EN 60076-1:1997

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Power transformers - Part 1: General (IEC 76-1:1993 modified)Leistungstransformatoren -- Teil 1: AllgemeinesTransformateurs de puissance -- Partie 1: GénéralitésPower transformers -- Part 1: General29.180Transformatorji. DušilkeTransformers. ReactorsICS:Ta slovenski standard je istoveten z:EN 60076-1:1997SIST EN 60076-1:1997en01-oktober-1997SIST EN 60076-1:1997SLOVENSKI
STANDARD
NORMEINTERNATIONALEINTERNATIONALSTANDARDCEIIEC76-1Deuxième éditionSecond edition1993-03Transformateurs de puissancePartie 1:GénéralitésPower transformersPart 1:General© CEI 1993 Droits de reproduction réservés — Copyright — all rights reservedAucune partie de cette publication ne peut être reproduite niutilisée sous quelque forme que ce soit et par aucun pro-cédé, électronique ou mécanique, y compris la photocopie etles microfilms, sans l'accord écrit de l'éditeur.No part of this publication may be reproduced or utilized inany form or by any means, electronic or mechanical,including photocopying and microfilm, without permissionin writing from the publisher.Bureau Central de la Commission Electrotechnique Internationale 3, rue de Varembé Genève, SuisseIEC•Commission Electrotechnique Internationale CODE PRIX^/International Electrotechnical Commission PRICE CODE/^Mew,ayHapoaHan 3nettrpoTexmoiecttan HOMHCCHA• Pour prix, voir catalogue en vigueurFor price, see current catalogueSIST EN 60076-1:1997

Publication 60076-1 de la CEIIEC Publication 60076-1(Deuxième édition - 1993)(Second edition - 1993)Transformateurs de puissance —Power transformers —Partie 1: GénéralitésPart 1: GeneralCORRIGENDUM 1Page 76Page 77Article A.1.2:Clause A.1.2:Au point h), dans la première ligne, au lieu deItem h), in the first line, instead of.(voir 2.1 b)).(see 2.1 b)).lireread.(voir 1.2.1 b)).(see 1.2.1 b)).Juin 1997June 1997SIST EN 60076-1:1997

76-1 ©IEC: 1993- 3 -CONTENTSPageFOREWORD 7Clause1Scope and service conditions
111.1Scope
111.2Service conditions 112Normative references 153Definitions 173.1General 173.2 Terminals and neutral point 173.3 Windings
193.4Rating
213.5 Tappings
233.6 Losses and no-load current 273.7 Short-circuit impedance and voltage drop 273.8 Temperature rise 313.9Insulation 313.10 Connections 313.11 Kinds of tests 333.12 Meteorological data with respect to cooling
334 Rating 334.1Rated power 334.2 Loading cycle 354.3 Preferred values of rated power 354.4 Operation at higher than rated voltage and/or at disturbed frequency 355Requirements for transformers having a tapped winding 375.1General - Notation of tapping range 375.2 Tapping voltage - tapping current. Standard categories of tapping voltagevariation. Maximum voltage tapping 375.3 Tapping power. Full-power tappings - reduced-power tappings 435.4Specification of tappings in enquiry and order 455.5Specification of short-circuit impedance 455.6 Load loss and temperature rise
476 Connection and phase displacement symbols for three-phase transformers 49SIST EN 60076-1:1997

76-1 ©IEC: 1993– 5 –7Rating plates 537.1Information to be given in all cases 537.2Additional information to be given when applicable
558Miscellaneous requirements
578.1Dimensioning of neutral connection
578.2 Oil preservation system
578.3 Load rejection on generator transformers
579 Tolerances 5710 Tests 6310.1 General requirements for routine, type and special tests 6310.2 Measurement of winding resistance
6510.3 Measurement of voltage ratio and check of phase displacement 6710.4 Measurement of short-circuit impedance and load loss 6710.5 Measurement of no-load loss and current
6910.6 Measurement of the harmonics of the no-load current
6910.7 Measurement of zero-sequence impedance(s) onthree-phase transformers 7110.8 Tests on on-load tap-changers
71AnnexesAInformation required with enquiry and order 75BExamples of specifications for transformers with tappings
81CSpecification of short-circuit impedance by boundaries
85DThree-phase transformer connections
87ETemperature correction of load loss
93FBibliography 95SIST EN 60076-1:1997

