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IS: 2026 (Part Iq.1977Indian Standard

SPECIFICATION .FORPOWER TRANSFORMERS

PART tt TEMPERA-W RE-RISE(First Revision)

Seventh Reprint JUNE 193( Incorporating Amend men t No. 1 )

UDC 621.3i4.222.6.017.71

@ Copyright 1982

BUREAU OF INDIAN STANDARDSMAN AKBHAVAN ,9 BAHAD UR SHAH ZAFAR MARGNEWDELHi llWO2

Cr5 June 19?7

( Reaffirmed 1996 )

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IS : 2626 (Part I I) - 1977Indian Standard

SPECIFICATION FORPOWER TRANSFORMERS

PART II TEMPERATU RE-RISE( First Revision)

Transformers Sectional Comm ittee, ETDC 16Ck&TUUl

SHRI U. K. PATWARDHANPrayog Electricals Pvt Ltd, Bomb ayMembers Representing

SH III S. AXMEE RJ ANSHRI N. S. S. AROKIASWAMY Bha ra t Hea vy Electricah Ltd (R&D Unit)Tamil Nadu Electricity Board, MadrasSHRI M. K. SUNDARAEAJ AN Alter n n te )&R I B. G. B- YDR B. N. J AYARAM (Alte r&e) Kirlosk ar Electric Co Ltd, BangaloreSHRI A. V. B-uSHRI J, S. IYER (Altematej Gujarat Electricity Board, VadodaraSHRI S. D. &I-Y Bomb ay Electric Sup ply and Transport Und er-

SHRI Y. K. PALVAN KAR Alternate) taking, BombayDIRECTOR(TRANSbIISSION)DEPUTY -OR (TRANSMS- Central Electricity Auth ority, New Delh iSION) (Ahmute)SHRI T. K. GHOSE Calcutta Electric Su pply Corporation Ltd, CalcuttaSm u P. K. BHA~AC~RJ EE (Alfemte)JOINT DIRFXXOR ~SUEMTATI ON) Research. De&m an d Stan dard s Oraan izat iou,Lu&low -DEPUTY DIRECXIR STANDARDS(E~~cnx1c.4~) (Ahmute)SHRI J. K. KHANNA

SHRI K. L. GARO (Altemufe)SHIUB. S. KOCHARSHRIR. D. J AIN (Alternate)SHRI J. R. MAHAJANSI-IRIP. K . PII~IP (Alter&)SHRI D. B. MEHTA

Directorate General of Sup plies and Disp osals(Inspection Win g), New Delh iRural Electrification Corporation Ltd, New DelhiIndian Electrical Man u fact u re-Ix Ass ociat ion,BombayTata Hydr a-Electric Power Su pp ly Co Ltd ,_BombaySHRIR. CHANDRAHOULIAhmute) (conlind on #age 2)

10 Cqyrigkf 1977BUREAU OF INDIAN STANDARDSThis pu blicat ion is protected u nd er the Znd iu n C@yrigkt Act (XIV of 1957) an dreproduction in wh ole or in part by any means except with written permk aion of tbepu blish er sh all be deemed to be an infringement of copyright u n der the sa id Act.

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IS : 2026 (P a r t U) - 1977(Continued from page 1)

h4embers Re@senting

SHRI D.\'.&RXE Bharat Heavy Electric& LtdSHRI ISHWAR CHANDRA (Alternate I)SHKI PRIM CHAND (Alternate 11)

SHR I I . S. P ATELSHRI V. N. PIZAHLAD

SHR I A. G. GURJ AR (Ahnate)SIIRI . N. RAMASWAMY

Hindu stan Brow n Boveri Ltd, BombayNational Electrical Indu stries Ltd , Bhop alDirectorate Genera l of Techn ical Developm ent,New Delhi

SHR I S. K. P ALIXAN (Alternate)SHR I CHANDRA K. ROI IATGISHRI D. P. SAHC~L

SHR I A. R . SALVI (Alternate)SH RI I. C. SXYGAK

SHR I R. C, QI.~NNA (Alternate)SHR I K. G. S HA~WKIIAPP A

SHR I P . S. RAMAN (Alternate)SHIU M. A. SHARIF F

S H R I B. C. ALVA (Alternate)S U P E R I N T E N D I N GN G I N E E R

(OPERATION)

Pradip Lamp Works, PatnaSiemens India Ltd, BombayDelhi Electric Sup ply Under taking, New DelhiNGEF Ltd, BangaloreKarn ataka J::r.ctricity Board , Bangalor eAnd hra Pra&=sh State Electricity Depar tmen t(Electricity Projects and Board ), Hyd erabad

S U P E R I N T E N D I K G N G I N E E RTECHNICAL (PROJ ECTS)(Alternate)

SHR I C. R. VARI ER Crompton Grcaves Ltd, BombaySHR I S. V. MANER IKAR (Ahmute)

SHRI S. P. SACHDEV, Direc tor Genera l , IS1 (Ex-o&cio Member)Director (Elec tech)

SecretarySHRI VIJ AI

Deputy Di rec tor (E lcc tech) , IS1

Panel for ?&vision of IS : 2026 Specification for Power Tran sformers,ETDC 16: P6Members

SH R~ S. V. MANER ~K.~R Cromp ton Greaves Ltd, BombaySHRI D. V. NARKE B h a r a t H e a v y E l e ct r i c a l s L t dSHR I ISH WAR CHANDRA (Alternate I)SHRI P R E M C H A ND (Alternate II)SHR I S. SR INIVA~AN Alternate III)

