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IS 2026 Part 6 ) : 1994

ndian StandardPOWERTR NSFORMERS

PART 5 TRANSFORMER/REACTOR BUSHINGS - MINIMUM EXTERNALCLEARANCE IN AIR - SPECIFICATIONSecond Reprint OCTOBER 1997

UDC 621*315*626*042*4 : 621*314.212+621-318.43

Q BI S 1994BUREAU OF INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SljAH ZAFAR MARG

NEW DELHI 110002

( Reaffirmed 1999 )

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Transformers Sectional Committee, ETD 16

FOREWORDThis Ir,dian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by theTransformers Sectional Committee had been approved by the Electrotechnical Division Council.In the preparation of this standard considerable assistance has been derived from IEC Pub 76-3-l( 1987 ) Power Transformer : Part 3 lnsulation levels and dielectric tests External clearances in airissued by the International Electrotechninal Commission ( IEC ).For the purpose of deciding whether a particular requirement of this standard is complied wilh, the finalvalue, observed or calculated, expressing the result of a test or analvsis shall be rounded off in accordancewith IS 2 : 1960 Rules for rounding off numerical values revised ). The number of significant placesretained in the rounded off value shall be the same as that of the specified value in this standard.

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I ndian St andardIS 2026 ( Part 5 ) : 1994

POWERTR NSFORMERSPART 5 TRANSFORMER/REACTOR BUSHINGS-MINIMUM EXTERNAL

CLEARANCE IN AIR - SPECIFICATIONP SCOPEThis Indian Standard specifies clearances in airbetween live paIts of bushings on oil-immer;sedpower transformers and reactors and to object atearth potential.

NOTE - Wherever the word transformer appearsin the text, read as transformer/reactor.2 REFERENCESThe Indian Standards listed in Annex A arenecessary adjuncts to this standard.3 GENERAL3.1 Clearances m air are understood to mean, inthis standard, distances where the electrostaticfield is free from disturbance by insulatorbodies. The standard does not deal with therequirements of effective flashover distance orcreepage distance along the bushing insulatorsnor does it consider the risk From intrusion ofbirds or animals.3.2 The insulation levels and dielectric testswhich are specified in IS 2026 ( Part 3 ) : 1981shall apply for oil immersed transformers to theinternal insulation only.3.3 It is reasonable that the rated withstandvoltage values which are specified for the inter-nal insulation of the transformer should also betaken as a reference for its external insulation.This may, however, not be true in all cases. Afailure of the non-self-restoring internal insula-tion is catastrophic and normally leads to thetransformer being out of service for a longperiod, while an external flashover may involveonly a short interruption of service withoutcausing lasting damage. Therefore, it may bethat, for increased safety, higher test voltagesare specified by the user for the internal insula-tion of the transformer than for the externalinsulation of other components in the system.If such a distinction is made, the externalclearances on the transformer should be referredinstead to those specified for external insulationwithstand voltages.3.4 When establishing the requirements of thepresent standard in the higher voltage ranges, ithas been recognised that the bushing endsnormally have a rounded electrode shape. Theclearance requirements are valid between such

rounded electrodes. It is assumed that conduc-tor clamps with their associated shield electrodesare suitably shaped so that they do not reducethe flashover voltage. It is also assumed that thearrangement of incoming conductors does notreduce the effective clearances provided by thetransformer itself.NOTE - If the user intends to make his connectionin a particular way which is likely to reduce theeffective clearances, this should be mentioned in theenquiry.

3.5 In general, the provision of adequateclearances in air becomes technically difficultmainly at high system voltages, particularly forrelatively small units, or when the installationspace is restricted. The principle follo\bed in therecommendations of this standard is to provideample, non-critical clearances which are satis-factory without further discussion or proofunder various system conditions and in differentclimates. Where extensive previous experiencehas indicated that smaller clearances areadequate, there is obviously no need to change.3.6 The recommended clearances are referredto the rated withstand voltage values of theinternal insulation of the transformer, unlessotherwise specified in the enquiry and order.When the clearances of the transformer areequal to or larger than the values specified inthis standard and the bushings have properlyselected ratings according to IS 2099 : 1986then the external insulation of the transformershall be regarded satisfactory without furthertesting.