Six Month's RuleReport on Voting14(CO)7514(CO)7776-1 © IEC: 1993- 7 -INTERNATIONAL ELECTROTECHNICAL COMMISSIONPOWER TRANSFORMERSPart 1: GeneralFOREWORD1)The IEC (International Electrotechnical Commission) is a worldwide organization for standardizationcomprising all national electrotechnical committees (IEC National Committees). The object of the IEC is topromote international cooperation on all questions concerning standardization in the electrical andelectronic fields. To this end and in addition to other activities, the IEC publishes International Standards.Their preparation is entrusted to technical committees; any IEC National Committee interested inthe subject dealt with may participate in this preparatory work. International, governmental andnon-governmental organizations liaising with the IEC also participate in this preparation. The IECcollaborates closely with the International Organization for Standardization (ISO) in accordance withconditions determined by agreement between the two organizations.2)The formal decisions or agreements of the IEC on technical matters, prepared by technical committees onwhich all the National Committees having a special interest therein are represented, express, as nearly aspossible, an international consensus of opinion on the subjects dealt with.3)They have the form of recommendations for international use published in the form of standards, technicalreports or guides and they are accepted by the National Committees in that sense.4)In order to promote international unification, IEC National Committees undertake to apply IEC InternationalStandards transparently to the maximum extent possible in their national and regional standards. Anydivergence between the IEC Standard and the corresponding national or regional standard shall be clearlyindicated in the latter.5)The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for anyequipment declared to be in conformity with one of its standards.This International Standard has been, prepared by IEC by technical committee 14: Powertransformers.This second edition cancels and replaces the first edition published in 1976 as well as thefirst edition of IEC 76-4 published in 1976.The text of this standard is based on the following documents:Full information on the voting for the approval of this standard can be found in the reporton voting indicated in the above table.IEC 76 consists of the following parts, under the general title: Power transformers.SIST EN 60076-1:1997

76-1 ©IEC: 1993– 9 –Part 1: 1993, General.Part 2: 1993, Temperature rise.Part 3: 1980, Insulation levels and dielectric tests.Part 5: 1976, Ability to withstand short circuit.Annexes A and E form an integral part of this standard.Annexes B, C and D are for information only.SIST EN 60076-1:1997

76-1 © IEC: 1993-11 -POWER TRANSFORMERSPart 1: General1 Scope and service conditions1.1 ScopeThis part of International Standard IEC 76 applies to three-phase and single-phase powertransformers (including auto-transformers) with the exception of certain categories of smalland special transformers such as:-single-phase transformers with rated power less than 1 kVA and three-phasetransformers less than 5 kVA;-instrument transformers;-transformers for static convertors;-traction transformers mounted on rolling stock;-starting transformers;- testing transformers;-welding transformers.When IEC standards do not exist for such categories of transformers, this part of IEC 76may still be applicable either as a whole or in part.For those categories of power transformers and reactors which have their own IECstandards, this part is applicable only to the extent in which it is specifically called up bycross-reference in the other standard.*At several places in this part it is specified or recommended that an `agreement' shall bereached concerning alternative or additional technical solutions or procedures. Suchagreement is to be made between the manufacturer and the purchaser. The mattersshould preferably be raised at an early stage and the agreements included in the contractspecification.1.2 Service conditions1.2.1 Normal service conditionsThis part of IEC 76 gives detailed requirements for transformers for use under thefollowing conditions:a) AltitudeA height above sea-level not exceeding 1 000 m (3 300 ft).Such standards exist for dry-type transformers (IEC 726), for reactors in general (IEC 289), for tractiontransformers and reactors (IEC 310), and are under preparation for static convertor transformers.SIST EN 60076-1:1997

76-1 © IEC: 1993- 13 -b) Temperature of ambient air and cooling mediumA temperature of ambient air not below -25 °C and not above +40 °C. For water-cooledtransformers, a temperature of cooling water at the inlet not exceeding +25 °C.Further limitations, with regard to cooling are given for:-oil-immersed transformers in IEC 76-2;-dry-type transformers in IEC 726.c)Wave shape of supply voltageA supply voltage of which the wave shape is approximately sinusoidal.NOTE - This requirement is normally not critical in public supply systems but may have to be consideredin installations with considerable convertor loading. In such cases there is a conventional rule that thedeformation shall neither exceed 5 % total harmonic content nor 1 % even harmonic content. Also note theimportance of current harmonics for load loss and temperature rise.d)Symmetry of three-phase supply voltageFor three-phase transformers, a set of three-phase supply voltages which areapproximately symmetrical.e)Installation environmentAn environment with a pollution rate (see IEC 137 and IEC 815) that does not requirespecial consideration regarding the external insulation of transformer bushings or of thetransformer itself.An environment not exposed to seismic disturbance which would otherwise requirespecial consideration in the design. (This is assumed to be the case when the groundacceleration level ag is below 2 m/s2.)*1.2.2 Provision for unusual service conditionsAny unusual service conditions which may lead to special consideration in the design of atransformer shall be stated in the enquiry and the order. These may be factors such ashigh altitude, extreme high or low temperature, tropical humidity, seismic activity, severecontamination, unusual voltage or load current wave shapes and intermittent loading. Theymay also concern conditions for shipment, storage and installation, such as weight orspace limitations (see annex A).Supplementary rules for rating and testing are given in other publications for:-Temperature rise and cooling in high ambient temperature or at high altitude:IEC 76-2 for oil-immersed transformers, and IEC 726 for dry-type transformers.-External insulation at high altitude: IEC 76-3 and IEC 76-3-1 for oil-immersedtransformers, and IEC 726 for dry-type transformers.See IEC 68-3-3.SIST EN 60076-1:1997

