IS I 3151 • 1982

Indian StandardSPECIFICATION FOR

EARTHING TRANSFORMERS

(First RevIsion)

o. FOR EW 0 R ·D.0.1 This Indian Standard ( First Revision) was adopted by the IndianStandards Institution on 26 April 1982, after the draft finalized by theTransformers Sectional COFmittee had been approved by the Electro-technical Division Council.0.2 Earthing transformers provide artificial neutral and are used for thefollowing purposes:

a) To earth an otherwise unearthed system;b) To connect an arc suppression coil; andc) To limit the CUI rent during a line to earth fault due to the imped-

ance of the coils and the possible additional resistors, permittingselective pl'Ctection.

0.2.1 Earthing transformers may .be connected in any of the mannersgiven below:

a) Inter connected star ( zig-zag )/-b) Inter connected star ( zig-zag )/starc) Inter connected star ( zig-zag )/,lelta

d) Star/deltaWherever ~econdarywinding is provided, it can be used for supplying

an auxiliary load.0.3 This standard was first published in 1965. The revision has beencarried Ollt to take into account the experience gained in the field sincethe publication of earlier ver~ion..

3

IS : 3151- 1982

0.4 The importance of carrying out short-circuit test on earthing trans-formers is realized. However-, in view of limiting testing facilities forcarrying out such tests in this country, this test has been included as aspecial test to be done by mutual agreement between the purchaser andthe manufacturer ( see 6.2 ).0.5 This standard is to be read in conjunction with IS : 2026 ( Part I )-1977., IS: 2026 (Part II )-1977t, 1S: 2026 (Part III )-198It, andIS: 2026 (Part IV )-1977§ to which a reference has been made. Should,however, any deviation exist between the requirements of IS: 2026( Part I )-1977., IS: 2026 ( Part II )-1977t, IS : 2026 ( Part" III )-1~8It,IS : 2026 ( Part IV )-1977§ and those of this standard, the provisions ofthe latter shall apply.0.6 In the preparation of this standard, assistance has been derived fromt.he following publications:

IEC Publication 289 ( 1968) - Reactors issued by InternationalElectrotechnical Commission.

BS 4944:1973 Reactors, arc s~ppression coils and earthing transfor-mers for electric power system issued by British Standards Institu-tion.

0.7 For the purpose of deciding whether a particular requirement of thisstandard is complied with, the final value, observed or calculated, expres-sing the result of a test, shall be rounded off in accordance with IS : 2-196011. The number of significant places retained in the rounded offvalue should be the same as that of the specified value in this standard.

1. SCOPE1.1 This standard specifies the requirements and tests for oil-immersedand dry type earthing transformers.

2. TERMINOLOGY2.0 For the purpose of this standard, the following definitions shallapply .

•Specification for power transformen: Part I General (first rerrision ).tSpecification for power transformers: Part II Temperature-rise (}irst revision ).tSpecification for power transformers: Part III Insulation level and dielectric teat

(second revisIon).§Specificationfor power transformen: Part IV Terminal marking. tappings and

connections (first revision).IIRulesfor rounding off numerical values ( revised ).

4

IS : 3151 • 1982

2.1 Rated Voltage - The voltage for which the earthing transformeris designed. Unless systemoperating conditiom justify the selectionof ahigher value, the rated voltage is equal to the line-to-linenominal voltageof the associated system.2.2 Contlnuou. Rated Curr~t - The continuous maximum permis-sible current in the neutral in permanent duty conditions, the apparatusbeing energized at rated voltage and frequency. This quantity isspecified only when the apparatus is designed for the connection of singlephase loads between tQMleutral and the line terminals. A continuousrated current in 3 phases may be specifiedwhen a load is connected to asecondary winding.2.3 Rated Short-Time Current - The fault current through theneutral which the apparatus is designed to,carry for a specified dura-tion.2.4 Zero-Sequence Impedance - The impedance in ohms per phaseat rated frequency corresponding at the appropriate reference tempera-ture to the flow of rated short time current between the line terminalsconnected together and the neutral.

