SIST IEC 60076-8:2001

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Power transformers - Application guideTransformateurs de puissance - Partie 8: Guide d'applicationPower transformers - Part 8: Application guide29.180Transformatorji. DušilkeTransformers. ReactorsICS:Ta slovenski standard je istoveten z:IEC 60076-8SIST IEC 60076-8:2001en01-julij-2001SIST IEC 60076-8:2001SLOVENSKI
STANDARD







NORMEINTERNATIONALECEIIECINTERNATIONALSTANDARD60076-8Première éditionFirst edition1997-10Transformateurs de puissance –Guide d'applicationPower transformers –Application guide Commission Electrotechnique Internationale International Electrotechnical
CommissionPour prix, voir catalogue en vigueurFor price, see current
catalogueÓ IEC 1997
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from the publisher.International Electrotechnical Commission3, rue de Varembé
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http: //www.iec.chCODE PRIXPRICE CODEXC



60076-8 ã IEC:1997– 3 –CONTENTSPageFOREWORD.5Clause
1General.7
2Characteristic properties of different three-phase winding combinations andmagnetic circuit designs.9
3Characteristic properties and application of auto-connected transformers.17
4Zero-sequence properties – neutral load current and earth fault conditions,magnetic saturation and inrush current.25
5Calculation of short-circuit currents in three-winding, three-phase transformers(separate winding transformers and auto-connected transformers)with earthed neutrals.51
6Parallel operation of transformers in three-phase systems.81
7Calculation of voltage drop for a specified load, three-winding transformer load loss.93
8Specification of rated quantities and tapping quantities.125
9Convertor applications with standard transformers.14710Guide to the measurement of losses in power transformers.151Annex A – Basic relations for single-phase and two-phase earth faults.165



60076-8 ã IEC:1997– 5 –INTERNATIONAL ELECTROTECHNICAL COMMISSION_________POWER TRANSFORMERS –APPLICATION GUIDEFOREWORD1)The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprisingall national electrotechnical committees (IEC National Committees). The object of the IEC is to promoteinternational co-operation on all questions concerning standardization in the electrical and electronic fields. Tothis end and in addition to other activities, the IEC publishes International Standards. Their preparation isentrusted to technical committees; any IEC National Committee interested in the subject dealt with mayparticipate in this preparatory work. International, governmental and non-governmental organizations liaisingwith the IEC also participate in this preparation. The IEC collaborates closely with the International Organizationfor Standardization (ISO) in accordance with conditions determined by agreement between the twoorganizations.2)The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, aninternational consensus of opinion on the relevant subjects since each technical committee has representationfrom all interested National Committees.3)The documents produced have the form of recommendations for international use and are published in the formof standards, technical reports 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.6)Attention is drawn to the possibility that some of the elements of this International Standard may be the subjectof patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.International Standard IEC 60076-8 has been prepared by IEC technical committee 14: Powertransformers.This first edition of IEC 60076-8 cancels and replaces IEC 60606 published in 1978. Thisedition constitutes a technical revision.The text of this standard is based on the following documents:FDISReport on voting14/260/FDIS14/297/RVDFull information on the voting for the approval of this standard can be found in the report onvoting indicated in the above table.IEC 60076 consists of the following parts, under the general title: Power transformers.Part 1: 1993, GeneralPart 2: 1993, Temperature risePart 3: 1980, Insulation levels and dielectric testsPart 5: 1976, Ability to withstand short circuitPart 8: 1997, Application guideAnnex A is for information only.