76-1 © I EC: 1993- 15 -2 Normative referencesThe following normative documents contain provisions which, through reference in thistext, constitute provisions of this part of IEC 76. At the time of publication, the editionsindicated were valid. All normative documents are subject to revision, and parties toagreements based on this part of IEC 76 are encouraged to investigate the possibility ofapplying the most recent edition of the normative documents indicated below. Members ofIEC and ISO maintain registers of currently valid International Standards.IEC 50(421): 1990, International Electrotechnical Vocabulary - Chapter 421: Powertransformers and reactorsIEC 68-3-3: 1991, Environmental testing - Part 3: Guidance. Seismic test methods forequipmentsIEC 76-2: 1993, Power transformers - Part 2: Temperature riseIEC 76-3: 1980, Power transformers - Part 3: Insulation levels and dielectric tests!EC 76-3-1: 1987, Power transformers - Part 3: Insulation levels and dielectric tests.External clearances in airIEC 76-5: 1976, Power transformers - Part 5: Ability to withstand short circuitIEC 137: 1984, Bushings for alternating voltages above 1 000 VIEC 354: 1991, Loading guide for oil-immersed power transformersIEC 529: 1989, Degrees of protection provided by enclosures (IP Code)IEC 551: 1987, Determination of transformer and reactor sound levelsIEC 606: 1978, Application guide for power transformersIEC 726: 1982, Dry-type power transformersIEC 815: 1986, Guide for the selection of insulators in respect of polluted conditionsIEC 905: 1987, Loading guide for dry-type power transformersISO 3: 1973, Preferred numbers - Series of preferred numbersISO 9001: 1987, Quality systems - Model for quality assurance in design/development,production, installation and servicingSIST EN 60076-1:1997

76-1 © IEC: 1993- 17 -3 DefinitionsFor the purpose of this part of IEC 76, the following definitions shall apply. Other termsuse the meanings ascribed to them in the International Electrotechnical Vocabulary (IEV).3.1 General3.1.1 power transformer: A static piece of apparatus with two or more windings which,by electromagnetic induction, transforms a system of alternating voltage and current intoanother system of voltage and current usually of different values and at the samefrequency for the purpose of transmitting electrical power. [IEV 421-01-01, modified]3.1.2 auto-transformer*: A transformer in which at least two windings have a commonpart. [IEV 421-01-11]3.1.3 booster transformer: A transformer of which one winding is intended to beconnected in series with a circuit in order to alter its voltage and/or shift its phase. Theother winding is an energizing winding. [IEV 42-01-12, modified]3.1.4 oil-immersed type transformer: A transformer of which the magnetic circuit andwindings are immersed in oil. [IEV 421-01-14]NOTE - For the purpose of this part any insulating liquid, mineral oil or other product, is regarded as oil.3.1.5 dry-type transformer: A transformer of which the magnetic circuit and windingsare not immersed in an insulating liquid. [IEV 421-01-16]3.1.6 oil preservation system: The system in an oil-immersed transformer by which thethermal expansion of the oil is accommodated. Contact between the oil and external airmay sometimes be diminished or prevented.3.2 Terminals and neutral point3.2.1 terminal: A conducting element intended for connecting a winding to externalconductors.3.2.2 line terminal: A terminal intended for connection to a line conductor of a network.[IEV 421-02-01]3.2.3 neutral terminal:a)For three-phase transformers and three-phase banks of single-phase transformers:The terminal or terminals connected to the common point (the neutral point) of astar-connected or zigzag connected winding.b)For single-phase transformers:The terminal intended for connection to a neutral point of a network. [IEV 421-02-02,modified]* Where there is a need to express that a transformer is not auto-connected, use is made of terms such asseparate winding transformer, or double-wound transformer (see IEV 421-01-13).SIST EN 60076-1:1997

76-1 ©IEC: 1993- 19 -3.2.4 neutral point: The point of a symmetrical system of voltages which is normally atzero potential.3.2.5 corresponding terminals: Terminals of different windings of a transformer,marked with the same letter or corresponding symbol. [IEV 421-02-03]3.3 Windings3.3.1 winding: The assembly of turns forming an electrical circuit associated with one ofthe voltages assigned to the transformer.NOTE - For a three-phase transformer, the 'winding' is the combination of the phase windings (see 3.3.3).[IEV 421-03-01, modified]3.3.2 tapped winding: A winding in which the effective number of turns can be changedin steps.3.3.3 phase winding: The assembly of turns forming one phase of a three-phasewinding.NOTE - The term 'phase winding' should not be used for identifying the assembly of all coils on a specificleg. [IEV 421-03-02, modified]3.3.4 high-voltage winding*: The winding having the highest rated voltage. [IEV 421-03-03]3.3.5 low-voltage winding*: The winding having the lowest rated voltage. [IEV 421-03-04]NOTE - For a booster transformer, the winding having the lower rated voltage may be that having thehigher insulation level.3.3.6 intermediate-voltage winding*: A winding of a multi-winding transformer having arated voltage intermediate between the highest and lowest winding rated voltages.[IEV 421-03-05]3.3.7 auxiliary winding: A winding intended only for a small load compared with therated power of the transformer. [IEV 421-03-08]3.3.8 stabilizing winding: A supplementary delta-connected winding provided in astar-star-connected or star-zigzag-connected transformer to decrease its zero-sequenceimpedance, see 3.7.3. [IEV 421-03-09, modified]NOTE - A winding is referred to as a stabilizing winding only if it is not intended for three-phaseconnection to an external circuit.* The winding which receives active power from the supply source in service is referred to as a 'primarywinding', and that which delivers active power to a load as a 'secondary winding'. These terms have nosignificance as to which of the windings has the higher rated voltage and should not be used except in thecontext of direction of active power flow (see IEV 421-03-06 and 07). A further winding in the transformer,usually with lower value of rated power than the secondary winding, is then often referred to as 'tertiarywinding', see also definition 3.3.8.SIST EN 60076-1:1997