3. REQ.UIltEMENTS

3.1 General - In addition to the requirements given in this standard,the earthing tIansformers shall also conform to the requirements ofIS : 2026 (Part I )-1977*, IS: 2026 (Part II )-1977t, IS: 2026(Part III )-1981: and IS : 2026 ( Part IV )-1977§, wherever applicable.

3.~ Ability to.Withstand Rated Short-Time Current

3.2.1 Earthing tr~nsformersshall be capable of withstanding the ratedshort-time current for a period of 30seconds, unlessotherwise specified.

3.3 Temperature Rise

3.3.0 The permissible temperature-rise of earthing transformers shallcorrespond to the respective purpose of use.

3.3.1 In the case of earthing transformers for auxiliary loading thetemperature-rise shall be the same as those for transformers [see IS : 2026( Part II )-1977t ].

*Specification for power transformers: Part I General (first"revisiDn ).tSpecification for power transformers: Part II Temperature-rise (first revisiDn ).tSpecification for power transformers: Part III Insulation level and dielectric tests

( second revision ).§Specification for power transformers: Part IV Terminal marking, tappings and

.connections (first revision ).

5

IS : 3151 .• 1982

3.3.2 In the case of earthing transformers for connection to arc sup-pression coil; the permissible temperature-ri£e shall be the same for whichthe arc suppression coil is designed.

3.3.3 If the earthing transformer limits the earth fault current, thetemperature-rise shall be the same as for a transformer during short-circuit. This can be ascertained by calculating the values of tem-perature in accordance with 9.1.5 ofIS : 2026 ( Part I )-1977 ••

4. TOLERANCES

4.1 The following tolerances .shall be permissible:

a) Zero sequence impedance 1 + 20with auxiliary winding ~ _ 0 percent of declared valueopen ( if provided ) J

b) No load losses 1c) Load losses and impedance ~ In accordanct' with Table 7 of

voltage ( in case of an ()auxiliary winding being IS : 2026 Part I -1977·present) J

5. RATING PLATE

5.1 The following information shall be given on the rating plate of theearthing transformers:

a) Kind of transformer ( earthing transformer );b) Nu~ber of this standard, RefIS : 3151;c) Name of the manufacturer;d) Manufacturer's serial number;e) Year of manufacture;f) Number of phases;g) Rated frequency;h) Rated voltages;j) Connection symbol;k) Type of cooling;

m) Total mass;n) Mass and volume of insulating oil or class of insulation (for dry

transformers ), as applicable;

·Specificatiop for power transformers: Part I General (first revisioll ).

6

IS : 3151 • 1982

p) Temperature rise;q) Insulation levels;r) Tranqportation mass (for transformers exceeding 5 tonne

mass );$) Untanking mass (for transformers exceeding 5 tonne mass);t) Insulating liquid ( if not mineral oil );u) Rated short time current and duration; andv) Measured zero-sequence impedance.

5.1.1 The following information shall also be marked when unit~upplies an auxiliary load:

a) Rated continuous current;b) Rated kVA; andc) Percentage impedance voltage ( two winding ).

.5.2 The earthing transformers may also be marked with the ISI Certifi-cation Mark.

Non: - The .useof the ISI Certification Mark is governed by the provisions of theIndian Standards Institution (Cf'rtification Marka) Act and the Rules and Rpgu-lations made therf'undf't. The ISI Mark on products covered by an Indian Standardconveys thl' auurance that they have been produced to comply with the require-ml'nts of that standard undl'r a wf'lI-definedsystf'm of intpection. testing and qualitycontrol which is devised and supl'rvist.'dby ISI and operated by the producer. ISImarkl'd products are also continuously checked by ISI for conformity to tbatstandard as a further safeguard. D"tail. of conditions under which a licence for theuse of thf' ISI Cl'rtification ~ark ·may be granted to man~acturers or processor.,may be obtained from the Indian Standards Institution.