60076-8 ã IEC:1997– 7 –POWER TRANSFORMERS –APPLICATION GUIDE1 General1.1 Scope and objectThis Standard applies to power transformers complying with the series of publicationsIEC 60076.It is intended to provide information to users about:–certain fundamental service characteristics of different transformer connections andmagnetic circuit designs, with particular reference to zero-sequence phenomena;–system fault currents in transformers with YNynd and similar connections;–parallel operation of transformers, calculation of voltage drop or rise under load, andcalculation of load loss for three-winding load combinations;–selection of rated quantities and tapping quantities at the time of purchase, based onprospective loading cases;–application of transformers of conventional design to convertor loading;–measuring technique and accuracy in loss measurement.Part of the information is of a general nature and applicable to all sizes of power transformers.Several chapters, however, deal with aspects and problems which are of the interest only forthe specification and utilization of large high-voltage units.The recommendations are not mandatory and do not in themselves constitute specificationrequirements.Information concerning loadability of power transformers is given in IEC 60354, for oil-immersed transformers, and IEC 60905, for dry-type transformers.Guidance for impulse testing of power transformers is given in IEC 60722.1.2 Normative referencesThe following normative documents contain provisions which, through reference in this text,constitute provisions of this International Standard. At the time of publication, the editionsindicated were valid. All normative documents are subject to revision, and parties toagreements based on this International Standard are encouraged to investigate the possibilityof applying the most recent edition of the normative documents indicated below. Members ofIEC and ISO maintain registers of currently valid International Standards.IEC 60050(421):1990, International Electrotechnical Vocabulary (IEV) – Chapter 421: Powertransformers and reactorsIEC 60076: Power transformersIEC 60076-1:1993, Power transformers – Part 1: General



60076-8 ã IEC:1997– 9 –IEC 60076-3:1980, Power transformers – Part 3: Insulation levels and dielectric testsIEC 60289:1988, ReactorsIEC 60354:1991, Loading guide for oil-immersed power transformersIEC 60722:1982, Guide to the lightning impulse and switching impulse testing of powertransformers and reactorsIEC 60905:1987, Loading guide for dry-type power transformersIEC 60909:1988, Short-circuit current calculation in three-phase a.c. systemsIEC 60909-1:1991, Short-circuit current calculation in three-phase a.c. systems – Part 1:Factors for the calculation of short-circuit currents in three-phase a.c. systems according toIEC 60909 (1988)IEC 60909-2:1992, Electrical equipment – Data for short-circuit current calculations inaccordance with IEC 60909 (1988)IEC 61378-1: 1997, Convertor transformers – Part 1: Transformers for industrial applicationsISO 9001: 1994, Quality systems – Model for quality assurance in design, development,production, installation and servicing2 Characteristic properties of different three-phase winding combinationsand magnetic circuit designsThis chapter is an overview of the subject. Additional information is given in clause 4 on zero-sequence properties.2.1 Y-, D-, and Z-connected windingsThere are two principal three-phase connections of transformer windings: star (Y-connection)and delta (D-connection). For special purposes, particularly in small power transformers,another connection named zigzag or Z is also used. Historically, several other schemes havebeen in use (such as "truncated delta", "extended delta", "T-connection", "V-connection", etc.).While such connections are used in transformers for special applications, they no longerappear in common power transmission systems.2.1.1 Advantages of a Y-connected windingThis type of winding:–is more economical for a high-voltage winding;–has a neutral point available;–permits direct earthing or earthing through an impedance;–permits reduced insulation level of the neutral (graded insulation);–permits the winding taps and tapchanger to be located at the neutral end of each phase;–permits single-phase loading with neutral current (see 2.2 and 4.8).