76-1 © IEC: 1993- 21 -3.3.9 common winding: The common part of the windings of an auto-transformer.[IEV 421-03-10]3.3.10 series winding: The part of the winding of an auto-transformer or the winding ofa booster transformer which is intended to be connected in series with a circuit.[IEV 421-03-11]3.3.11 energizing winding: The winding of a booster transformer which is intended tosupply power to the series winding. [IEV 421-03-12]3.4 Rating3.4.1 rating: Those numerical values assigned to the quantities which define theoperation of the transformer in the conditions specified in this part of IEC 76 and on whichthe manufacturer's guarantees and the tests are based.3.4.2 rated quantities: Quantities (voltage, current, etc.), the numerical values of whichdefine the rating.NOTES1 For transformers having tappings, rated quantities are related to the principal tapping (see 3.5.2),unless otherwise specified. Corresponding quantities with analogous meaning, related to other specifictappings, are called tapping quantities (see 3.5.10).2Voltages and currents are always expressed by their r.m.s. values, unless otherwise specified.3.4.3 rated voltage of a winding (Ur): The voltage assigned to be applied, or developedat no-load, between the terminals of an untapped winding, or of a tapped windingconnected on the principal tapping (see 3.5.2). For a three-phase winding it is the voltagebetween line terminals. [IEV 421-04-01, modified]NOTES1The rated voltages of all windings appear simultaneously at no-load when the voltage applied to one ofthem has its rated value.2For single-phase transformers intended to be connected in star to form a three-phase bank, the ratedvoltage is indicated as phase-to-phase voltage, divided by 'T for exemple Ur = 400/'3 kV.3 For the series winding of a three-phase booster transformer which is designed as an open winding (see3.10.5) the rated voltage is indicated as if the winding were connected in star, for example Ur = 23/5 kV.3.4.4 rated voltage ratio: The ratio of the rated voltage of a winding to the rated voltageof another winding associated with a lower or equal rated voltage. [IEV 421-04-02]3.4.5 rated frequency (fr): The frequency at which the transformer is designed tooperate. [IEV 421-04-03, modified]SIST EN 60076-1:1997

76-1 ©IEC: 1993– 23 –3.4.6 rated power (Sr): A conventional value of apparent power assigned to a windingwhich, together with the rated voltage of the winding, determines its rated current.NOTES1Both windings of a two-winding transformer have the same rated power which by definition is the ratedpower of the whole transformer.2 For a multi-winding transformer, half the arithmetic sum of the rated power values of all windings(separate windings, not auto-connected) gives a rough estimate of its physical size as compared with atwo-winding transformer.3.4.7 rated current (id: The current flowing through a line terminal of a winding which isderived from rated power Sr and rated voltage Ur for the winding. [IEV 421-04-05,modified]NOTES1For a three-phase winding the rated current Ir is given by:SrIr -‘/-3— A
x Ur2For single-phase transformer windings intended to be connected in delta to form a three-phase bankthe rated current is indicated as line current divided by '13 for example:I500
Ar _^3.5 Tappings3.5.1 tapping: In a transformer having a tapped winding, a specific connection of thatwinding, representing a definite effective number of turns in the tapped winding and,consequently, a definite turns ratio between this winding and any other winding with fixednumber of turns.NOTE - One of the tappings is the principal tapping, and other tappings are described in relation to theprincipal tapping by their respective tapping factors. See definitions of these terms below.3.5.2 principal tapping: The tapping to which the rated quantities are related.[IEV 421-05-02]3.5.3 tapping factor (corresponding to a given tapping):The ratio:UdUr(tapping factor) or100UdUr(tapping factor expressed as a percentage).whereUr is the rated voltage of the winding (see 3.4.3);Ud is the voltage which would be developed at no-load at the terminals of the winding, at the tappingconcerned, by applying rated voltage to an untapped winding.NOTE - This definition is not appropriate in relation to a series winding of a booster transformer(see 3.1.3), and in that case the percentage notation would be referred to the voltage of the energizingwinding or of the winding of an associated system transformer. [IEV 421-05-03, modified]SIST EN 60076-1:1997