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IS : 2026 (P a r t II) - 1977

Indian StandardSPECIFICATION FOR

POWER TRANSFORMERSPART II TEMPERATU RE-RISE

( First Rev ision)0. FOREWORD

0.1 This Indian Stan dard (Pa rt II) was adopted by the Indian Stan dar dsInst itut ion on 24 Febru ar y 1977, after the dra ft fina lized by the Tra ns-form ers Sectional Comm ittee ha d been appr oved by th e Electr otechnicalDivision Council.0.2 The firs t revision of IS : 2026-1962* ha s been un derta ken with a view t obring it in line with th e revision of IE C Pu b 76 (1967) P ower tr an sform ers .0.3 In t his revision th e requiremen ts for power tr an sform ers a re coveredin. four pa rt s as follows :

Part I GeneralPart II Temperature-risePa rt III Insu lation levels an d dielectr ic test sPart IV Terminai markings, tappings and connections0.4 This st an da rd (Pa rt II) h as been based on IE C Pu b 76-2 (1976) Powertr an sform ers, P ar t 2 Tempera tu re-rise, issued by th e Int erna tiona l Electr o-technical Commission.0.5 This part shall be rea d in conjunction with IS : 2026 (Part I)-1977?,IS : 2026 (Pa rt III)-1977: an d IS : 2026 (Part IV)-I977$0.6 For th e pur pose of deciding wheth er a par ticular requiremen t of th isst an da rd iscomplied with, th e final value, observed or calculated, expressingth e resu lt of a test , shall be roun ded off in accordan ce with IS : 2-196011.The nu mber of significan t p laces ret ained in th e roun ded off value shouldbe th e sam e as th at of th e specified value in th is sta nda rd.

* Specif~catiou for power fransformers.t Specifkation for power transformers: Part I General.r Spesihation for power transformers: P art II I Insulation levels and diekctr ic tests.8 Specification for power trausformcrs: P art #I V Terminal markings, tapping~and connec-tiOnS.

II Rules for rounding off nunpical values (ret ied).3

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IS : 20 26 (Par t I I) - 1 9 771. SCOPE1 .1 Th i s stand ard (Part II) covers temp erature-rise requirements of powertransformers.2. IDENTIFICATION ACCORDING TO COOLING METHOD2.1 Iden t i fi ca t i on Sym bo ls - Transformers shall be identified accordingto the cooling method emp loyed. Letter svmbols for use in connectionwith each cooling method shall be as given in Table 1.

SOTE - In transformers w ith forced directed oil circulation a certain p roportim ofthe forced ojl flow is channelled so as to pass through the windings. Certain wind ings,however, may have a non-directed oil flow, for instance, separate tappin g win dings,auxiliary windings and stabilizing windings.

TABLE 1 LETTER SYMBOLS (Clau se 2.1)i) Kittd of Coolitlg Medium Symbol

a) Mineral oil or equ ivalent flammab le synthetic 0insulating liquidb) N on-flammable synthetic insulating liquidc) Gas kd j \ Vater Wc) Air Aii ) hiud of Circulation

al Naturalbj Forced (oil not directed)c) Forced (directed oil)

2.2 Ar r a n g e m e n t o f S y m b o ls - Transformers shall be identified by foursymbols for each cooling method for which a rating is assigned by themanufacturer.2.2.1 Dry-type transformers without protective enclosures are identified

by two symbols only for the cooling medium that is in contact with thewindings of the surface coating of wind ings with an overall coating (forexample, epoxy resin).2.2.2 The order in which the symbols are used shall be as given in Table 2.Oblique strokes shall be used to separate the group symbols for d ifferentcooling methods.2.2.3 For example, an oil-immersed transformer with forced directed oilcirculation and forced air circulation shall be designated ODAF.2.2.4 For oil-imm ersed transformers in which the alternatives of natu ral

or forced cooling with non-directed oil flow are possible, typical designations.are :ONAN/ ONAF ONAN/ OFAF

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IS : 20 26 (Par t I I) - 19 772 . 2 . 5 . The cooling m ethod of a dry-type transformer with out a protectivewith a ventilated enclosure an d w ith natu ral air cooling is: AN2.2.6 For a d ry-type transformer in a non-ven tilated protective enclosureair cooling inside and ou tside the enclosure the designation is :

ANAN

rst Letter

TABLE 2 OR DER OF SYMBOLS(Clause 2.2.2)

2nd Letter 3rd Let&r 4th Letterof cooling med i- Kind of circulation Kind of cooling Kind of circulationindicating the COOL medium indicatingmediu m that is int with the win d- the cooling mediumthat is in contactwith the externalcooling systems

OF -TEMPERATURE-RISENo r m a l T e m p e r a t u r e -R is e Li m i t s - The temperatu re-rises of thedin gs, cores and oil, of transformers designed for operation at altitudeseedin g those given in 3 of IS : 2026 (Part I)-1977* and with coolingium tempetitures as described in 3 of IS : 2026 (Part I)-1977* shall notceed the limits specified in Tables 3 and 4 wh en tested in accordan ce with 4.

