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BIS 2003

B U R E A U O F I N D I A N S T A N D A R D S

MANAKBHAVAN, 9BAHADURSHAHZAFARMARGNEWDELHI110002

IS : 2026 (Part II) 1977

(Reaffirmed 2001)

Edition 2.2

(1984-10)

Price Group 5

Indian StandardSPECIFICATION FOR

POWER TRANSFORMERS

PART II TEMPERATURE-RISE

( First Revision )

(Incorporating Amendment Nos. 1 & 2)

UDC 621.314.222.6.017.71

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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 the

publisher shall be deemed to be an infringement of copyright under the said Act.

Indian Standard

SPECIFICATION FORPOWER TRANSFORMERS

PART II TEMPERATURE-RISE

( First Revision )

Transformers Sectional Committee, ETDC 16

Chairman

SHRIU. K. PATWARDHAN

Prayog Electricals Pvt Ltd, Bombay

Members Representing SHRIS. AMMEERJAN Bharat Heavy Electricals Ltd (R&D Unit)

SHRIN. S. S. AROKIASWAMY Tamil Nadu Electricity Board, Madras

SHRIM. K. SUNDARARAJAN(Alternate )

SHRIB. G. BHAKEY Kirloskar Electric Co Ltd, Bangalore

DRB. N. JAYARAM(Alternate )

SHRIA. V. BHEEMARAU Gujarat Electricity Board, Vadodara

SHRIJ. S. IYER(Alternate )

SHRIS. D. CHOTRANEY Bombay Electric Supply and Transport Undertaking,Bombay

SHRIY. K. PALVANKAR(Alternate )

DIRECTOR(TRANSMISSION) Central Electric Authority, New DelhiDEPUTYDIRECTOR

(TRANSMISSION) (Alternate )

SHRIT. K. GHOSE Calcutta Electric Supply Corporation Ltd, Calcutta

SHRIP. K. BHATTACHARJEE(Alternate )

JOINTDIRECTOR(SUB-STATION) Research, Designs and Standards Organization,Lucknow

DEPUTYDIRECTORSTANDARDS(ELECTRICAL) (Alternate )

SHRIJ. K. KHANNA Directorate General of Supplies and Disposals(Inspection Wing), New Delhi

SHRIK. L. GARG(Alternate )SHRIB. S. KOCHAR Rural Electrification Corporation Ltd, New Delhi

SHRIR. D. JAIN(Alternate )

SHRIJ. R. MAHAJAN Indian Electrical Manufacturers Association, Bombay

SHRIP. K. PHILIP(Alternate )

SHRID. B. MEHTA Tata Hydro-Electric Power Supply Co Ltd, Bombay

SHRIR. CHANDRAMOULI(Alternate )

( Continued on page2 )

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( Continued from page1 )

Members Representing

SHRID. V. NARKE Bharat Heavy Electricals Ltd

SHRIISHWARCHANDRA(Alternate I )SHRIPREMCHAND(Alternate II )

SHRII. S. PATEL Hindustan Brown Boveri Ltd, Bombay

SHRIV. N. PRAHLAD National Electrical Industries Ltd, Bhopal

SHRIA. G. GURJAR(Alternate )

SHRIK. N. RAMASWAMY Directorate General of Technical Development,New Delhi

SHRIS. K. PALHAN(Alternate )

SHRICHANDRAK. ROHATGI Pradip Lamp Works, Patna

SHRID. P. SAHGAL Siemens India Ltd, Bombay

SHRIA. R. SALVI(Alternate )

SHRII. C. SANGAR Delhi Electric Supply Undertaking, New Delhi

SHRIR. C. KHANNA(Alternate )

SHRIK. G. SHANMUKHAPPA NGEF Ltd, Bangalore

SHRIP. S. RAMAN(Alternate )

SHRIM. A. SHARIFF Karnataka Electricity Board, Bangalore

SHRIB. C. ALVA(Alternate )

SUPERINTENDINGENGINEER(OPERATION)

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

SUPERINTENDINGENGINEERTECHNICAL(PROJECTS) (Alternate )

SHRIC. R. VARIER Crompton Greaves Ltd, BombaySHRIS. V. MANERIKAR(Alternate )

SHRIS. P. SACHDEV, Director General, ISI ( Ex-officio Member )

Director (Elec tech)

Secretary

SHRIVIJAIDrputy Director (Elec tech), ISI

Panel for Revision of IS : 2026 Specification for Power Transformers,ETDC 16 : P6

