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CONTENTS, Page,FOREWORD 5,Clause, 1 General 7, 2 Characteristic properties of different three phase winding combinations and. magnetic circuit designs 9, 3 Characteristic properties and application of auto connected transformers 17. 4 Zero sequence properties neutral load current and earth fault conditions . magnetic saturation and inrush current 25, 5 Calculation of short circuit currents in three winding three phase transformers. separate winding transformers and auto connected transformers . with earthed neutrals 51, 6 Parallel operation of transformers in three phase systems 81. 7 Calculation of voltage drop for a specified load three winding transformer load loss 93. 8 Specification of rated quantities and tapping quantities 125. 9 Convertor applications with standard transformers 147. 10 Guide to the measurement of losses in power transformers 151. Annex A Basic relations for single phase and two phase earth faults 165. 60076 8 IEC 1997 5 , INTERNATIONAL ELECTROTECHNICAL COMMISSION. , POWER TRANSFORMERS , APPLICATION GUIDE, FOREWORD. 1 The IEC International Electrotechnical Commission is a worldwide organization for standardization comprising. all national electrotechnical committees IEC National Committees The object of the IEC is to promote. international co operation on all questions concerning standardization in the electrical and electronic 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 in the subject dealt with may. participate in this preparatory work International governmental and non governmental organizations liaising. with the IEC also participate in this preparation The IEC collaborates closely with the International Organization. for Standardization ISO in accordance with conditions determined by agreement between the two. organizations , 2 The formal decisions or agreements of the IEC on technical matters express as nearly as possible an. international consensus of opinion on the relevant subjects since each technical committee has representation. from all interested National Committees , 3 The documents produced have the form of recommendations for international use and are published in the form. of 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 International. Standards transparently to the maximum extent possible in their national and regional standards Any. divergence between the IEC Standard and the corresponding national or regional standard shall be clearly. indicated in the latter , 5 The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any. equipment 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 subject. of 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 Power. transformers , This first edition of IEC 60076 8 cancels and replaces IEC 60606 published in 1978 This. edition constitutes a technical revision , The text of this standard is based on the following documents . FDIS Report on voting, 14 260 FDIS 14 297 RVD, Full information on the voting for the approval of this standard can be found in the report on. voting indicated in the above table , IEC 60076 consists of the following parts under the general title Power transformers . 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, Part 8 1997 Application guide. Annex A is for information only , 60076 8 IEC 1997 7 . POWER TRANSFORMERS , APPLICATION GUIDE,1 General,1 1 Scope and object. This Standard applies to power transformers complying with the series of publications. IEC 60076 , It is intended to provide information to users about . certain fundamental service characteristics of different transformer connections and. magnetic 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 and. calculation of load loss for three winding load combinations . selection of rated quantities and tapping quantities at the time of purchase based on. prospective 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 for. the specification and utilization of large high voltage units . The recommendations are not mandatory and do not in themselves constitute specification. requirements , 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 references, The following normative documents contain provisions which through reference in this text . constitute provisions of this International Standard At the time of publication the editions. indicated were valid All normative documents are subject to revision and parties to. agreements based on this International Standard are encouraged to investigate the possibility. of applying the most recent edition of the normative documents indicated below Members of. IEC and ISO maintain registers of currently valid International Standards . IEC 60050 421 1990 International Electrotechnical Vocabulary IEV Chapter 421 Power. transformers and reactors,IEC 60076 Power transformers. IEC 60076 1 1993 Power transformers Part 1 General. 