For mu lti-win din g transformers, the temp erature-rise of the top oil refersthe specified loading comb ination for wh ich the total losses are h ighest.ividual win din g temp erature-rises shall be considered relative to thatd loading comb ination kh ich is the m ost severe for the particular.nd ing und er consideration.3.1.1 The meth od of ve&ication of the forced directed oil flow shall beto agreement between the man ufacturer and the pu rchaser, normally3.1.2 The temp erature-rises of transformers imm ersed in non -flamm ableng liquids and using insulating materials wh ose temperaturelasses are &&rent from A may b e raised by an amoun t to be agreed byufacturer and pu rchaser.3.1.3 In certain types of transformers with concentric win din gs and

al axes of core an d win din gs two or more win din gs can be arranged*Specification for pow er ran sformers:art I Cjeneral.

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IS : 2026 (Pa r t II) - 1977on e above the other. If these win din gs are iden tical, the arithmetic meanvalue of their temperature-rises shall not exceed the permissible tem perature-rises given in Tables 3 and 4. If the win din gs are not iden tical, their tem-perature-rises shall be sub ject to agreemen t between the man ufacturer andthe purchaser. In this case, the axial d imensions of each win din g are tobe considered.

TABLE 3 TBMP_TFUI FU IFWIS ~iS FOR DRY-TYPE(Clauses 3.1, 3.1.3, 3.3, 4.3 a nd 4.5.1)

SLNo.(1)

PART

(2)CooLrNGhfETHOD

(3)

3

ii)

Wjndings (Tempera tu re- Air, na tu ra l orrise mea sur ed by re- forcedsistance method)

Cores and other partsa) Adjacent to wind - Allingsb) Not adjacent to Allwindings

TEMPERATURECLASS O F

INSUL_~TI~~N*(9

--

4W

Same values as forwindingsThe temperature shallin no case reach avalue thatwill damagethe core itself, otherparts or adjacentmaterials

NOTE - Insulating mat erials may be used separa tely or in combination provided thatin any application each mat erial shall not be subjected to a tempera tur e in excess of thatfor which it is suita ble, if operat ed un der ra ted conditions.*In accordance with IS : i271-1958 Class ification of insu lat ing ma ter ial for electricalma chinery an d appar atu s in relation to their therm alstability in service.tF or certa in insulatin g mat erials, tem pera tu re-rises in excess of 140C ma y be adoptedby agreement between t he manu facturer and the purchaser.

3.2 R e d u c e d T e m p e r a t u r e -R i se s fo r T r a n s fo r m e r s D e si gn e d fo rH5&~~ng-Medm m Tem pe+yes o r Spec+ l Cooling-Medium- If the transformer is designed for servrce where the tempera-ture of the cooling-med ium exceeds one of the maximu m values shownin 3 of IS : 2026 (Part I)-1977* by no more than lo%, the allow ab le tem-perature-rises for the wind ings, cores and oil shall be redu ced.

*Specificationor pow er ran sformers: art I General .

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IS : 2026 (P a r t II) - 1977Any site conditions which may either impose restrictions on the aircooling or produce high ambient air temp eratures shall be specified by thepurchaser.

3.3 R e d u c e d T e m p e r a t u r e -R & e s fo r T r a n s fo r m e r s D e si gn e d f orHigh Al t i t ude s - Unless otherwise agreed between the manufacturer andthe purchaser for air-cooled transformers designed for opera tion at an altitudegreater than 1 000 m but tested at norm al altitudes, the limits of temp era-ture-rise given in Tables 3 and 4 shall be reduced by the following amountsfor each 500 m by which the intend ed working altitude exceeds 1 000 m :a) Oil-immersed, natu ral air-cooled t ransformers 2-O percentb) Dry-type, natural air-cooled transformers 2.5 percentc) Oil-immersed, forced air-cooled transformers 3-O percentd) Dry-type, forced air-cooled transformers 5.0 percentNorel - If air-cooled transformers, wh ich are design ed for operation below 1 000 m,are tested at altitudes above 1 000 m , the measured temperature-rises are to be reducedby the above mention ed amoun ts for each 50 m by wh ich the test altitude exceeds1OOOm.NOTE Z-These reductions in temperature-rise limits or in measured temp erature-rises are not app licable to water-cooled transformers.

4. TEST OF TEMP ER ATURE -RISE (TYP E TEST)4 .1 Me a su r e m e n t o f Te mp e r a t u r e o f Coo ling Ai r4.1.1 General - The cooling-air temp erature shall be measured by meansof several therm ometers arranged accord ing to 4.1.2 and 4.1.3. They shallbe protected from draugh t and abnormal heat radiation.

To avoid errors due to the time lag between variations in the temperatu reof the transformer and that of the cooling air, the thermom eters may beinserted in cup s filled w ith liquid, having a time constant of about two hours.The value to be adopted for the temperature of the cooling air for a testis the average of the readings taken on these therm ometers at equal intervalsof time du ring the last quarter of the test period.The temp erature of the cooling air shou ld be as constant as possibledu ring the test period, especially du ring the last quarter.4.1.2 Natural Air-Cooling - The thermometers (at least three) shall beplaced at different points around the transformer, at a level approximatelyfzsazy up the cooling surface, at a d istance of 1 to 2 m from the cooling.4.1.3 Forced Air-Cooling - If there is a well defined flow of air from thesurround ings toward s the intakes of the coolers, without mu ch recirculationof warm air, the thermom eters should be placed in this intake stream.They should be-far enough away from the tank and cooler su rfaces to pre-vent disturbance by rad iant heating (distance of 1 m to 2 m).If these conditions cannot be fulfilled, the temp eratures shall be measu red

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IS : 2026 (P a r t II) - 1977nd the complete tr%nsformer, outside the recirculation streams, pre-rably on the side withou t cooler, if any.M ea s u r e m e n t o f T e m p e r a t u r e o f C oo li n g Wa t e r - The cooling

peratu re shall be measured at the intake of the cooler and theeratu re shall be taken as the average of at least three readings taken atproximately equal intervals not greater than one hou r. The readings shallin the last quarter of the test per iod.D e t e r m i n a t i on o f Win d i n g Te m p e r a t u r e - The winding tempera-res shall in principle be ascertained using the resistance method .