Members

SHRIS. V. MANERIKAR Crompton Greaves Ltd, Bombay

SHRID. V. NARKE Bharat Heavy Electricals Ltd

SHRIISHWARCHANDRA(Alternate I )

SHRIPREMCHAND(Alternate II )

SHRIS. SRINIVASAN(Alternate III )

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Indian Standard

SPECIFICATION FOR

POWER TRANSFORMERSPART II TEMPERATURE-RISE

( First Revision )

0. F O R E W O R D

0.1 This Indian Standard (Part II) was adopted by the IndianStandards Institution on 24 February 1977, after the draft finalized bythe Transformers Sectional Committee had been approved by the

Electrotechnical Division Council.0.2 The first revision of IS : 2026-1962* has been undertaken with aview to bring it in line with the revision of IEC Pub 76 (1967) Powertransformers.

0.3 In this revision the requirements for power transformers arecovered in four parts as follows :

Part I General

Part II Temperature-rise

Part III Insulation levels and dielectric tests

Part IV Terminal markings, tappings and connections0.4 This standard (Part II) has been based on IEC Pub 76-2 (1976)Power transformers, Part 2 Temperature-rise, issued by theInternational Electrotechnical Commission.

0.5 This part shall be read in conjunction with IS : 2026 (Part I)-1977,IS : 2026 (Part III)-1977 and IS : 2026 (Part IV)-I977.

0.6 This edition 2.2 incorporates Amendment No. 2 (October 1984).Side bar indicates modification of the text as the result of incorporationof the amendment. Amendment No. 1 had been incorporated earlier.

0.7 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.

*Specification for power transformers.Specification for power transformers: Part I GeneralSpecification for power transformers: Part III Insulation levels and dielectric tests.Specification for power transformers: Part IV Terminal markings, tappings and

connections.||Rules for rounding off numerical values ( revised ).

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1. SCOPE

1.1 This standard (Part II) covers temperature-rise requirements ofpower transformers.

2. IDENTIFICATION ACCORDING TO COOLING METHOD

2.1 Identification Symbols Transformers shall be identifiedaccording to the cooling method employed. Letter symbols for use inconnection with each cooling method shall be as given in Table 1.

NOTE In transformers with forced directed oil circulation a certain proportion of theforced oil flow is channelled so as to pass through the windings. Certain windings,however, may have a non-directed oil flow, for instance, separate tapping windings,auxiliary windings and stabilizing windings.

2.2 Arrangement of Symbols Transformers shall be identified byfour symbols for each cooling method for which a rating is assigned bythe manufacturer.

2.2.1 Dry-type transformers without protective enclosures areidentified by two symbols only for the cooling medium that is in contactwith the windings of the surface coating of windings with an overall

coating (for example, epoxy resin).2.2.2 The order in which the symbols are used shall be as given inTable 2. Oblique strokes shall be used to separate the group symbolsfor different cooling methods.

2.2.3 For example, an oil-immersed transformer with forced directedoil circulation and forced air circulation shall be designated ODAF.

2.2.4 For oil-immersed transformers in which the alternatives ofnatural or forced cooling with non-directed oil flow are possible, typicaldesignations are :

TABLE 1 LETTER SYMBOLS

( Clause2.1 )i) Kind of Cooling Medium Symbol

a) Mineral oil or equivalent flammable syntheticinsulating liquid

O

b) Non-flammable synthetic insulating liquid L

c) Gas G

d) Water W

e) Air A

ii) Kind of Circulation

a) Natural Nb) Forced (oil not directed) F

c) Forced (directed oil) D

ONAN/ONAF ONAN/OFAF

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2.2.5 The cooling method of a dry-type transformer without aprotective enclosure or with a ventilated enclosure and with naturalair cooling is designated by :

AN

2.2.6 For a dry-type transformer in a non-ventilated protectiveenclosure with natural air cooling inside and outside the enclosure thedesignation is :

ANAN

3. LIMITS OF TEMPERATURE-RISE

3.1 Normal Temperature-Rise Limits The temperature-rises of

the windings, cores and oil, of transformers designed for operation ataltitudes not exceeding those given in 3of IS : 2026 (Part I)-1977* andwith cooling medium temperatures as described in 3 of IS : 2026(Part I)-1977* shall not exceed the limits specified in Tables 3 and 4when tested in accordance with 4.

3.1.1 The method of verification of the forced directed oil flow shall besubject to agreement between the manufacturer and the purchaser,normally at the time of tender.