60076 8 IEC 1997 9 , IEC 60076 3 1980 Power transformers Part 3 Insulation levels and dielectric tests. IEC 60289 1988 Reactors, IEC 60354 1991 Loading guide for oil immersed power transformers. IEC 60722 1982 Guide to the lightning impulse and switching impulse testing of power. transformers and reactors, IEC 60905 1987 Loading guide for dry type power transformers. IEC 60909 1988 Short circuit current calculation in three phase a c systems. IEC 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 to. IEC 60909 1988 , IEC 60909 2 1992 Electrical equipment Data for short circuit current calculations in. accordance with IEC 60909 1988 , IEC 61378 1 1997 Convertor transformers Part 1 Transformers for industrial applications. ISO 9001 1994 Quality systems Model for quality assurance in design development . production installation and servicing, 2 Characteristic properties of different three phase winding combinations. and magnetic circuit designs, This 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 windings. There 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 have. been 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 longer. appear in common power transmission systems ,2 1 1 Advantages of a Y connected winding. This 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 IEC 1997 11 ,2 1 2 Advantages of a D connected winding. This 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 in. that winding ,2 1 3 Advantages of a Z connected winding. This type of winding , permits neutral current loading with inherently low zero sequence impedance It is used. for earthing transformers to create an artificial neutral terminal of a system . reduces voltage unbalance in systems where the load is not equally distributed between. the phases , 2 2 Characteristic properties of combinations of winding connections. The notation of winding connections for the whole transformer follows the conventions in. IEC 60076 1 clause 6 , This subclause is a summary of the neutral current behaviour in different winding. combinations Such conditions are referred to as having zero sequence components of. current 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 connected. together externally ,2 2 1 YNyn and YNauto, Zero sequence current may be transformed between the windings under ampere turn balance . meeting low short circuit impedance in the transformer System transformers with such. connections may in addition be provided with delta equalizer winding see 4 7 2 and 4 8 . 2 2 2 YNy and Yyn, Zero 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 therefore. constitutes a magnetizing current for the iron core and is controlled by a zero sequence. magnetizing impedance This impedance is high or very high depending on the design of the. magnetic circuit see 2 3 The symmetry of the phase to neutral voltages will be affected and. there 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 compensating. circulating current to flow in the delta winding The impedance is low approximately equal to. the positive sequence short circuit impedance between the windings . If there are two star windings with earthed neutrals including the case of auto connection with. common neutral there is a three winding loading case for zero sequence current This is dealt. with in 4 3 2 and 4 7 2 and in clause 5 , 60076 8 IEC 1997 13 . 2 2 4 Yzn or ZNy, Zero sequence current in the zigzag winding produces an inherent ampere turn balance. between the two halves of the winding on each limb and provides a low short circuit. impedance , 2 2 5 Three phase banks of large single phase units use of delta connected tertiary windings. In some countries transformers for high voltage system interconnection are traditionally made. as banks of single phase units The cost mass and loss of such a bank is larger than for a. corresponding three phase transformer as long as it can be made The advantage of the bank. concept is the relatively low cost of providing a spare fourth unit as a strategic reserve It may. also be that a corresponding three phase unit would exceed the transport mass limitation . The three single phase transformers provide independent magnetic circuits representing high. magnetizing impedance for a zero sequence voltage component . It may be necessary to provide a delta equalizer winding function in the bank or there may be. a need for auxiliary power at relatively low voltage from a tertiary winding This can be. achieved by external busbar connection from unit to unit in the station The external connection. represents an additional risk of earth fault or short circuit on the combined tertiary winding of. the bank ,2 3 Different magnetic circuit designs, The 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 by. horizontal yokes , IEC 1119 97, Figure 1 Three limb core form magnetic circuit. The five limb core form magnetic circuit see figure 2 has three limbs with windings and two. unwound side limbs of lesser cross section The yokes connecting all five limbs also have a. reduced cross section in comparison with the wound limbs . 60076 8 IEC 1997 15 , IEC 1120 97, Figure 2 Five limb core form magnetic circuit. The conventional shell form three phase design has a frame with the three wound limbs. horizontal and having a common centre line see figure 3 The core steel limbs inside the. windings have an essentially rectangular cross section and the adjoining parts of the magnetic. circuit surround the windings like a shell , IEC 1121 97. Figure 3 Three phase conventional shell form magnetic circuit. A new three phase shell form magnetic circuit is the seven limb core in which the wound limbs. are oriented in a different way see figure 4 , IEC 1122 97. Figure 4 Three phase seven limb shell form magnetic circuit. 