The temp eratu re-of a winding ( 0, ) at the end of a test period shall beculated from its measu red resistance (R,) at tha t temperatu re and itsred resistance (RX) at some other temp eratu re ( 8, ) using the formula :S,= 3 (235+8,)-235 for copper

1e,= 2 (225+8,) -225 for aluminiumI

where 8r and 6, are m easured in C,The resistance (RJ is generally the cold resistance measu red in accordancewith 16.2 of IS : 2026 (Part I)-1977*. Theresistance (Z&) is measuredeither after switching off the sup ply, having regard to the corrections in-d icated in 4.9, or withou t interruption of the sup ply by means of the super-position meth od (see Append ix A) which consists of injecting into the wind-ing a dc measuring current of low value sup erposed on the load cur rent.In case of transformers with concentric windings an d vertical axes ofcore and windings where two or more identical wind ings are arranged oneabove the other, all these windings can be connected in series for the testof temp erature-rise. The measu red temp erature-rise of these series-con-nected w indings shall not exceed th e appropriate value given in Tables3 and 4.If the wind ings have a resistance of 0.005 ohm or greater the windingtemp eratu res shall be ascertained by mean s of the resistance method .Practical difficulties due to the short time available for the measurement ofhot resistances may affect the accuracy of such measurement by abou t one

percent.For wind ing having a value less than O-005 ohm , the resistance methodmay be inaccurate. In such casesthe temp erature-rise at the end of atemp eratu re test shall be determined as follows :The temp eratu re of the oil shall be measu red by a thermometer placedin a thermom eter pocket. The temperature-rise so determined shall notexceed the limiting value for oil given in Table 4.Any one method shall be used for the determination of the wind ing tem-peratures.lpecScatibn for power ransformers:Part I General.

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16 : 2026 (Pa r t I I ) - 19774 .4 M e a s u r e m e n t o f T o p O il T e m p e r a t u r e - T h e temperature of thetop oi l shall be measured by a thermom eter placed in an oil-filled thermo-meter pocket on the cover or in the outlet pip e to the cooler, bu t in th e caseofsrparate coolers it shall be located in the outlet pip e adjacent to the trans-former. Should the tank n ot be comp letely filled with oil, the pocket shallbe long enough , or placed in an approp riate position on the tank, to ensuretrue measuremen t of the top oil temperature; alternatively, an opening shallbe provided through w hich the thermom eter can be inserted. The tem-peratu re-rise so determ ined shall not exceed the limitin g value for oil givenin Table 4.4.5 D u r a t i o n o f T e s t o f T e m p e r a t u r e -R i s e - The test shall be conti-nu ed un til the requirements of one of the following m ethods have b een m et.The method shall bc chosen by the manufacturer.

4.5.1 Method (1 Evidence shall be obtained that the highest temp erature-rise will not errcecl the value given in Table 3 or 4, as app ropriate. even ifthe test were continu ed un til thermal equ ilibrium is reached . Temperaturesshall be taken \ rhere possible during operation, as well as wh en the supp lyto the transformer is switched -off. The test shall not be regard,ed as com-pleted un til the temperatu re-rise incremen t is less than 3C in 1 hou r.The method shown in Fig. 1 shall be employed for the determin ation ofthe final temperature-rise.4.5.2 Method b - It shall bc dem onstrated that the top oil temperature-rise does not vary m ore th an 1C p er hour d urin g 4 consecutive hou rlyreadin gs. If the test is performed initially w ith reduced cooling or withou tany cooling, it shall be contin ued for sufficient time w ith full cooling toprevent errors in the measurement of the final oil temperature-rise.

4.6 Te s t Me th od fo r Dr y -Type Tr a n s fo r m e r s* -Th e method shahbe one involvin g excitation of the core at norm al flu x den sity. The in pu ttest curren t It shall be held constan t at a value as near as possib le to therated value Ix and at least equal to 90 percent of this value, and the runcontin ued u ntil the temp erature-rise, et, of the wind ings, is steady.The temp erature-rise of the win din gs above the temperature of the coolir@air. for rated load condition s, 0x, is calculated from the formu la :

The value of q shall be taken as follows :AN transformers = I.6.4F tran sform ers = I.8NOTE - For l oad in g m eth od scc 4.8.

*It is assure-d that the transform er has n o tapp ings. or if it has, th at the t,est is mad e on th eprincipal tapping. If the test is mad e on ahother tapping, it is necessary to replace in thetext the word s rated voltage and rated current by approp riate tapp ing voltage an dapp ropriate tapp ing current.IO

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IS : 2026 (P ar t I I ) - 19774 .7 Tes t Me thod fo r Oi l-Imm er sed Type Tra n s fo rm er s p - Tem-peratu re-rise tests of oil-immersed type transformers include the determina-tion of top oil tcmpcraturc-rise and of wind ing temp erature-rises.