76-1 ©IEC: 1993- 25 -3.5.4 plus tapping: A tapping whose tapping factor is higher than 1. [IEV 421-05-04]3.5.5 minus tapping: A tapping whose tapping factor is lower than 1. [IEV 421-05-05]3.5.6 tapping step: The difference between the tapping factors, expressed as apercentage, of two adjacent tappings. [IEV 421-05-06]3.5.7 tapping range: The variation range of the tapping factor, expressed as apercentage, compared with the value `100'.NOTE - If this factor ranges from 100 + a to 100 – b, the tapping range is said to be: +a %, –b % or ±a %,if a = b. [IEV 421-05-07]3.5.8 tapping voltage ratio (of a pair of windings): The ratio which is equal to therated voltage ratio:-multiplied by the tapping factor of the tapped winding if this is the high-voltagewinding;-divided by the tapping factor of the tapped winding if this is the low-voltage winding.[IEV 421-05-08]NOTE - While the rated voltage ratio is, by definition, at least equal to 1, the tapping voltage ratio can belower than 1 for certain tappings when the rated voltage ratio is close to 1.3.5.9 tapping duty: The numerical values assigned to the quantities, analogous to ratedquantities, which refer to tappings other than the principal tapping (see clause 5,and IEC 606). [IEV 421-05-09, modified]3.5.10 tapping quantities: Those quantities the numerical values of which define thetapping duty of a particular tapping (other than the principal tapping).NOTE - Tapping quantities exist for any winding in the transformer, not only for the tapped winding,(see 5.2 and 5.3).The tapping quantities are:-tapping voltage (analogous to rated voltage, 3.4.3);-tapping power (analogous to rated power, 3.4.6);-tapping current (analogous to rated current, 3.4.7). [IEV 421-05-10, modified]3.5.11 full-power tapping: A tapping whose tapping power is equal to the rated power.[IEV 421-05-14]3.5.12 reduced-power tapping: A tapping whose tapping power is lower than the ratedpower. [IEV 421-05-15]3.5.13 on-load tap-changer: A device for changing the tapping connections of awinding, suitable for operation while the transformer is energized or on load. [IEV 421-11-01]SIST EN 60076-1:1997

76-1 © IEC: 1993- 27 -3.6 Losses and no-toad currentNOTE - The values are related to the principal tapping, unless another tapping is specifically stated.3.6.1 no-load loss: The active power absorbed when rated voltage (tapping voltage) atrated frequency is applied to the terminals of one of the windings, the other winding orwindings being open-circuited. [IEV 421-06-01, modified]3.6.2 no-load current: The r.m.s. value of the current flowing through a line terminal of awinding when rated voltage (tapping voltage) is applied at rated frequency, the otherwinding or windings being open-circuited.NOTES1For a three-phase transformer, the value is the arithmetic mean of the values of current in the threephases.2 The no-load current of a winding is often expressed as a percentage of the rated current of thatwinding. For a multi-winding transformer this percentage is referred to the winding with the highest ratedpower. [IEV 421-06-02, modified]3.6.3 load loss: The absorbed active power at rated frequency and referencetemperature (see 10.1), associated with a pair of windings when rated current (tappingcurrent) is flowing through the line terminals of one of the windings, and the terminals ofthe other winding are short-circuited. Further windings, if existing, are open-circuited.NOTES1 For a two-winding transformer there is only one winding combination and one value of load loss. For amulti-winding transformer there are several values of load loss corresponding to the different two-windingcombinations (see clause 6 of IEC 606). A combined load loss figure for the complete transformer isreferred to a specified winding load combination. In general, it is usually not accessible for directmeasurement in testing.2 When the windings of the pair have different rated power values the load loss is referred to ratedcurrent in the winding with the lower rated power and the reference power should be mentioned.3.6.4 total losses: The sum of the no-load loss and the load loss.NOTE - The power consumption of the auxiliary plant is not included in the total losses and is statedseparately. [IEV 421-06-05, modified]3.7 Short-circuit impedance and voltage drop3.7.1 short-circuit impedance of a pair of windings: The equivalent series impedanceZ= R + jX, in ohms, at rated frequency and reference temperature, across the terminals ofone winding of a pair, when the terminals of the other winding are short-circuited andfurther windings, if existing, are open-circuited. For a three-phase transformer theimpedance is expressed as phase impedance (equivalent star connection).SIST EN 60076-1:1997