6. TESTS6.0 General - Tests shall be conducted in accordance with IS: 2026(Part I )-1977*, IS: 2026 (Part' II )-1977t, IS: 2026 (Part 111)-1981: and IS: 2026( Part IV )-1977§, unless otherwise specified.,6.1Classification of Tests

6.1.1 Type Tests - The followingshall constitute type tests:a) Measurement of win:\ing re~istance,b) Measurement of no-load lo~sand no-load current,

·Specification for power transformers: Part I General (first rn;si",. ).tSpecification for power transformers: Part II Temperature-rise ( /irst r.oisi",.).tSpecification for 'power tralbformers: Part III Insulation level dielectric test.(suond m.ision).§SpecificaUQDfor power transformers: Part IV Terminal markings, tappings and

.connections(first r.visi4n ).

7

IS : 3151 - 1982

c) Measurement of zero-sequence impedance,d) Measurement of insulation resistance,e) Induced voltage withstand test,f) Separate source voltage withstand test,g) Lightning impulse voltage withstand test,h) Temperature rise ( 3.3 ).

6.1.1.1 The following tests shalI also be applicable when the unitsupplies an auxiliary load:

a) Measurement of impedance voltage and load losses, andb) Voltage ratio and voltage vedor relationship.

6.1.2 Routine Tests - The following shall constitute routine tests:

a) Measurement of winding resistance,b) Measurement of no-load loss and no-load current,c) Measurement of zero-sequence impedance,d) Measurement of insulation resistance,e) Induced voltage withstand test, andf) Separate source voltage withstand test.

6.1.2.1 The following routine tests shall also be applicable when theunit supplies an auxiliary load:

a) Impedance voltage and load loss, andb) Voltage ratio and voltage vector relationship.

6.1.3 Special Test - When agreed between the purchaser and thesupplier short circuit te~t may be carried out as a special test in accord-ance with 6.2.

6.2 Short-Circuit Test

6.2.1 This is a special type test which may be carried out by mutualagreement between the purchaser and the supplier. Type test certificatesmay be accepted in lieu of the type test.

6.2.2 The earthing transformer shall be' excited at rated voltage froma three-phase supply and a line-to-earth fault shall be applied by connec-ting one line terminal to the neutral either directly or through the earth-ing impedance as appropriate, in such a manner as to obtain maximumasymmetry on the first half wave of the fault current Ten short-circuitapplications shall be made at suitable intervals, each for the duratioil of0'2 seconds ( 10 cycles ).

B

IS : 3151 - 1981

6.2.2.1 Tile nl,iJir}' of th,. ('nl/hjn1~ Iramfonlll'r 10 with~land thet('~t ~hall he df'tf'llllilll'" in till' ~:llIle 1Il:lIl11N as spc('ifi,~t1 in IS: 2026(Pnrl I )-)!}77·.

6.2.2.2 For I':HtJ.ill,L~ trnn~r()rlllf'rs clI1ployed 10 connert an arc-suppn'~sion roil two slllnt-,'ircllit nPl'lirntiolls arl' "(,l'llled sullicicnl.

NOT!: -•. /Iy :>I:"'f'III"'" ""(""""" rl.f' I",rd"" •.r ;1",111 ••. ~III'I'Ji('r.a Ihrt'("l'h:>,r "'.\1In:\Y hI' rrpl:\fl'd "r a ~illJ.:II'·pl",,(· L•." uf "qual 'r\'l~rilr.

6.2.3 In rase allxiliary windinR is \\Sed in transformer for supplying anauxiliary 10<\<1, suhjrrt to :l~rCf'JJ1rnl Iwt"'r!'ll tll(' purchaser and thesupplirr, as :In :lltl'rll:lte to I('sts of 6.2.2, the trallSforlller may be testedfor shor.t circllit trsl in accordance with IS: 2026 (ParI I )-1~'77•. Inthat Ca~e the transformer shall he excited at rated voltage from a three-phase supply and auxiliary winding shall be shorl circuited.

·Sprcific:\lion for !'own tr:1mf"rlll"rs: I':>rl I Gem'nll (firsl rtlisi071).

9

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