XOTE 1 -During the test the power required for the pu mp s and fans may bemeasured.IOTE 2 -For loadin g meth od SEC .8.

4 . 7 . 1 To! Oil Tenrjerature-Rise - Tbc top oil temp erature-rise shall beobtained by subtracting the cooling medium test temp erature from themeasured top oil tcmpcrdtu rc, the transformer being supp lied w ith the totallosses. The input p owrr shall be maintained at a steady value.If the total losses (taken as the sum of the measured load losses, correctedto the refcrcncc temperature and th e measured no-load lossts) [see 16.1and 16.4 of IS : 2026 (Part I)-1977t] cannot be obtained , d ifferent losses, as

Ilear ac po%siblr to the above losses, bu t in any cave not less than 80 percent,&all be sup plied and the following correction factor ap plied to the top oiltemp erature-rise SO dctermincd :c

Iolal losses .YPest losses 1Thealue 0f.r shall be as follows :For natural air circulation : 0.8For forced air circulation and water cooling : I.0

SOTE - I n c a s t ' s wh ere irstin g f acililies are not available for carrying out the test at80 percent of the total losses, the losses at wh ich the test is to be condu cted and the detail sof the test shall be subject of agreement betw een the manufacturer and the purchaser.4.7.2 W inding Temperature-R ises - Winding temperatu re-rises shall beobtained on all wind ings by subtracting the external cooling med ium testtemperature from the average temperature of the wind ings as measured byresistance, after circulating the rated curren t at rated frequency in theGxling un der test.If the rated current cannot be sup plied, the tests may be performed witha current not less than 90 percent of the rated current.-4lternatively, a curren t p rovid ing the total losses may be sup plied. I,neither case the following correction factor shall be applied to the determinetltemp erature-rise of the wind ings above average oil temp erature :

cRated current _y

_ Test curren t 1The value of y shall be as follows : For natural-and forced non -directed oil circulation : 1%For forced d irected oil circulation : 2.0

*It is assured th at th e transformer has no tappings, or if it has, th at the test is mad e on theprincipal tapping. If the test is mad e on another tapping, it is necessary to replace in thetext the words rated voltage and rated current by app ropriate tapp ing voltage andappropriate tapping current.tSpecification for pow er transformers: Part I Gen eral.

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IS : 2026 (P a r t II) - 1977NATE - In casts where testing facilities are not availabl e for carrying out th e test at

90 percent of rated current the current at which the test is to be condu cted and the detailsof the test &al1 be subject of agreement between the manufacturer and the purchaser.The average oil temperature may be determined by any of the followingways :a) For all cooling meth ods the average oil temp erature in the stirround-ings of the different win din gs shall be calculated accordin g toFig. 2 from th e resistance R where th e straight line L cuts th e ordi-nate. The win din g resistances shall be determ ined after switching-offthe sup ply accordin g to 4.9.

tL R2

\ \t, w ,, Y \

TEST READINGS(corrected for diffrrmcein average oil t rmpora -

--tI

__ -i

i i-At+At+At4AatYI I I IIp ; 1 ( 1AR I cRESISTANCE .INCREMENT TIME

(All three scales are linear)F I G. 2 METHO D FOR DETERMIN ING HE WINDIN G RESISTANCEAT THEINSTANTOF SWITCHING -OFF HE SUPPLY, ANDTHE AVERAGEOILTEMPERATURE

b) In transformers with ON , O F and OD cooling the average oiltemperature shall be determined as the difference between the topoil temperature and half the temperature-drop in the cooling cqu ip-ment.For tanks with tub es or radiators mou nted on them, the temperature-drop shall be taken as the difference between the surface tempera-tures at the top and the bottom of a cooling tub e or radiator elemen t,

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IS : 2828 (P&t ll) - 1977

c>

th e tu be or radiat or element chosen being as nea r as pra cticable tothe middle of a side of the tank.In the case of a separ at e cooler, th e tem pera tu re-drop sha ll be tak enas th e difference in read ings of th erm omet ers in therm omet er pocketsadjacent to th e main tan k in the inlet an d outlet pipes to an d fromthe cooler.For transformers having rated powers up to 2 500 kVA with naturaloil circulat ion an d plain or corr ugat ed ta nk s or with tu bes or radia-tors mount ed on th e ta nk s, th e average oil tem perat ur e-rise is ta kenappr oxima tely as 0.8 times top oil tempera tu re-rise.

Allowan ces sha ll be ma de for variat ion in average oil tem perat ur e dur ingtest s, as explained in deta il in 48.3.- It sha ll be accepted th at th e average oil temp erat ur e-rise ma y vary withth e losses accord ing to the law given in 4.7.1 for t he top oil tem pera tu re-rise.