76-1 © IEC: 1993- 29 -In a transformer having a tapped winding, the short-circuit impedance is referred to aparticular tapping. Unless otherwise specified the principal tapping applies.NOTE - This quantity may be expressed in relative, dimensionless form, as a fraction z of the referenceimpedance Zref, of the same winding of the pair. In percentage notation:Zz = 100 ZrefwhereU2Zref=Sr(Formula valid for both three-phase and single-phase transformers).U is the voltage (rated voltage or tapping voltage) of the winding to which Zand Zref belong.Sr is the reference value of rated power.The relative value is also equal to the ratio between the applied voltage during a short-circuit measurementwhich causes the relevant rated current (or tapping current) to flow, and rated voltage (or tapping voltage).This applied voltage is referred to as the short-circuit voltage [IEV 421-07-01) of the pair of windings. It isnormally expressed as a percentage. [IEV 421-07-02, modified]3.7.2 voltage drop or rise for a specified load condition: The arithmetic differencebetween the no-load voltage of a winding and the voltage developed at the terminals ofthe same winding at a specified load and power factor, the voltage supplied to (one of) theother winding(s) being equal to:- its rated value if the transformer is connected on the principal tapping (the no-loadvoltage of the former winding is then equal to its rated value);- the tapping voltage if the transformer is connected on another tapping.This difference is generally expressed as a percentage of the no-load voltage of theformer winding.NOTE - For multi-winding transformers, the voltage drop or rise depends not only on the load and powerfactor of the winding itself, but also on the load and power factor of the other windings (see IEC 606).[IEV 421-07-03]3.7.3 zero-sequence impedance (of a three-phase winding): The impedance,expressed in ohms per phase at rated frequency, between the line terminals of athree-phase star-connected or zigzag-connected winding, connected together, and itsneutral terminal. [IEV 421-07-04, modified]NOTES1 The zero-sequence impedance may have several values because it depends on how the terminals ofthe other winding or windings are connected and loaded.2 The zero-sequence impedance may be dependent on the value of the current and the temperature,particularly in transformers without any delta-connected winding.3 The zero-sequence impedance may also be expressed as a relative value in the same way as the(positive sequence) short-circuit impedance (see 3.7.1).SIST EN 60076-1:1997

76-1 © IEC: 1993– 31 –3.8 Temperature riseThe difference between the temperature of the part under consideration and thetemperature of the external cooling medium. [IEV 421-08-01, modified]3.9 InsulationFor definitions relating to insulation see IEC 76-3.3.10 Connections3.10.1 star connection (Y-connection): The winding connection so arranged that eachof the phase windings of a three-phase transformer, or of each of the windings for thesame rated voltage of single-phase transformers associated in a three-phase bank, isconnected to a common point (the neutral point) and the other end to its appropriate lineterminal. [IEV 421-10-01, modified]3.10.2 delta connection (D-connection): The winding connection so arranged that thephase windings of a three-phase transformer, or the windings for the same rated voltageof single-phase transformers associated in a three-phase bank, are connected in series toform a closed circuit. [IEV 421-10-02, modified]3.10.3 open-delta connection: The winding connection in which the phase windings ofa three-phase transformer, or the windings for the same rated voltage of single-phasetransformers associated in a three-phase bank, are connected in series without closingone corner of the delta. [IEV 421-10-03]3.10.4 zigzag connection (Z-connection): The winding connection in which one end ofeach phase winding of a three-phase transformer is connected to a common point (neutralpoint), and each phase winding consists of two parts in which phase-displaced voltagesare induced.NOTE - These two parts normally have the same number of turns. [IEV 421-10-04, modified]3.10.5 open windings: Phase windings of a three-phase transformer which are notinterconnected within the transformer. [IEV 421-10-05, modified]3.10.6 phase displacement of a three-phase winding: The angular difference betweenthe phasors representing the voltages between the neutral point (real or imaginary) andthe corresponding terminals of two windings, a positive-sequence voltage system beingapplied to the high-voltage terminals, following each other in alphabetical sequence if theyare lettered, or in numerical sequence if they are numbered. The phasors are assumed torotate in a counter-clockwise sense. [IEV 421-10-08, modified]NOTE - The high-voltage winding phasor is taken as reference, and the displacement for any otherwinding is conventionally expressed by the 'clock notation', that is, the hour indicated by the windingphasor when the H.V. winding phasor is at 12 o'clock (rising numbers indicate increasing phase lag).3.10.7 connection symbol: A conventional notation indicating the connections of thehigh-voltage, intermediate-voltage (if any), and low-voltage windings and their relativephase displacement(s) expressed as a combination of letters and clock-hour figure(s).[IEV 421-10-09, modified]SIST EN 60076-1:1997