60076-8 ã IEC:1997– 11 –2.1.2 Advantages of a D-connected windingThis type of winding:–is more economical for a high-current, low-voltage winding;–in combination with a star-connected winding, reduces the zero-sequence impedance inthat winding.2.1.3 Advantages of a Z-connected windingThis type of winding:–permits neutral current loading with inherently low zero-sequence impedance. (It is usedfor earthing transformers to create an artificial neutral terminal of a system);–reduces voltage unbalance in systems where the load is not equally distributed betweenthe phases.2.2 Characteristic properties of combinations of winding connectionsThe notation of winding connections for the whole transformer follows the conventions inIEC 60076-1, clause 6.This subclause is a summary of the neutral current behaviour in different windingcombinations. Such conditions are referred to as having "zero-sequence components" ofcurrent and voltage. This concept is dealt with further in clauses 4 and 5.The statements are also valid for three-phase banks of single-phase transformers connectedtogether externally.2.2.1 YNyn and YNautoZero-sequence current may be transformed between the windings under ampere-turn balance,meeting low short-circuit impedance in the transformer. System transformers with suchconnections may in addition be provided with delta equalizer winding (see 4.7.2 and 4.8).2.2.2 YNy and YynZero-sequence current in the winding with earthed neutral does not have balancing ampere-turns in the opposite winding, where the neutral is not connected to earth. It thereforeconstitutes a magnetizing current for the iron core and is controlled by a zero-sequencemagnetizing impedance. This impedance is high or very high, depending on the design of themagnetic circuit (see 2.3). The symmetry of the phase-to-neutral voltages will be affected andthere may be limitations for the allowable zero-sequence current caused by stray-flux heating(see 4.8).2.2.3 YNd, Dyn, YNyd (loadable tertiary) or YNy + d (non-loadable delta equalizer winding)Zero-sequence current in the star winding with earthed neutral causes compensatingcirculating current to flow in the delta winding. The impedance is low, approximately equal tothe positive-sequence short-circuit impedance between the windings.If there are two star windings with earthed neutrals (including the case of auto-connection withcommon neutral), there is a three-winding loading case for zero-sequence current. This is dealtwith in 4.3.2 and 4.7.2, and in clause 5.



60076-8 ã IEC:1997– 13 –2.2.4 Yzn or ZNyZero-sequence current in the zigzag winding produces an inherent ampere-turn balancebetween the two halves of the winding on each limb, and provides a low short-circuitimpedance.2.2.5 Three-phase banks of large single-phase units – use of delta connected tertiary windingsIn some countries, transformers for high-voltage system interconnection are traditionally madeas banks of single-phase units. The cost, mass, and loss of such a bank is larger than for acorresponding three-phase transformer (as long as it can be made). The advantage of the bankconcept is the relatively low cost of providing a spare fourth unit as a strategic reserve. It mayalso be that a corresponding three-phase unit would exceed the transport mass limitation.The three single-phase transformers provide independent magnetic circuits, representing highmagnetizing impedance for a zero-sequence voltage component.It may be necessary to provide a delta equalizer winding function in the bank, or there may bea need for auxiliary power at relatively low-voltage from a tertiary winding. This can beachieved by external busbar connection from unit to unit in the station. The external connectionrepresents an additional risk of earth fault or short circuit on the combined tertiary winding ofthe bank.2.3 Different magnetic circuit designsThe most common magnetic circuit design for a three-phase transformer is the three-limb core-form (see figure 1). Three parallel, vertical limbs are connected at the top and bottom byhorizontal yokes.Figure 1 – Three-limb, core-form magnetic circuitThe five-limb, core-form magnetic circuit (see figure 2) has three limbs with windings and twounwound side limbs of lesser cross-section. The yokes connecting all five limbs also have areduced cross-section in comparison with the wound limbs.IEC
1119/97



60076-8 ã IEC:1997– 15 –IEC
1120/97Figure 2 – Five-limb, core-form magnetic circuitThe conventional shell-form three-phase design has a frame with the three wound limbshorizontal and having a common centre line (see figure 3). The core-steel limbs inside thewindings have an essentially rectangular cross-section and the adjoining parts of the magneticcircuit surround the windings like a shell.Figure 3 – Three-phase conventional shell-form magnetic circuitA new three-phase shell-form magnetic circuit is the seven-limb core, in which the wound limbsare oriented in a different way (see figure 4).Figure 4 – Three-phase seven-limb shell-form magnetic circuitIEC
1121/97IEC
1122/97