4.8 Loa d ing Me thods* - At t he choice of th e ma nu factu rer , for t wo-winding tr an sform ers an y one of th e load ing met hods accord ing to 4.8.1to 4.8.3 ma y be applied for oil-immersed type t ra nsform ers a nd either of th eloadin g met hods accord ing to 4.8.1 an d 4.8.2 for dr y-type t ra ns form ers.4.8.1 Direct Loading Method - One winding of th e tr an sform er sh all beexcited at ra ted voha ge with t he oth er conn ected to a suita ble load suchth at ra ted cur ren ts flow in both windings. No correct ion for avera ge oil

tem perat ur e n eed be applied to th e winding temp erat ur e-rise of oil-immersedtype transformer.4.8.2 Back-to-Back Method - Two tr an sform ers, one of which is th e tr an s-form er un der test , sha ll be conn ected in par allel a nd excited at the ra tedvoltage of the transformer under test. By means of different voltage ratiosor an injected voltage, ra ted cur rent sha ll be made to flow in the tra ns-former under test. No corr ection for avera ge oil tem pera tu re need beapplied t o th e winding tem perat ur e-rise of oil-immersed type tr an sform ers.48.3 Short-Circuit Method - To determ ine th e tem pera tu re-rise ofth e oil,

th e sum of th e no-load an d load losses at th e reference tem pera tu re sha ll besup plied to th e tr an sform er, one of its windings being excited an d a n.orh ershort-circuited at its ter mina ls. The top oil temperature-rise and theaverage oil tempera tu re-rise sha ll be recorded.The inpu t sha ll th en be redu ced to a value which resu lts in the circulat ionof ra ted cur ren t at ra ted frequency in th e windings, an d th is value sha ll bema inta ined for 1 hour. The temperat ur e of th e windings shall th en bedeterm ined by th e resista nce met hod. The drop in average oil temperatureduring th is hour is ta ken into accoun t when calculating th e temperat ur e-

&e of th e windings above th e average pi1 temp erat ur e..*It is assu redhat the transformer has nd tappin gs, or if it has, that the test is mad e on theprincipal tapping. If the test is mad e on another tapping, it IS necessary to replace in thetext the word s rated voltage and rated current by app ropriate tapp ing voltage andapp ropriate tapp ing current.

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IS : 2026 (P a r t II) - 1977The temp eratu re-rise of the windings above the average oil temp eratu re,determined in the second part of the test, add ed to the average oil tempera-ture-rise, determined in the first part of the test, shall give the temperatu re-rise of the windings above th e cooling med ium temp eratu re for total losses alrated current, rated frequency and rated voltage.Alternatively, the temp eratu re-rise of the windings above the coolingmed ium temperatu re for total losses at rated current and rated voltage ma!be derived from the temperatu re-rise of the windings at the end of the runwith total losses, mentioned in the first sentence of 4.8.3, by correcting thedifference between the temperatu re of the wind ings and the average oiltemperatu re to the cond itions correspond ing to rated current and ratedfrequency.4.8.4 Loading of Multi- Winding Transformers - In the case of multi-wind ing transformers wh ere more than tw o wind ings can be loaded simu l-

taneou sly in service the temp eratu re-rise tests shall norm ally be performedby separate two-wind ing tests.In certain c a s e s he rated pow ers of the individu al wind ings m ay precludethe testing of the transformer at the full total losses.For such cases the correction of the top oil tem perature-rise of oil-imm er-sed type transformers shall be made as described in 4.7.1.Calculated corrections accord ing to 4.7.2 shall be made, where necessary,to determine the individu al winding temperature-rises at the specified load-ing combination which is the most severe for the particular winding. Inthese calculations the manu facturer may take account of the stray andedd y curren t losses at this loading combination.4.9 T e m p e r k t n r e C or r e c t io n fo r C oo li n g o f T r a n s fo r m e r s Aft e rs wi t ch in g -o ff t h e Su p p Iy

4.9.1 General -Winding temperatu re measurem ent may be mad e whilethe tran sformer is in operation by the superposition method mentionedin 4.3 or by taking resistance readings after the sup ply to transformer isswitched-off.In the latter case, to provide for the interval between the instant ofswitching-off the supply and th e measurem ent of the temperature, a correc-tion shall be app lied so as to-obtain as nearly as practicable the temperatu reat the instant of switching-off the sup ply.Readings shall be taken as soon as possible after switching-off the sup ply,but allowing sufficient time for the ind uctive effect to disappear , as indicatedfrom the cold resistance measurem ents [see 16.2.1 of IS : 2026 (Part I)-1977*1.When the supply to transformer is switched-off, the fans and water pu mp sshall be stopped but the oil pu mp s shall remain running.Correction of the temp eratu re-rise as determined by the resistance methodto the instant of switching-off the sup ply shall be made by extrapolation back

*Spcciftcation or power ransforrncrs: art I General.15

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IS : 2026 (Par t I I ) - 1977to the instant of switching-off the sup ply from time/ temp erature curves ortime/ resistance curves.

4.9.2 Method of Extrapolation Using Linear Scales-The correction shallbe determined app roximately by making a series of resistance measurementsand from this plotting a time/ resistance curve, which is extrapolated backto the instant of switching-off the sup ply. The highest wind ing temperatu reshall then be cakulated from the resistance at the instant of switching-offthe supply.This extrapolation shall be done according to Fig. 2, where the resistancesR are determined at equal intervals of time At.In taking actual resistance measurements by some bridge methods it ismore accurate to determine times for fixed changes in resistance AR, thatis, time as noted at the moment the indicator of the pre-set bridge passesthrough zero.