3.1.2 The temperature-rises of transformers immersed in non-flammable synthetic insulating liquids and using insulating materialswhose temperature classes are different from A may be raised by anamount to be agreed by manufacturer and purchaser.

3.1.3 In certain types of transformers with concentric windings andvertical axes of core and windings two or more windings can bearranged one above the other. If these windings are identical, thearithmetic mean value of their temperature-rises shall not exceed thepermissible temperature-rises given in Tables 3 and 4. If the windings

TABLE 2 ORDER OF SYMBOLS

( Clause2.2.2 )

1st Letter 2nd Letter 3rd Letter 4th Letter

Kind of cooling med-ium indicating thecooling medium thatis in contact with thewindings

Kind of circulation Kind of cooling med-ium indicating thecooling medium thatis in contact with theexternal coolingsystems

Kind of circulation

*Specification for power transformers: Part I General

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are not identical, their temperature-rises shall be subject to agreementbetween the manufacturer and the purchaser. In this case, the axialdimensions of each winding are to be considered.

3.2 Reduced Temperature-Rises for Transformers Designed forHigh Cooling-Medium Temperatures or Special Cooling-MediumConditions If the transformer is designed for service where thetemperature of the cooling-medium exceeds one of the maximum valuesshown in 3of IS : 2026 (Part I)-1977* by no more than 10C, the allowabletemperature-rises for the windings, cores and oil shall be reduced.

TABLE 3 TEMPERATURE-RISE LIMITS FOR DRY-TYPETRANSFORMERS

( Clauses3.1, 3.1.3, 3.3, 4.3 and4.5.1 )

SLNO.

PART COOLINGMETHOD

TEMPERATURECLASSOF

INSULATION*

TEMPERATURE-RISE

(1) (2) (3) (4) (5)

C

i) Windings (Temperature-

rise measured by resis-tance method)

Air, natural

or forced

A

EBFHC

50

657090

115140

ii) Cores and other parts

a) Adjacent to windings

b) Not adjacent towindings

All

All

a) Same values as forwindings

b) The temperature shallin no case reach avalue that will damagethe core itself, otherparts or adjacentmaterials

NOTE Insulating materials may be used separately or in combination provided thatin any application each material shall not be subjected to a temperature in excess ofthat for which it is suitable, if operated under rated conditions.

*In accordance with IS : 1271-1958 Classification of insulating material for electricalmachinery and apparatus in relation to their thermal stability in service.

For certain insulating materials, temperature-rises in excess of 140C may beadopted by agreement between the manufacturer and the purchaser.

*Specification for power transformers: Part I General.

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If the rated power is 10 MVA or greater, the reduction shallcorrespond to the excess temperature. For smaller rated powers the

allowable temperature-rises shall be reduced as follows :a) by 5C if the excess temperature is less than or equal to 5C, and

b) by 10C if the excess temperature is greater than 5C and lessthan or equal to 10C.

Where for air-cooled transformers the excess temperature specifiedabove exceeds 10C, or for water-cooled transformers the temperatureof the cooling water exceeds 30C, the allowable temperature-risesshall be agreed to between the manufacturer and the purchaser.

Any site conditions which may either impose restrictions on the air

cooling or produce high ambient air temperatures shall be specified bythe purchaser.

TABLE 4 TEMPERATURE-RISE LIMITS FOR OIL-IMMERSED TYPETRANSFORMERS

( Clauses3.1, 3.1.3, 3.3, 4.3, 4.4 and4.5.1 )

SLNO.

PART TEMPERATURE-RISEEXTERNALCOOLING

MEDIUM

Air Water

(1) (2) (3)

C

(4)

C

i) Windings (temperature-rise measured by resistancemethod) temperature classof insulation A

55, when the oil circula-tion is natural orforced non-directed

60, when the oil circula-tion is natural orforced non-directed

60, when the oil circula-

tion is forced directed

65, when the oil circula-

tion is forced directed

ii) Top oil (temperature-risemeasured by thermometer)

50, when the transformeris equipped with aconservator or sealed

55, when the transformeris equipped with aconservator or sealed

45, when the transformeris neither equippedwith a conservatornor sealed

50, when the transformeris neither equippedwith a conservatornor sealed

iii) Cores, metallic parts, andadjacent materials

The temperature shall inno case reach a value thatwill damage the core

itself, other parts oradjacent materials

The temperature shall inno case reach a value thatwill damage the core

itself, other parts oradjacent materials

NOTE The temperature-rise limits of the windings (measured by resistance method)are chosen to give the same hot-spot temperature-rise with different types of oilcirculation. The hot-spot temperature-rise cannot normally be measured directly.Transformers with forced directed oil flow have a difference between the hot-spot andthe average temperature-rise in the windings which is smaller than that intransformers with natural or forced but not directed oil flow. For this reason thewindings of transformers with forced directed oil flow can have temperature-rise limits(measured by resistance method) which are 5C higher than in other transformers.