60076-8 ã IEC:1997– 17 –The principal difference between the designs, to be discussed here, lies in their behaviourwhen subjected to an asymmetrical three-phase set of voltages having a non-zero sum i.e.having a zero-sequence component.This condition may also be described as starting from a zero-sequence current withoutbalancing ampere-turns in any other winding. Such a current appears as a magnetizing currentfor the magnetic circuit and is controlled by a magnetizing impedance, across which a zero-sequence voltage drop is developed.The usual types of magnetic circuits behave as follows.2.3.1 Three-limb core-form magnetic circuitIn the three-limb core-form transformer, positive and negative sequence flux components in thewound limbs (which have a zero sum at every instant) cancel out via the yokes, but the residualzero-sequence flux has to find a return path from yoke to yoke outside the excited winding.This external yoke leakage flux sees high reluctance and, for a given amount of flux (a givenapplied zero-sequence voltage), a considerable magnetomotive force (high magnetizingcurrent) is required. In terms of the electrical circuit, the phenomenon therefore represents arelatively low zero-sequence (magnetizing) impedance. This impedance varies in a non-linearway with the magnitude of the zero-sequence component.Conversely, uncompensated zero-sequence current constitutes a magnetizing current which iscontrolled by the zero-sequence magnetizing impedance. The result is a superposedasymmetry of the phase-to-neutral voltages, the zero-sequence voltage component.The zero-sequence yoke leakage flux induces circulating and eddy currents in the clampingstructure and the tank, generating extra stray losses in these components. There could also beincreased eddy losses in the windings caused by the abnormal stray flux. There are limitationsto the magnitude of any long duration neutral current which is allowable in service. This isconsidered in 4.8.2.3.2 Five-limb core-form, or shell-form magnetic circuitIn a five-limb core-form, or a shell-form transformer, there are return paths available for thezero-sequence flux through unwound parts of the magnetic circuit (side limbs of five-limb core,outside parts of the shell frame plus, and for the seven-limb shell-form core, the two unwoundinter-winding limbs). The zero-sequence flux sees low magnetic reluctance equivalent to a veryhigh magnetizing impedance, similar to that of normal positive-sequence voltage. This appliesup to a limit, where the unwound parts of the magnetic circuit reach saturation. Above that, theimpedance falls off, resulting in peaked, distorted current.A three-phase bank of single-phase transformers reacts similarly. The magnetic circuits areseparate and independent at any applied service voltage.Due to the phenomena described above, it is customary to provide such transformers ortransformer banks with a delta-connected stabilizing winding (see clause 4).3 Characteristic properties and application of auto-connected transformers3.1
By definition, an auto-connected transformer is a transformer in which at least twowindings have a common part (see 3.1.2 of IEC 60076-1).



60076-8 ã IEC:1997– 19 –The single line diagram of an auto-transformer is shown in figure 5. The high-voltage side ofthe transformer (identified with U1, I1 in the figure) consists of the common winding togetherwith the series winding. The low-voltage side (U2, I2) consists of the common winding alone.The high- and low-voltage systems are electrically connected.UIUIS1122==UUUIII121212-=-=a()()UUIUIIS121221-=-=aIEC
1123/97U1I1U1 - U2U2I1 - I2I2i1I2Figure 5 – Auto-connected transformer, single-line diagram3.2
The reduction factor or auto-factor, aThe auto-transformer is physically smaller and has lower losses than a separate windingtransformer for the same throughput power. The relative saving is greater the closer thetransformation ratio is to unity. The two windings (series and common) represent the sameequivalent power ratings or, expressed in other terms, balancing ampere-turns. The relationsshown in figure 5 immediately explain the reduction factor, a, of the auto-connection. If S is therated power of the auto-connected windings, noted on the rating plate, then the transformer issimilar, with regard to physical size and mass, to a separate winding transformer having ratedpower a ´ S. This is often referred to with expressions such as intrinsic rated power orequivalent two-winding rating.ExampleAn auto-connected transformer 420/240 kV, 300 MVA, is comparable with a separatewinding transformer having a rated power of:((420 – 240)/420) ´ 300 = 129 MVAIf the transformer in addition is provided with a non-auto-connected tertiary winding of 100MVA rated power (YNauto d 300/300/100 MVA), then its equivalent two-winding rating willbe(129 + 129 + 100)/2 = 179 MVA3.3
Short-circuit impedance and leakage flux effectsThe short-circuit impedance of a transformer may be described physically in terms of thereactive power in the leakage field. This in turn depends on the physical size and geometry ofthe windings.



