OVERVOLTAGE

B-3.1 If a low voltage winding, which is physically adjacent to the highvoltage winding, is left without connection to earth or with only ahigh-impedance connection to earth while the high voltage winding isenergized, there is a risk of power frequency overvoltage bycapacitance division.

B-3.2 The risk is obvious for a single-phase winding, but it can alsoexist for a three-phase winding if the primary winding voltage becomesasymmetric, as occurs during earth faults. Under particularcircumstances resonance conditions may arise.

B-3.3 Tertiary winding and stabilizing windings in large transformersare also subjected to the same risk. It is the responsibility of the userto prevent a tertiary winding being accidentally left with too high animpedance to earth. A stabilizing winding should normally bearranged for permanent connection to earth (tank) either externally orinternally.

The overvoltage is determined by capacitances between windingsand between windings and earth. These can be measured at lowfrequency from the terminals of the transformer in differentcombinations, and they can also be calculated with sufficient accuracy.

A P P E N D I X C

( Clause 15.1 )

INFORMATION ON TRANSFORMER INSULATION AND DIELECTRIC TESTS TO BE SUPPLIED WITH AN ENQUIRY

AND WITH A TENDER

C-1. For all windings:

a) Value of Um.

b) Rated withstand voltages constituting the insulation level for lineterminals.

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c) Whether the winding is to have uniform or non-uniforminsulation and in the case of non-uniform insulation, the powerfrequency withstand voltage of the neutral.

d) Whether a rated impulse withstand level is assigned to theneutral, and, in such case, the appropriate withstand voltage.

e) Whether the lightning impulse test on the line terminals shall beextended to include a chopped impulse test.

C-2. For Transformers Having a High-Voltage Winding with Um ≥ 300kV:

a) Whether the transformer shall be specified and tested accordingto Method I or Method 2 ( see 5.4 ).

b) If the transformer shall be specified according to Method 2, achoice shall be made between alternative procedures for theinduced overvoltage withstand test in accordance with 11.4.

C-3. It is further recommended that test connections and proceduresshould be discussed before the time of testing particularly with regardto the connection for induced overvoltage test on complicatedtransformers with non-uniformly insulated high-voltage winding( see 11.3.4, Note ) and the method to be used for impulse tests onhigh-power low-voltage windings and neutral terminals ( see 12.3 ).

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( Continued from page 2 )

Members Representing

SHRI S. D. CHOTRANEYSHRI Y. K. PALVANKAR ( Alternate )

Bombay Electric Supply and TransportUndertaking, Bombay

DIRECTOR (SUBSTATIONS) Central Electricity Authority, New DelhiSHRI P. K. DWIVEDI National Thermal Power Corporation Ltd,

New DelhiSHRI A. K. GUPTA

SHRI MOINUDDIN ( Alternate )UP State Electricity Board, Lucknow

SHRI INDERJIT SINGH KALRA Bhakra Beas Management Board,Chandigarh

SHRI S. V. MANERIKAR Crompton Greaves Ltd, BombaySHRI I. S. PATEL

SHRI M. S. DHARWADKAR ( Alternate )Hindustan Brown Boveri Ltd, Howrah

SHRI P. K. PHILIP Transformers & Electricals Kerala Ltd,Ernakulam

SHRI V. N. PRAHLADSHRI J. R. MAHAJAN ( Alternate )

Voltas Ltd (Motor and Transformer Plant),Bombay

SHRI S. G. RAMACHANDRA Kirloskar Electric Co Ltd, BangaloreSHRI N. J. RONGHE

SHRI A. J. KHAN ( Alternate )Maharashtra State Electricity Board, Bombay

SHRI A. M. SAHNISHRI R. CHANDRAMOULI ( Alternate )

Tata-Hydro Electric Power Supply Co Ltd,Bombay

SHRI P. K. SAXENA Rural Electrification Corporation Ltd, NewDelhi

SHRI K. G. SHANMUKHAPPASHRI K. V. JAYADEV ( Alternate )

NGEF Ltd, Bangalore

SHRI R. SRINIVASAN Research, Designs and StandardsOrganization, Lucknow

SHRI B. A. SUBRAMANYAM The General Electric Co of India Ltd,Allahabad

Bureau of Indian StandardsBIS is a statutory institution established under the Bureau of Indian Standards Act, 1986 to promoteharmonious development of the activities of standardization, marking and quality certification ofgoods and attending to connected matters in the country.

CopyrightBIS has the copyright of all its publications. No part of these publications may be reproduced in anyform without the prior permission in writing of BIS. This does not preclude the free use, in the courseof implementing the standard, of necessary details, such as symbols and sizes, type or gradedesignations. Enquiries relating to copyright be addressed to the Director (Publications), BIS.

Review of Indian StandardsAmendments are issued to standards as the need arises on the basis of comments. Standards are alsoreviewed periodically; a standard along with amendments is reaffirmed when such review indicatesthat no changes are needed; if the review indicates that changes are needed, it is taken up forrevision. Users of Indian Standards should ascertain that they are in possession of the latestamendments or edition by referring to the latest issue of ‘BIS Catalogue’ and ‘Standards : MonthlyAdditions’.This Indian Standard has been developed by Technical Committee : ETDC 16

Amendments Issued Since Publication

Amend No. Date of IssueAmd. No. 1 March 1994

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Telegrams: Manaksanstha(Common to all offices)

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