The decreases in resistance, AR, correspond ing to equal time interval.are put down horizontaJly at the app ropr iate points of the ordinate and givethe straight line L. R, is the resistance of the wind ing at the instant ofswitching-off the sup ply.4.9.3 Method of Extrapolation Using Log- Linear Scales - The difference ARbetween the measured resistance and the resistance R, correspond ing to thetemp erature to wh ich the wind ing is cooling after switching-off the sup plyshall be draw n on log-linear grap h paper with time as linear and AR asthe logarithm ic axis. The resistance R shall be chosen in such a way thatthe test points plotted ap pear almost on a straight line. The resistance atthe instan t of switching-off the sup ply shall then be equal to R + AR,,,wh ere AR,-, is found -by drawing a straight line throu gh the point on thegraph and extrapolating it back to zero time.4.9.4 Tmjwrature cf Hot-Spot in Winding - For the purpose of calibratingwinding-temperatu re indicators, the temp erature of the hot-spot in a wind-ing shall be taken as the sum of the temperature at the top oil (&+coolingair temperature) and 1 *l times the temperature-rise of the winding aboveaverage oil temperature.

APPENDIX A(CZuwe4.3)TEMP EBATURE-RUE OF WINDINGS BY RESISTANCEMEASURED BY THE SUPER PO SITIO N METHO D

A-l. PRI NCIP LE OF THE METH ODA-l.1 A small auxiliary dc cur rent, sup plied preferably from a storagebattery is shperposed on the ac load current in the transformer wind ingun der consideration. Measurements are mad e pf the magn itude of thesup erposed cur rent circulating in the wind ing and also the voltage d rop at

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IS : 20 26 (Par t I I) - 1 97 7the terminals of the winding due to this direct current. These measurem entsare mad e at least at the beginning and end of the temperature-rise test andare used to determine the mean temperature of the windings by the variationof resistance, the accuracy being in the ord er of 1C.The method is equally applicable to temp erature-rise tests carried ou t bymeans of the direct loading, back-to-back or short-circuit methods.The test arrangements vary accord ing to the wind ing connections; thetwo most frequent connections are described in A-2 and A-3.A-2. STAR WINDINGS WITH NEUTRAL BROUGHT OUTA-2.1 The injection of dc through the wind ing neu tral does not present anydifficulty. Nevertheless it is necessary to arrange for a return neu tral pointwh ich w ill generallv be that of the sup ply transformer or a neu tral pointof the external circuit, or, in the case of a shor t-circuited wind ing, theshor t-circuit connection at the phase terminals. One point of the injectioncircuit shou ld preferably be earthed .Fig. 3 shows the circuit for a star/ star transformer with the neutrals brou ghtout from both wind ings, the temperature test being carried out by the short-circuit method.For each of the tw o windings, the sum I of the dc currents injected intothe three phases is measu red by means of a millivoltmeter connected across ashunt placed in the injection circuit.For the energized w inding, a microamm eter is connected between theneutral point of the wind ipg and the neutral point of three reactors connec-ted in star to the phase terminals. These reactors are for the purpose oflimiting the circulation of alternating current, and to &is effect, voltagetransformers may be used. Provided that the resistance of the voltagetransformer winding in sach phase is equal, then the microamm eter measu resa current i proportional to the sum of the dc voltages at the terminals of thethree phases of the transformer.

If R s the mean resistance of the three-ph ase wind ings of the transformerund er test, ri the resistance per phase of the reactors or voltage transformerand r the circuit resistance, thenR= f (3r+5)

In the case of the windings on short-circuit, r again being the resistanceof the voltage measuring circuit the relation becomes simp ly :R=f~3r

A-3. DELTA WINDING OR STAR WINDING WITH OUT ACCE!W-BLE NEUTRALA-3.1 Such wind ing connections do not lend themselves to the injection ofdc when they are short-circuited. The phase terminals shall necessarily be

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IS : 2026 (P a r t II) - 1977connected to an externah,circuit such as a sup ply transformer or a loadingcircuit.The dc injection may be mad e through one of the phase terminals bu t thethree phases of the wind ing do not play symmetrical roles and intervene inthe measurement with different weights.Return of the dc may often be effected by an accessible neutral point ofthe external circuit, which is then earthed , The injection circuit mustinclud e in series a reactor capable of withstand ing the ac ph ase voltage.In the absence of such an available neutral point in the internal circuit,return of the dc may be effected in anoth er way, for example, by a secondphase terminal. One point in the injection circuit is then fixed at earthpotential and reactors interposed between this point and the two-ph aseterminals used.Fig. 4 show s the circuit for a delta-connected winding and where theneutral point is available on the sup ply transformer.The dc is injected via terminal U,, and th e measuring shu nt shall be placedbetween the terminal and the point ofinjection. The shunt is thus at phasevoltage and the millivoltmeter shall be read at a distance.The direct current I which flows via terminal Vi divides equally betweenthe two-phase windings each of resistance R connected to this terminal andflows out via terminals Vi and W,.To measure the dc voltage between terminal U, and the combination ofterminals I, and W,, three reactors are arranged as show n in the figure.These may consist of voltage tr ansformers sup plemented , if necessary, byadd itional resistors, the primary pu rpose of wh ich is to ensure that overallresistances r, and r3 are equal. Adequate capacitances are connectedbetween the ends of the reactances and earth to suppress residual alternat-ing voltages.If i is the current in the microamm eter, the mean resistance of the twophases included in the measurement is :

A-4. MEASURING EQUIPMENTAA.1 S h u n t s - The shun ts placed in the neutral connection do not pre-sent any difficulty. For the shu nt inserted in a phase conductor, in thecase of a delta-connected winding, it is necessary to consider possible ther-mocoup le effects due to the alternating current which is of the order of 100times the dc measuring current. It shall therefore be constru cted withparticular care to completely eliminate these therm ocouple effects.A-1.2 Mic r oa m m e te r a n d Mil livol tme t e r - The se instruments shallbe accurate and linear. The accuracy class shall be 0.5. Roth instrumentsshall be filtered so tha t the presence of alternating current does not affect theinstrument.