76-1 ©IEC: 1993- 33 -3.11 Kinds of tests3.11.1 routine test: A test to which each individual transformer is subjected.3.11.2 type test: A test made on a transformer which is representative of othertransformers, to demonstrate that these transformers comply with specified requirementsnot covered by routine tests.NOTE - A transformer is considered to be representative of others if it is fully identical in rating andconstruction, but the type test may also be considered valid if it is made on a transformer which has minordeviations of rating or other characteristics. These deviations should be subject to agreement between themanufacturer and the purchaser.3.11.3 special test: A test other than a type test or a routine test, agreed by themanufacturer and the purchaser.3.12 Meteorological data with respect to cooling3.12.1 monthly average temperature: Half the sum of the average of the daily maximaand the average of the daily minima during a particular month - over many years.3.12.2 yearly average temperature: one-twelfth of the sum of the monthly averagetemperatures.4 Rating4.1 Rated powerThe transformer shall have an assigned rated power for each winding which shall bemarked on the rating plate. The rated power refers to continuous loading. This is areference value for guarantees and tests concerning load losses and temperature rises.If different values of apparent power are assigned under different circumstances, forexample, with different methods of cooling, the highest of these values is the rated power.A two-winding transformer has only one value of rated power, identical for both windings.When the transformer has rated voltage applied to a primary winding, and rated currentflows through the terminals of a secondary winding, the transformer receives the relevantrated power for that pair of windings.SIST EN 60076-1:1997

76-1 © IEC: 1993- 35 -The transformer shall be capable of carrying, in continuous service, the rated power (for amulti-winding transformer: the specified combination(s) of winding rated powers) underconditions listed in 1.2 and without exceeding the temperature-rise limitations specifiedin IEC 76-2.NOTE - The interpretation of rated power according to this subclause implies that it is a value of apparentpower input to the transformer - including its own absorption of active and reactive power. The apparentpower that the transformer delivers to the circuit connected to the terminals of the secondary winding underrated loading differs from the rated power. The voltage across the secondary terminals differs from ratedvoltage by the voltage drop (or rise) in the transformer. Allowance for voltage drop, with regard to loadpower factor, is made in the specification of the rated voltage and the tapping range (see clause 2 ofIEC 606).This is different from the method used in transformer standards based on US tradition (ANSI/IEEEC57.12.00), where 'rated kVA' is 'the output that can be delivered at . rated secondary voltage .According to that method, allowance for voltage drop has to be made in the design so that the necessaryprimary voltage can be applied to the transformer. In addition, ANSI/IEEE specifies, under 'Usual serviceconditions': 'load power factor is 80 % or higher' (quotation from 1987 edition).4.2 Loading cycleIf specified in the enquiry or the contract, the transformer may, in addition to its ratedpower for continuous loading, be assigned a temporary load cycle which it shall becapable of performing under conditions specified in IEC 76-2.NOTE - This option is to be used in particular to give a basis for design and guarantees concerningtemporary emergency loading of large power transformers.In the absence of such specification, guidance on loading of transformers complying withthis part may be found in IEC 354 and in IEC 905.The bushings, tap-changers and other auxiliary equipment shall be selected so as not torestrict the loading capability of the transformer.NOTE - These requirements do not apply to special purpose transformers, some of which do not needloading capability above rated power. For others, special requirements will be specified.4.3 Preferred values of rated powerFor transformers up to 10 MVA, values of rated power should preferably be taken from theR10 series given in ISO 3 (1973): preferred numbers: series of preferred numbers.(.100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1 000, etc.).4.4 Operation at higher than rated voltage and/or at disturbed frequencyMethods for the specification of suitable rated voltage values and tapping range to copewith a set of loading cases (loading power and power factor, corresponding line-to-lineservice voltages) are described in IEC 606.Within the prescribed value of Um*, a transformer shall be capable of continuous servicewithout damage under conditions of `overfluxing' where the ratio of voltage over frequencyexceeds the corresponding ratio at rated voltage and rated frequency by no more than5 %.Urn the highest voltage for equipment applicable to a transformer winding (see IEC 76-3).SIST EN 60076-1:1997

76-1 ©IEC: 1993- 37 -5 Requirements for transformers having a tapped winding5.1 General - Notation of tapping rangeThe following subclauses apply to transformers in which only one of the windings is atapped winding.In a multi-winding transformer, the statements apply to the combination of the tappedwinding with either of the untapped windings.In auto-connected transformers, tappings are sometimes arranged at the neutral whichmeans that the effective number of turns is changed simultaneously in both windings. Forsuch transformers, the tapping particulars are subject to agreement. The requirements ofthis clause should be used as far as applicable.Unless otherwise specified, the principal tapping is located in the middle of the tappingrange. Other tappings are identified by their tapping factors. The number of tappings andthe range of variation of the transformer ratio may be expressed in short notation by thedeviations of the tapping factor percentages from the value 100 (for definitions of terms,see 3.5).Example: A transformer with a tapped 160 kV winding having altogether 21 tappings,symmetrically placed, is designated:(160±10x 1,5%)/66kVIf for some reason the tapping range is specified asymmetrically around the rated voltage,we may get:+12x1,5%160 _8x1,5%> /66kVNOTE - This way of short notation is only a description of the arrangement of the tapped winding anddoes not imply actual variations of applied voltage on that winding in service. This is dealt with in 5.2and 5.3.Regarding the full presentation on the nameplate of data related to individual tappings,see clause 7.Some tappings may be 'reduced-power tappings' due to restrictions in either tappingvoltage or tapping current. The boundary tappings where such limitations appear arecalled 'maximum voltage tapping' and 'maximum current tapping' (see figure 1).5.2 Tapping voltage - tapping current. Standard categories of tappingvoltage variation. Maximum voltage tappingThe short notation of tapping range and tapping steps indicates the variation range of theratio of the transformer. But the assigned values of tapping quantities are not fully definedby this alone. Additional information is necessary. This can be given either in tabular formwith tapping power, tapping voltage and tapping current for each tapping, or as text,indicating 'category of voltage variation' and possible limitations of the range within whichthe tappings are `full-power tappings'.SIST EN 60076-1:1997