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3.3 Reduced Temperature-Rises for Transformers Designed forHigh Altitudes Unless otherwise agreed between the manufacturerand the purchaser for air-cooled transformers designed for operation atan altitude greater than 1 000 m but tested at normal altitudes, the

limits of temperature-rise given in Tables 3 and 4 shall be reduced bythe following amounts for each 500 m by which the intended workingaltitude exceeds 1 000 m :

NOTE1 If air-cooled transformers, which are designed for operation below 1 000 m,are tested at altitudes above 1 000 m, the measured temperature-rises are to bereduced by the above mentioned amounts for each 500 m by which the test altitude

exceeds 1 000 m.NOTE2 These reductions in temperature-rise limits or in measured temperature-rises are not applicable to water-cooled transformers.

3.4 Choice of Tapping for Temperature-Rise Test Temperature-rise test shall be performed at the tapping as desired by the purchaser.If nothing has been stated by the purchaser, the test shall be carried outas indicated below:

a) For tapping ranges less than or equal to 10 percent tapping onnegative side, the test shall be performed on the lowest tap atappropriate current relating to this tapping; and

b) For tapping ranges exceeding 10 percent on negative side, the testshall be performed at 10 percent tapping with appropriatecurrent relating to this tapping.

NOTE 1 While above mentioned tapping limits may be applicable in respect oftemperature-rise test only, there shall be no injury to the transformer when deliveringthe appropriate rated current on any tap including the extreme negative tap.

NOTE 2 A transformer may be expected to operate without permanent injury so longas it is operating within the absolute temperature limits and other conditions specifiedin IS : 6600-1972 Guide for loading of oil immersed transformers.

4. TEST OF TEMPERATURE-RISE (TYPE TEST)

4.1 Measurement of Temperature of Cooling Air4.1.1 General The cooling-air temperature shall be measured bymeans of several thermometers arranged according to 4.1.2and 4.1.3.They shall be protected from draught and abnormal heat radiation.

To avoid errors due to the time lag between variations in thetemperature of the transformer and that of the cooling air, thethermometers may be inserted in cups filled with liquid, having a timeconstant of about two hours.

The value to be adopted for the temperature of the cooling air for atest is the average of the readings taken on these thermometers atequal intervals of time during the last quarter of the test period.

a) Oil-immersed, natural air-cooled transformersb) Dry-type, natural air-cooled transformersc) Oil-immersed, forced air-cooled transformersd) Dry-type, forced air-cooled transformers

2.0 percent2.5 percent3.0 percent5.0 percent

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The temperature of the cooling air should be as constant as possibleduring the test period, especially during the last quarter.

4.1.2 Natural Air-Cooling The thermometers (at least three) shall

be placed at different points around the transformer, at a levelapproximately half way up the cooling surface, at a distance of 1 to 2 mfrom the cooling surface.

4.1.3 Forced Air-Cooling If there is a well defined flow of air fromthe surroundings towards the intakes of the coolers, without muchrecirculation of warm air, the thermometers should be placed in thisintake stream. They should be far enough away from the tank andcooler surfaces to prevent disturbance by radiant heating (distanceof 1 m to 2 m).

If these conditions cannot be fulfilled, the temperatures shall be

measured around the complete transformer, outside the recirculationstreams, preferably on the side without cooler, if any.4.2 Measurement of Temperature of Cooling Water Thecooling water temperature shall be measured at the intake of the coolerand the temperature shall be taken as the average of at least threereadings taken at approximately equal intervals not greater than onehour. The readings shall be taken in the last quarter of the test period.

4.3 Determination of Winding Temperature The winding temp-eratures shall in principle be ascertained using the resistance method.

The temperature of a winding ( ) at the end of a test period shall be

calculated from its measured resistance ( R2 ) at that temperature andits measured resistance ( R1 ) at some other temperature ( ) using theformula :

for copper

for aluminium

where and are measured in C.

The resistance ( R1) is generally the cold resistance measured inaccordance with 16.2of IS : 2026 (Part I)-1977*. The resistance ( R2 ) ismeasured either after switching off the supply, having regard to thecorrections indicated in 4.9, or without interruption of the supply bymeans of the super-position method ( seeAppendix A ) which consists ofinjecting into the winding a dc measuring current of low valuesuperposed on the load current.