60076-8 ã IEC:1997– 21 –For an auto-transformer with its reduced dimensions, the reactive power in the leakage field isnaturally smaller than for a separate winding transformer with the same rated power. Itsimpedance, expressed as a percentage, will then be correspondingly lower. The auto-connection factor, a, is also a benchmark for the percentage impedance.However, it may also be observed that if the percentage impedance of an auto-transformer isspecified with an elevated value (with a view to limiting fault-current amplitudes in thesecondary-side system) then this transformer will, from a design point of view, be a physicallysmall unit with a quite large leakage field. This will be reflected as higher additional losses(winding eddy loss as well as stray field loss in mechanical parts) and possibly even saturationeffects due to leakage flux circulating in part through the magnetic circuit. Such effects wouldrestrict the loadability of the unit above rated conditions, but this is not revealed by standardtests.The transformer loading guide, IEC 60354, takes these phenomena into account whenseparating between large and medium power transformers. Auto-transformers are to beclassified according to their equivalent power rating, and the corresponding percentageimpedance, instead of by the rating-plate figures.3.4
System restrictions, insulation co-ordinationThe direct electrical connection between the primary and secondary (three-phase) systemsimplies that they will have a common neutral point and that the three-phase connection of theauto-transformer is in star. In practice, the systems will normally be effectively earthed and theneutral point of the auto-transformer will usually be specified with reduced insulation level.–If the transformer neutral is to be directly earthed, the necessary insulation level is verylow (see 5.5.2 of IEC 60076-3).–It may alternatively be foreseen that not all neutrals of several transformers in a stationwill be directly earthed. This is in order to reduce the prospective earth fault currents. Theunearthed neutrals will, however, usually be provided with a surge arrester for protectionagainst transient impulses. The specified arrester rated voltage and the insulation level ofthe neutral will be co-ordinated with the power frequency voltage appearing at the unearthedneutral during a system earth fault.–In extra-high-voltage systems with long overhead lines, the possibility of successfulsingle-pole reclosing may be improved by specially tuned reactor earthing. This requires arelatively high insulation of the transformer neutral, which is connected via the tuning reactorto earth.The series winding of an auto-transformer sometimes presents design difficulties for theinsulation across the winding. It is assumed that the X-terminal, the low-voltage side-line terminal,stays at low potential at the incidence of a transient overvoltage on the high-voltage side-lineterminal. The stress corresponding to the whole impulse insulation level of the high-voltage sidewill therefore be distributed along the series winding only. This represents a correspondinglyhigher turn-to-turn voltage, compared with an overvoltage across the low-voltage side, distributedalong the common winding.3.5
Voltage regulation in system-interconnection autotransformersVariation of the voltage ratio in an auto-connected transformer may be arranged in differentways. Some of these follow the underlying principles of 5.1 of IEC 60076-1. Others do notbecause the number of effective turns is changed in both windings simultaneously.



60076-8 ã IEC:1997– 23 –The tapping turns will be either at the neutral terminal or at the joint between the common andthe series windings (common point) (see figure 6).3.5.1
Tapping turns at the neutralRegulation at the neutral simultaneously increases or decreases the number of turns in both the high-voltageand low-voltage windings but the ratio between the windings changes. This type of regulation will be insufficientin the sense that it requires many regulating turns for the specified range of variation of ratio. Therefore, thevolts per turn in the transformer will vary considerably across the tapping range (variable flux). The phenomenongets more pronounced the closer the ratio of the transformer approaches unity (low a value). This has to becovered by a corresponding over-dimensioning of the magnetic circuit. It will also result in unequal voltages perstep.The obvious advantage of regulation in the neutral is that the tapping winding and the tap-changer will be closeto neutral potential and require only low insulation level to earth.Figure 6 – Tapping turns at the common neutral3.5.2
Tapping turns at the X-terminalRegulation arranged at the auto-interconnection in the transformer (the low-voltage side-lineterminal) requires the tapping winding and tapchanger to be designed with the insulation levelof the X-terminal. They will be directly exposed to steep-front voltage transients from lightningor switching surges. Figure 7 shows a number of different arrangements.IEC
1124/97X