76-1 ©IEC: 1993- 39 -The extreme categories of tapping voltage variation are:-constant flux voltage variation (CFVV), and-variable flux voltage variation (VFVV).They are defined as follows:CFVVThe tapping voltage in any untapped winding is constant from tapping to tapping. Thetapping voltages in the tapped winding are proportional to the tapping factors.VFVVThe tapping voltage in the tapped winding is constant from tapping to tapping. The tappingvoltages in any untapped winding are inversely proportional to the tapping factor.CbVV (Combined voltage variation)In many applications and particularly with transformers having a large tapping range, acombination is specified using both principles applied to different parts of the range:combined voltage variation (CbVV). The change-over point is called `maximum voltagetapping'. For this system the following applies:CFVV applies for tappings with tapping factors below the maximum voltage tappingfactor.VFVV applies for tappings with tapping factors above the maximum voltage tappingfactor.Graphic presentation of tapping voltage variation categories:CFVV figure 1a) - VFVV figure 1b) - CbVV figure 1c)Symbols:UA , IATapping voltage and tapping current in the tapped winding.UB , /BTapping voltage and tapping current in the untapped winding.SABTapping power.AbscissaTapping factor, percentage (indicating relative number of effective turns intapped winding).1Indicates full-power tappings throughout the tapping range.2Indicates `maximum-voltage tapping', `maximum current tapping' andrange of reduced power tappings.SIST EN 60076-1:1997

1j2SAB76-1 ©IEC: 1993- 41 -10010090100110Tapping factor1EC 260/93Figure la) - Constant flux voltage variation CFVVOptional maximum current tapping shown100UA1/BII iII'/2Figure lb) - Variable flux voltage variation VFVV^//Optional maximum current tapping shown/^/I//\12 SAB90100110'Tapping factorMEC 261193\1/A2100100100SIST EN 60076-1:1997

76-1 © IEC: 1993- 43 -100100100Figure 1c) - Combined volt-age variation CbVVThe change-over point isshown in the plus tappingrange. It constitutes both amaximum voltage tapping(UA) and a maximum currenttapping (/8 constant, notrising above the change-overpoint). An additional, optionalmaximum current tapping (inthe CFVV range) is alsoshown.90100110Tapping factorIEC 262/935.3 Tapping power. Full-power tappings - reduced-power tappingsAll tappings shall be full-power tappings, except as specified below.In separate-winding transformers up to and including 2 500 kVA with a tapping range notexceeding ± 5 % the tapping current in the tapped winding shall be equal to rated currentat all minus tappings. This means that the principal tapping is a `maximum currenttapping', see below.In transformers with a tapping range wider than ± 5 %, restrictions may be specified onvalues of tapping voltage or tapping current which would otherwise rise considerablyabove the rated values. When such restrictions are specified, the tappings concerned willbe `reduced-power tappings'. This subclause describes such arrangements.When the tapping factor deviates from unity, the tapping current for full-power tappingsmay rise above rated current on one of the windings. As figure la) illustrates, this appliesfor minus tappings, on the tapped winding, under CFVV, and for plus tappings on theuntapped winding under VFVV (figure 1 b)). In order to limit the correspondingreinforcement of the winding in question, it is possible to specify a maximum currenttapping. From this tapping onwards the tapping current values for the winding are thenspecified to be constant. This means that the remaining tappings towards the extremetapping are reduced-power tappings (see figures 1 a), lb) and lc)).SIST EN 60076-1:1997

76-1 © IEC: 1993– 45 –Under CbVV, the 'maximum voltage tapping', the change-over point between CFVV andVFVV shall at the same time be a 'maximum current tapping' unless otherwise specified.This means that the untapped winding current stays constant up to the extreme plustapping (figure 1c).5.4 Specification of tappings in enquiry and orderThe following data are necessary to define the design of the transformer.a)Which winding shall be tapped.b)The number of steps and the tapping step (or the tapping range and number ofsteps). Unless otherwise specified it shall be assumed that the range is symmetricalaround the principal tapping and that the tapping steps in the tapped winding are equal.If for some reason the design has unequal steps, this shall be indicated in the tender.c)The category of voltage variation and, if combined variation is applied, thechange-over point ('maximum voltage tapping', see 5.2).d)Whether maximum current limitation (reduced power tappings) shall apply, and ifso, for which tappings.Instead of items c) and d), tabulation of the same type as used on the rating plate may beused to advantage (see example in annex B).The specification of these data may be accomplished in two different ways:–either the user may specify all data from the beginni
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