In case of transformers with concentric windings and vertical axes ofcore and windings where two or more identical windings are arrangedone above the other, all these windings can be connected in series for the

*Specification for power transformers: Part I General

2

1

2

R2

R1------- 235( 1) 235+=

2

R2

R1------- 225(

1) 225+=

1 2

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test of temperature-rise. The measured temperature-rise of theseseries-connected windings shall not exceed the appropriate value givenin Tables 3 and 4.

If the windings have a resistance of 0.005 ohm or greater the winding

temperatures shall be ascertained by means of the resistance method.Practical difficulties due to the short time available for themeasurement of hot resistances may affect the accuracy of suchmeasurement by about one percent.

For winding having a value less than 0.005 ohm, the resistancemethod may be inaccurate. In such cases the temperature-rise at theend of a temperature test shall be determined as follows :

The temperature of the oil shall be measured by a thermometerplaced in a thermometer pocket. The temperature-rise so determinedshall not exceed the limiting value for oil given in Table 4.

Any one method shall be used for the determination of the windingtemperatures.

4.4 Measurement of Top Oil Temperature The temperature ofthe top oil shall be measured by a thermometer placed in an oil-filledthermometer pocket on the cover or in the outlet pipe to the cooler, butin the case of separate coolers it shall be located in the outlet pipeadjacent to the transformer. Should the tank not be completely filledwith oil, the pocket shall be long enough, or placed in an appropriateposition on the tank, to ensure true measurement of the top oil

temperature; alternatively, an opening shall be provided throughwhich the thermometer can be inserted. The temperature-rise sodetermined shall not exceed the limiting value for oil given in Table 4.

4.5 Duration of Test of Temperature-Rise The test shall becontinued until the requirements of one of the following methods havebeen met. The method shall be chosen by the manufacturer.

4.5.1 Method a Evidence shall be obtained that the highesttemperature-rise will not exceed the value given in Table 3 or 4, asappropriate, even if the test were continued until thermal equilibriumis reached. Temperatures shall be taken where possible during

operation, as well as when the supply to the transformer isswitched-off. The test shall not be regarded as completed until thetemperature-rise increment is less than 3C in 1 hour.

The method shown in Fig. 1 shall be employed for the determinationof the final temperature-rise.

4.5.2 Method b It shall be demonstrated that the top oiltemperature-rise does not vary more than 1C per hour during 4consecutive hourly readings. If the test is performed initially withreduced cooling or without any cooling, it shall be continued forsufficient time with full cooling to prevent errors in the measurementof the final oil temperature-rise.

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FIG. 1 METHODFORDETERMININGTHEFINALTEMPERATURE-RI

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4.6 Test Method for Dry-Type Transformers* The method shallbe one involving excitation of the core at normal flux density. The inputtest current Itshall be held constant at a value as near as possible to therated value INand at least equal to 90 percent of this value, and the run

continued until the temperature-rise, , of the windings, is steady.The temperature-rise of the windings above the temperature of the

cooling air, for rated load conditions, , is calculated from the formula :

The value of qshall be taken as follows :

NOTE For loading method see4.8.

4.7 Test Method for Oil-Immersed Type Transformers* Tempe-rature-rise tests of oil-immersed type transformers include the determi-nation of top oil temperature-rise and of winding temperature-rises.

NOTE1 During the test the power required for the pumps and fans may be measured.

NOTE2 For loading method see4.8.

4.7.1 Top Oil Temperature-Rise The top oil temperature-rise shallbe obtained by subtracting the cooling medium test temperature fromthe measured top oil temperature, the transformer being supplied with

the total losses. The input power shall be maintained at a steady value.If the total losses (taken as the sum of the measured load losses,

corrected to the reference temperature and the measured no-loadlosses) [ see 16.1 and 16.4 of IS : 2026 (Part I)-1977 ] cannot beobtained, different losses, as near as possible to the above losses, but inany case not less than 80 percent, shall be supplied and the followingcorrection factor applied to the top oil temperature-rise so determined :

The value of xshall be as follows :

NOTE In cases where testing facilities are not available for carrying out the test at80 percent of the total losses, the losses at which the test is to be conducted and the detailsof the test shall be subject of agreement between the manufacturer and the purchaser.

*It is assured that the transformer has no tappings, or if it has, that the test is madeon the principal tapping. If the test is made on another tapping, it is necessary to replacein the text the words rated voltage and rated current by appropriate tapping voltage

and appropriate tapping current.