60076-8 ã IEC:1997– 25 –IEC
1125/97XXXa)b)c)a)The number of turns in the common winding remains unchanged. This is a logical choice if the low-voltagesystem voltage remains relatively constant while the high-voltage system voltage is more variable.b)This alternative is the opposite to a). The number of turns facing the high-voltage system voltage remainsconstant, while the effective number of turns of the low-voltage side varies.c)The number of turns is constant on the high-voltage side, but for a specific number of reconnected turns, theratio varies more than in case b). Case b) on the other hand permits plus-minus utilization of the tappingwinding by reversing it as indicated in the figure.Figure 7 – Tapping turns at the lower voltage terminal4 Zero-sequence properties – neutral load current and earth fault conditions,magnetic saturation and inrush currentThis clause outlines the characteristics of three-phase transformers and banks of single-phasetransformers with regard to asymmetrical three-phase service conditions.There are differences depending on the geometry of the magnetic circuit and on thecombination of three-phase connections of the windings.The asymmetrical conditions comprise transient disturbances as well as asymmetries duringcontinuous service, giving rise to:–temporary loss of symmetry of three-phase voltages and, consequently, of the symmetryof magnetization of the core;–temporary or permanent asymmetry of load currents, particularly current in the neutral,which will affect the voltage stability, leakage flux and core magnetization.4.1 Introduction of the symmetrical components of a three-phase systemA short explanation of the conventional analytical method called symmetrical components,which is frequently referred to in power system analysis, is given in 4.1.1. For furtherinformation on this method and its application, see textbooks on power system analysis.



60076-8 ã IEC:1997– 27 –A further explanation regarding the practical aspects of earthing of the system throughtransformer neutrals is given in 4.1.2.4.1.1 Principles and terminology of symmetrical components of voltage and currentThe method, as conventionally applied, presupposes synchronous and sinusoidal voltages andcurrents, linked by circuit elements in the form of constant impedance or admittance, withequal value for the three phases. These assumptions imply that all circuit equations are linear,and that changes of variables by linear transformations are possible. One such transformationis that of symmetrical components.In the general asymmetrical case, the three individual phase voltages or phase current haveunequal amplitudes and are not spaced equally in time (not 120 electrical degrees apart). Thesum of the momentary values may be different from zero. The phasor picture is anasymmetrical star. The vectorial sum of the three phasors does not necessarily form a closedtriangle (non-zero sum).It is however always possible to replace the original three asymmetrical variables by acombination of the following three symmetrical components:–a positive sequence component having a fully symmetrical, ordinary set of three-phasevoltages or currents;–a negative sequence component having another symmetrical set, but with opposite phasesequence;–a zero sequence component having the same phasor value in all three phases with nophase rotation.The two first components each have zero sum at every instant. The third componentrepresents the residual, non-zero sum of the original variables, with one-third appearing ineach phase.The advantage of the method of symmetrical components for calculation of voltages andcurrents is that the original system of three coupled equations with three unknown variables isreplaced by three separate, single-phase equations with one unknown, one for eachcomponent. Each equation makes use of the relevant impedance or admittance parameters forthe respective component.The solution of the equations for the separate symmetrical components are then superposedback, phase by phase, to obtain the phase voltages or currents of the real system.The algorithms for transformation of the original phase quantities into symmetrical componentsand back again can be found in appropriate textbooks.4.1.2 Practical aspectsThe properties of the components have the following practical cons
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