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BUREAU OF INDIA@ STANDARDSfleedquertm :Menak Bhavan, 9 Bahadur Shah Zafar Marg. NEW DELHI 11 (KXt2Tefaphorias 331 01 31 lolagrams : Manaksansths

331 13 76 (Common to all OffiiTelephoneegion81 Offices :Central : Manak Bhavan, 9, Bahadur Shah Zafar Marg.NEW DELHI l10002 Eastern : 1114 C.I.T. Suhama Vll M,V.I.P. Road, .Maniktola, CALCUTTA 700064Northern : SC0 445-448, Sector 35-C CHANEBGARH 180036

tW= : C.I.T. Campus, IV Cross Road, MADRAS 600113*Manakalaya, E9 MIDC. Marol. Andhari (East), BOMBAY 4OfX93--&*Ilcb Offices :.PushpA, NurmohamedShaikh Marg, Khanpur, ANMADABAD 38ooolt Paanya Industrial Araa;lstStaga. Bangalore-Tumkur Road.BANGALORE 680968GaB~o$~~&35t+fldgr, Bltadbhada Road. T.T. Nagar.Plot No. 82183, Labis Ruad, BHUBANESHWAR 761002Kalai Katftir Building>,&48-A Avanad Road, COIMBATORE 841837Quality Marking Carma! N.H. IV, N.1.T.. FARIDABAD t2lb91Savitri Complex, 116 0. T.Road, GHAZIABAD 20100153/5 Ward No. 29, R.G. Barua Road. 6th y-lane.

GUWAHATI 781OC3 BS-8-58C L. N. Gupta Marg, ( Nampally Station Road)HYDERABAD 59goglR14 Yudhist& Marg, C Schar& JAIPUR 392&117/418 B Sarvodaya Nagar, KANPUR 2o8005Plot No. A-9, House No. 581/83. Sindhu Nagar, Kanuur React.LUCKNOW 226005Patliputra%dustrial Estate, PATNA 89W13D?tri& a;m&;;tre Complex, Bagh-e-Ali Maidan.SRfNM

T. C. No, 14/1421, University P. 0.. Palayam,THIR WANANTHAPURAM 895034inspection Offices (With Sala Paint) :Pushpanjali. First Floor, 205-A Wast High Court~Road.Shankar Nagar Square, NAGPUR 449910Institution of Engineers (India) Buffding, 1332 Shivaji Nagar.PUNE 411006Sales Office Calcu~is at 5 Chominghaa Approa&,P. 0. Princep Street, CALCUTTAt Salds OffNa is at Novelty Chambers, Grant Road, BOMBAYf sSl~/Gffi~ is at Unity Building, Narasimharaja Square,BANGALORE

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Reprography Unit, BIS. New Delhi, India

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AMENDMENT NO. 2 OCTOBER 1984 .TO

I S: 2026( Par t 2) - 1977 SPECI FI CATI ONTRAHSFORMERS

PART 2 TEMPERATURE- RI SE(First Revisial

(Page 6, cZcruse.3.1) - Delete paraclause.

2 of this

[Page 7, Tab&z 4, co2 (1) 072d (2)] - Substitute

FOR POJ ER

tk follovilq for tk existing mtter a.gainst l No. i1sod II) azri emmkr subsequent Serial Aos. accordin@:c.

(1, (2)i) Windings (temperature-rise measured byresistance method) temperature class of

insulation A(Rzge 8, clame 3.3) - Add the folloving nev

clause after this clause:"3.4 Choice of TaD~~_f~r_T~meeLe~ise Test -aa----- -m-w--Temperature-rise test shall be performed at the tappingas desired by the purchaser. If nothing has been stated'Bf he purchaser, the test shall be carried out asindicated belov:

a) For tapping ranges less than or equal to10 percent tapping OE negative side, the testshall be performed on the losrest ap at appro-priate current relating to this tapping; and

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b) Fos tappir .): ran ,-es exceedin: 10 per c~~.t c::negat ive side, ti:e tea:; sha ll k perform 3at -10 Dercent tk ;?ing vitn appr opr iat e c?L;rrer.:relat i.ng to th is xpping.

ROTE 1 - ;Sni7 e above rcent ioned ta pping3fi3its my be ap?li.csble in respect of ten per-at ur e-rise test mly, th ere sha ll be no injur yto the trmrforrizr unen deliverir ,g th e appro-pria te ra ted cur zxt on ar !y ta p including th eCXke!iIe ne -olive Lap.

NOT2 2 - A t rz?.sfGr?;.er rxa y be evected toopera te h-ith C3t p5rm r:ent injur y so long as itis opera ting vit k.in the obsolute temperat ur elimit s an d. oth er ccnditions specifi.e3. in1~:6600-1072 GuiZe for loadin s of oil irmrse2transfomers .

kpr ography U nlit, IS, New Dcllli,India