ANAF

transformers =transformers =

1.61.8

Specification for power transformers: Part I General

For natural air circulation:For forced air circulation and water cooling:

0.81.0

t

N

N

t

IN

It-------

q

=

Total losses

Test losses--------------------------------

x

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4.7.2 Winding Temperature-Rises Winding temperature-rises shallbe obtained on all windings by subtracting the external coolingmedium test temperature from the average temperature of thewindings as measured by resistance, after circulating the rated

current at rated frequency in the winding under test.If the rated current cannot be supplied, the tests may be performed

with a current not less than 90 percent of the rated current.

Alternatively, a current providing the total losses may be supplied. Ineither case the following correction factor shall be applied to the deter-mined temperature-rise of the windings above average oil temperature :

The value ofyshall be as follows :

NOTE In cases where testing facilities are not available for carrying out the test at90 percent of rated current the current at which the test is to be conducted and the detailsof the test shall be subject of agreement between the manufacturer and the purchaser.

The average oil temperature may be determined by any of thefollowing ways :

a) For all cooling methods the average oil temperature in the surro-undings of the different windings shall be calculated according toFig. 2 from the resistance Rwhere the straight line L cuts theordinate. The winding resistances shall be determined afterswitching-off the supply according to 4.9.

b) In transformers with ON, OF and OD cooling the average oiltemperature shall be determined as the difference between thetop oil temperature and half the temperature-drop in the coolingequipment.For tanks with tubes or radiators mounted on them, the temp-erature-drop shall be taken as the difference between the surfacetemperatures at the top and the bottom of a cooling tube orradiator element, the tube or radiator element chosen being as

near as practicable to the middle of a side of the tank.In the case of a separate cooler, the temperature-drop shall betaken as the difference in readings of thermometers inthermometer pockets adjacent to the main tank in the inlet andoutlet pipes to and from the cooler.

c) For transformers having rated powers up to 2 500 kVA withnatural oil circulation and plain or corrugated tanks or with tubesor radiators mounted on the tanks, the average oil temperature-rise is taken approximately as 0.8 times top oil temperature-rise.

For natural and forced non-directed oil circulation:For forced directed air circulation:

1.62.0

Rated current

Test current--------------------------------------

y

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Allowances shall be made for variation in average oil temperatureduring tests as explained in detail in 4.8.3.

It shall be accepted that the average oil temperature-rise may varywith the losses according to the law given in 4.7.1 for the top oiltemperature-rise.

4.8 Loading Methods* At the choice of the manufacturer, for two-winding transformers any one of the loading methods according

to 4.8.1to 4.8.3may be applied for oil-immersed type transformers andeither of the loading methods according to 4.8.1and 4.8.2for dry-typetransformers.

4.8.1Direct Loading Method One winding of the transformer shallbe excited at rated voltage with the other connected to a suitable loadsuch that rated currents flow in both windings. No correction foraverage oil temperature need be applied to the winding temperature-rise of oil-immersed type transformer.

FIG. 2 METHODFORDETERMININGTHEWINDINGRESISTANCEATTHEINSTANTOFSWITCHING-OFFTHESUPPLY,ANDTHEAVERAGEOILTEMPERATURE

*It is assured that the transformer has no tappings, or if it has, that the test is madeon the principal tapping. If the test is made on another tapping, it is necessary to replace

in the text the words rated voltage and rated current by appropriate tapping voltageand appropriate tapping current.

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4.8.2Back-to-Back Method Two transformers, one of which is thetransformer under test, shall be connected in parallel and excited atthe rated voltage of the transformer under test. By means of differentvoltage ratios or an injected voltage, rated current shall be made to

flow in the transformer under test. No correction for average oiltemperature need be applied to the winding temperature-rise ofoil-immersed type transformers.

4.8.3 Short-Circuit Method To determine the temperature-rise of theoil, the sum of the no-load and load losses at the reference temperatureshall be supplied to the transformer, one of its windings being excitedand another short-circuited at its terminals. The top oil temperature-rise and the average oil temperature-rise shall be recorded.

The input shall then be reduced to a value which results in the

circulation of rated current at rated frequency in the windings, and thisvalue shall be maintained for 1 hour. The temperature of the windingsshall then be determined by the resistance method. The drop in averageoil temperature during this hour is taken into account when calculatingthe temperature-rise of the windings above the average oil temperature.