NOTES1 The impulse withstand strength of the externalinsulation is polarity dependent, in contract to whatis assumed for the internal insulation. The testsprescribed for the internal insulation of the trans-former do not automatically verify that the externalinsulation is satisfactory. The recommendedclearances are dimensioned for the more onerouspolarity ( positive ).2 It is recognised that in some casesgood experiencewith well documented national practice allows thecontinuing use of established smaller clearances with-out further discussion or testing.

3.7 If a clearance smaller than that accordingto the paragraph above has been used for acontract, a type test may be required on anarrangement simulating the actual clearance, or1

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IS 2026 Part 5 ) : 1994Table 1 Recommeded Minimum Clearances from Bushing Live Parts on Power TransformersHaving Windings witb Highest Voltage for Equipment U, 300 kV, SpecifiedAccording to Method 1 Altitude Up to 1 000 m )Clauses 4.1 and 4.2 )

Highest Voltagefor EF;pment( rms )

1)kV

Rated Short Dura-tion Power Fre-quency WithstandVoltage( rms )2)kV .

Rated Lightn- Minimum Clearanceing Impulse r------- h-_-----~Withstand Phase-to- Phase-to-Voltage Earth Earth

p3) 4) 5)kV mm mm

46

9.50

1 0501 0501 175

1 9002 3002 3002 700

2 2502 6502 6503 100

1 3001 425

2 7003 100

3 1003 500

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IS 2025 Part 5 ) : 1994Table 3 Recommended Minimum Clearances from Bushing Live Parts on Power TransformersHaving Windings with Highest Voltage for Equipment iJ, > 300 kV, SpecifiedAccording to Method 2 Altitude Up to 1 000 m )

Clauses 4.3, 4.3.1.3.~ and-4.3.2Highest Voltage Rated Switchingfor Eq;i;ment Impulse Withstand Rated LightningVoltage Impulse With-

rms) peak )stand Voltage

peak 1

1) 2)kV kV75

365

36295

42

1 5

5251175

765

3)kV65

95

(4) (5)mm mm

1 900 2 250

Voltage(6)mm16001 750

1 5

1175

2 300 2 650 1 9502 700 3 100

220013 24001425

155

18

195

3 100 3 5co 2 6503 700 4 200 2 8505 000 5 800 3 3005 800 6700

3 600

LMinimum Clearance~---_-----_----h ---------7Phase-to-Earth, Phase-to-Phase, To OtherBased on Swit- Based on Swit- Winding,thing Impulse thing Impulse Based onWithstand WithstandVojtage LightningVoltage ImpulseWithstand

Dm 1

5

4

3

2

1i i i i i i i

500 1000 1500 2ooq

6

kV

Fan. I CLEARANCEPHASE-TO-EARTH BASEDON RATED SWITCHING IMPULSEWITHSTAND VOLTAGE

4

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IS 2026 Part 5 ) : 1994

ma

1 1500 2000 2500FIG. 2 CLEARANCEHASE-TO-PHASEBASEDON SWITCHING IMPULSEVOLTAGE APPEARING

BETWEENPHASE

to IS 2165 ( Part 2 ) : 1983 ( see Table 4 ). Anappropriate test procedure involves positivepolarity impulses for a configuration phase-to-earth, and opposite polarity impulses for phase-to-phase clearances ( see 4.3.3 ). This has beenconsidered for the clearance values given inTable 3.4.3.1 Clearance Phase-t o-Eart h, Phase-t o-N eut raland Phase-t o-Phase Betw een Phases of t he SameWindingThe clearance from the high voltage bushingtop to earth ( tank, conservator, cooling equip-ment, switchyard structures ) or to the neutralterminal is determined from column 4 ofTable 3. The clearance between bushing caps ofdifference phases is determined from column 5of Table 3.4.3.2 Clearance Betw een Termi nal s of D i fferentWindingsThe clearance between terminals of differentwindings of the transformer shall be checkedwith regard to both switching, impulse andlightning impulse conditions. The switchingimpulse withstand requirement is based on theprecalculated difference voltage which appears