The temperature-rise of the windings above the average oil temp-erature, determined in the second part of the test, added to the averageoil temperature-rise, determined in the first part of the test, shall give thetemperature-rise of the windings above the cooling medium temperaturefor total losses at rated current, rated frequency and rated voltage.

Alternatively, the temperature-rise of the windings above thecooling medium temperature for total losses at rated current and ratedvoltage may be derived from the temperature-rise of the windings atthe end of the run with total losses, mentioned in the first sentenceof4.8.3, by correcting the difference between the temperature of thewindings and the average oil temperature to the conditionscorresponding to rated current and rated frequency.

4.8.4 Loading of Multi-Winding Transformers In the case of multi-winding transformers where more than two windings can be loadedsimultaneously in service the temperature-rise tests shall normally beperformed by separate two-winding tests.

In certain cases the rated powers of the individual windings maypreclude the testing of the transformer at the full total losses.

For such cases the correction of the top oil temperature-rise ofoil-immersed type transformers shall be made as described in 4.7.1.

Calculated corrections according to 4.7.2 shall be made, wherenecessary, to determine the individual winding temperature-rises atthe specified loading combination which is the most severe for theparticular winding. In these calculations the manufacturer may take

account of the stray and eddy current losses at this loading combination.

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4.9 Temperature Correction for Cooling of Transformers AfterSwitching-Off the Supply

4.9.1 General Winding temperature measurement may be made

while the transformer is in operation by the superposition methodmentioned in 4.3or by taking resistance readings after the supply totransformer is switched-off.

In the latter case, to provide for the interval between the instant ofswitching-off the supply and the measurement of the temperature, acorrection shall be applied so as to obtain as nearly as practicable thetemperature at the instant of switching-off the supply.

Readings shall be taken as soon as possible after switching-off thesupply, but allowing sufficient time for the inductive effect todisappear, as indicated from the cold resistance measurements

[ see 16.2.1of IS : 2026 (Part I)-1977* ].When the supply to transformer is switched-off, the fans and waterpumps shall be stopped but the oil pumps shall remain running.

Correction of the temperature-rise as determined by the resistancemethod to the instant of switching-off the supply shall be made byextrapolation back to the instant of switching-off the supply fromtime/temperature curves or time/resistance curves.

4.9.2 Method of Extrapolation Using Linear Scales The correctionshall be determined approximately by making a series of resistancemeasurements and from this plotting a time/resistance curve, which is

extrapolated back to the instant of switching-off the supply. Thehighest winding temperature shall then be calculated from theresistance at the instant of switching-off the supply.

This extrapolation shall be done according to Fig. 2, where theresistances Rare determined at equal intervals of time t.

In taking actual resistance measurements by some bridge methodsit is more accurate to determine times for fixed changes in resistanceR, that is, time as noted at the moment the indicator of the pre-setbridge passes through zero.

The decreases in resistance, Rn corresponding to equal timeinterval are put down horizontally at the appropriate points of theordinate and give the straight line L. R2 is the resistance of thewinding at the instant of switching-off the supply.

4.9.3 Method of Extrapolation Using Log-Linear Scales The diffe-rence Rbetween the measured resistance and the resistance R, corres-ponding to the temperature to which the winding is cooling afterswitching-off the supply shall be drawn on log-linear graph paper withtime as linear and Ras the logarithmic axis. The resistance Rshall bechosen in such a way that the test points plotted appear almost on astraight line. The resistance at the instant of switching-off the supply

*Specification for power transformers: Part I General.

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shall then be equal to R+ Ro, where Ro is found by drawing a straightline through the point on the graph and extrapolating it back to zero time.

4.9.4 Temperature of Hot-Spot in Winding For the purpose ofcalibrating winding-temperature indicators, the temperature of thehot-spot in a winding shall be taken as the sum of the temperature atthe top oil ( + cooling air temperature ) and 1.1 times thetemperature-rise of the winding above average oil temperature.

A P P E N D I X A

( Clause4.3 )

TEMPERATURE-RISE OF WINDINGS BY RESISTANCEMEASURED BY THE SUPERPOSITION METHOD

A-1. PRINCIPLE OF THE METHOD

A-1.1A small auxiliary dc current, supplied preferably from a storagebattery is superposed on the ac load current in the transformerwinding under consideration. Measurements are made of themagnitude of the superposed current circulating in the winding andalso the voltage drop at the terminals of the winding due to this directcurrent. These measurements are made at least at the beginning andend of the temperature-rise test and are used to determine the meantemperature of the windings by the variation of resistance, theaccuracy being in the order of 1C.

The method is equally applicable to temperature-rise tests carried outby means of the direct loading, back-to-back or short-circuit methods.

The test arrangements vary according to the winding connections;the two most frequent connections are described inA-2andA-3.

A-2. STAR WINDINGS WITH NEUTRAL BROUGHT OUT

A-2.1 The injection of dc through the winding neutral does not presentany difficulty. Nevertheless it is necessary to arrange for a returnneutral point which will generally be that of the supply transformer ora neutral point of the external circuit, or, in the case of a short-circuited

winding, the short-circuit connection at the phase terminals. One pointof the injection circuit should preferably be earthed.

Fig. 3 shows the circuit for a star/star transformer with the neutralsbrought out from both windings, the temperature test being carried outby the short-circuit method.

For each of the two windings, the sum Iof the dc currents injectedinto the three phases is measured by means of a millivoltmeterconnected across a shunt placed in the injection circuit.

For the energized winding, a microammeter is connected betweenthe neutral point of the winding and the neutral point of three reactorsconnected in star to the phase terminals. These reactors are for the

r

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purpose of limiting the circulation of alternating current, and to thiseffect, voltage transformers may be used. Provided that the resistanceof the voltage transformer winding in each phase is equal, then themicroammeter measures a current iproportional to the sum of the dc

voltages at the terminals of the three phases of the transformer.If R is the mean resistance of the three-phase windings of the

transformer under test, r1 the resistance per phase of the reactors orvoltage transformer and rthe circuit resistance, then

In the case of the windings on short-circuit, r again being theresistance of the voltage measuring circuit the relation becomes simply :

A-3. DELTA WINDING OR STAR WINDING WITHOUT ACCES-SIBLE NEUTRAL

A-3.1 Such winding connections do not lend themselves to theinjection of dc when they are short-circuited. The phase terminals shallnecessarily be connected to an external circuit such as a supplytransformer or a loading circuit.

The dc injection may be made through one of the phase terminalsbut the three phases of the winding do not play symmetrical roles andintervene in the measurement with different weights.

Return of the dc may often be effected by an accessible neutral pointof the external circuit, which is then earthed. The injection circuit mustinclude in series a reactor capable of withstanding the ac phase voltage.

In the absence of such an available neutral point in the internalcircuit, return of the dc may be effected in another way, for example, bya second phase terminal. One point in the injection circuit is then fixedat earth potential and reactors interposed between this point and thetwo-phase terminals used.

Fig. 4 shows the circuit for a delta-connected winding and where theneutral point is available on the supply transformer.

The dc is injected via terminal U1, and the measuring shunt shall beplaced between the terminal and the point of injection. The shunt isthus at phase voltage and the millivoltmeter shall be read at a distance.

The direct current I which flows via terminal U1 divides equallybetween the two-phase windings each of resistance Rconnected to thisterminal and flows out via terminals V1and W1.

To measure the dc voltage between terminal U1and the combinationof terminalsV1andW1, three reactors are arranged as shown in the figure.

Ri

I-- 3r r1+( )=

R

i

I--

3r=

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FIG. 3 STAR/STARTRANSFORMERWITHNEUTRALSBROUGHT

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FIG. 4 DELTAWINDINGWITHNEUTRALPOINTAVAILABLEAT

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These may consist of voltage transformers supplemented, ifnecessary, by additional resistors, the primary purpose of which is toensure that overall resistances r2 and r3 are equal. Adequatecapacitances are connected between the ends of the reactances and

earth to suppress residual alternating voltages.If iis the current in the microammeter, the mean resistance of the

two phases included in the measurement is :

A-4. MEASURING EQUIPMENT

A-4.1 Shunts The shunts placed in the neutral connection do notpresent any difficulty. For the shunt inserted in a phase conductor, inthe case of a delta-connected winding, it is necessary to consider possiblethermocouple effects due to the alternating current which is of the orderof 100 times the dc measuring current. It shall therefore be constructedwith particular care to completely eliminate these thermocouple effects.

A-4.2 Microammeter and Millivoltmeter These instrumentsshall be accurate and linear. The accuracy class shall be 0.5. Bothinstruments shall be filtered so that the presence of alternating currentdoes not affect the instrument.

Ri

I-- 2r1 r2+( )=

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Bureau of Indian Standards

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This Indian Standard has been developed by Technical Committee : ETDC 16

Amendments Issued Since Publication

Amend No. Date of Issue

Amd. No. 1 Incorporated earlier

Amd. No. 2 October 1984

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