between the two terminals under considerationduring the switching impulse test [ see 14 ofIS 2026 ( Part 3 ) : 1981 ] according to the turnsratio between the windings. This voltage.valuegives the clearance which is required with regardto the switching impulse condition. Fig. 2 isused to find the recommended clearance if theterminals receive opposite polarity voltages andthe ratio between their voltage amplitudes is 2or less. In other cases, Fig. 1 applies.NOTE;If Fig. 1 and Fig. 2 are compared, it appenrsthat a phase-to-phase clearance withstands a highervoltage difference than the same.distance would doin a phase-to-earth configuration. The reason is thatin the phase-to-phase configuration the two terminalsare supposed to have opposite polarity, and themaximum dielectric gradient at either of them ( whichis largely determined by the voltage to earth ) isrelatively lower.It is assumed also that the electrodes have arounded shape. The clearance shall, however,also fulfil the lightning impulse withstandrequirement, which presupposes that the lowervoltage winding terminal is at earth potentialwhen rated lightning impulse withstand voltage isapplied to the high voltage terminal. The dis-tance requirement in column 6 of Table 3 andFig. 3 corresponding to this rated lightning

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IS 2026 Part 5 ) : 1994

FIG 3 CLEARANCE BASED ON LIGHTNING IMPULSE VOLTAGEimpulse voltage, has therefore, to be fulfilledbetween the two terminals. The higher of thetwo clearance estimates shall apply. Thes\\,itching impulse test on the transformer willinduce voltages between phases of other star-connected windings as well. It shall be checkedwliether this condition requires a larger phase-to-phase clearance in such a winding than asprescribed for this \vinding alone such as in 4.1.4.3.3 Type Test Procedur eIf a tape test on a reduce clearance is to beconducted, the test procedure shall be asfoilows:

A test on a configuration phase-to-earth orphase-to-netural, or towards a terminal of alower voltage winding shall consist of aswitching impulse test, dry, with positivepolarity on the line terminal of the winding( the higher voltage winding ). The counterelectrode shall be earthed. If the testedterminal belongs to a three-phase winding,the other line terminals shall also beearthed.

NOTE - This test ih not generally feasible oncomplete three-phase transformers and may, thcre-for-e, have to be conducted on ;I model simulating theactual configuration of the transformer.Tests on the phase-to-phase clearances of athree-phase transformer shall consist of switch-ing impulse tests, dry, with half of the specified

test voltage, positive, on one line terminal, theother half, negative on another line terminal,and the third terminal earth [ see 10, 12 and 14of IS 2165 ( Part 2 ) : 1983 ] The combinationsof phase-to-earth and phase-to-phase test volt-ages, according to IS 2165 ( Part 2 ) : 1983 arereproduced in Table 4. The values forU, = 525 kV and 800 kV are subject toreview.When the outer phases are placed symmetricallywith respect to the middle phase, it is sufficientto make two separate tests, one with positivepolarity on the middle phase, and the otherwith positive polarity on an outer phase, themiddle phase having negative polarity. If theline terminal arrangement is asymmetrical, itmay be necessary to perform more than twotests.Each test shall consist of 15 applications ofimpulse voltage with wave shape 250/2 500 inaccordance with IS 2071 ( Part 1 ) : 1993.

NOTE - The above test procedure for phase-to-phase external clearances, which follows IS 2165( Part 2 ) : 1983 differs in several respects from theswitching impulse test procedure specified for theinternal insulation of the transformer in 14 ofIS 2165 ( Part 2 ) . 1983. The two test procedures donot replace each other.6

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Burenti 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 of goods andattending to connected matters in the country.CopyrightBIS has the copyright of all its publications. No part of these publications may be reproduced in any formwithout the prior permission in writing of BPS. This does not preclude the free use, in the course ofimplementing the standard, of necessary details, such as symbols and sizes, type or grade designations.Enquiries relating to copyright be addressed to the Director (Publication), BIS.Review of Indian StandardsAmendments are issued to standards as the need arises on the basis of comments. Standards are also reviewedperiodically; a standard along with amendments is reaffirmed when such review indicates that no changes areneeded; if the review indicates that changes are needed, it is taken up for revision. Users of Indian Standardsshould ascertain that they are in possession of the latest amendments or edition by referring to the latest issueof BIS Handbook and Standards Monthly Additions.This Indian Standard has been developed from Dot: No. ETD 16 ( 3486 J

Amendments Issued Since PublicationAmend No. Date of Issue Text Affected

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