60076 8 IEC 1997 17 , The principal difference between the designs to be discussed here lies in their behaviour. when 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 without. balancing ampere turns in any other winding Such a current appears as a magnetizing current. for 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 circuit, In the three limb core form transformer positive and negative sequence flux components in the. wound limbs which have a zero sum at every instant cancel out via the yokes but the residual. zero 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 given. applied zero sequence voltage a considerable magnetomotive force high magnetizing. current is required In terms of the electrical circuit the phenomenon therefore represents a. relatively low zero sequence magnetizing impedance This impedance varies in a non linear. way with the magnitude of the zero sequence component . Conversely uncompensated zero sequence current constitutes a magnetizing current which is. controlled by the zero sequence magnetizing impedance The result is a superposed. asymmetry of the phase to neutral voltages the zero sequence voltage component . The zero sequence yoke leakage flux induces circulating and eddy currents in the clamping. structure and the tank generating extra stray losses in these components There could also be. increased eddy losses in the windings caused by the abnormal stray flux There are limitations. to the magnitude of any long duration neutral current which is allowable in service This is. considered in 4 8 , 2 3 2 Five limb core form or shell form magnetic circuit. In a five limb core form or a shell form transformer there are return paths available for the. zero 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 unwound. inter winding limbs The zero sequence flux sees low magnetic reluctance equivalent to a very. high magnetizing impedance similar to that of normal positive sequence voltage This applies. up to a limit where the unwound parts of the magnetic circuit reach saturation Above that the. impedance falls off resulting in peaked distorted current . A three phase bank of single phase transformers reacts similarly The magnetic circuits are. separate and independent at any applied service voltage . Due to the phenomena described above it is customary to provide such transformers or. transformer banks with a delta connected stabilizing winding see clause 4 . 3 Characteristic properties and application of auto connected transformers. 3 1 By definition an auto connected transformer is a transformer in which at least two. windings have a common part see 3 1 2 of IEC 60076 1 . 60076 8 IEC 1997 19 , The single line diagram of an auto transformer is shown in figure 5 The high voltage side of. the transformer identified with U 1 I 1 in the figure consists of the common winding together. with the series winding The low voltage side U 2 I 2 consists of the common winding alone . The high and low voltage systems are electrically connected . I1, I U2I2 S, U11, U1 U2, U1 U 2 I I, 2 1 I2, U1, U1 I2. U 1 U 2 I 1 U 2 I 2 I 1 S, U2, I1 I2, i1 I2, IEC 1123 97. Figure 5 Auto connected transformer single line diagram. 3 2 The reduction factor or auto factor , The auto transformer is physically smaller and has lower losses than a separate winding. transformer for the same throughput power The relative saving is greater the closer the. transformation ratio is to unity The two windings series and common represent the same. equivalent power ratings or expressed in other terms balancing ampere turns The relations. shown in figure 5 immediately explain the reduction factor of the auto connection If S is the. rated power of the auto connected windings noted on the rating plate then the transformer is. similar with regard to physical size and mass to a separate winding transformer having rated. power S This is often referred to with expressions such as intrinsic rated power or. equivalent two winding rating , Example, An auto connected transformer 420 240 kV 300 MVA is comparable with a separate. winding transformer having a rated power of , 420 240 420 300 129 MVA. If the transformer in addition is provided with a non auto connected tertiary winding of 100. MVA rated power YNauto d 300 300 100 MVA then its equivalent two winding rating will. be, 129 129 100 2 179 MVA, 3 3 Short circuit impedance and leakage flux effects. The short circuit impedance of a transformer may be described physically in terms of the. reactive power in the leakage field This in turn depends on the physical size and geometry of. the windings , 60076 8 IEC 1997 21 , For an auto transformer with its reduced dimensions the reactive power in the leakage field is. naturally smaller than for a separate winding transformer with the same rated power Its. impedance expressed as a percentage will then be correspondingly lower The auto . connection factor is also a benchmark for the percentage impedance . However it may also be observed that if the percentage impedance of an auto transformer is. specified with an elevated value with a view to limiting fault current amplitudes in the. secondary side system then this transformer will from a design point of view be a physically. small 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 saturation. effects due to leakage flux circulating in part through the magnetic circuit Such effects would. restrict the loadability of the unit above rated conditions but this is not revealed by standard. tests , The transformer loading guide IEC 60354 takes these phenomena into account when. separating between large and medium power transformers Auto transformers are to be. classified according to their equivalent power rating and the corresponding percentage. impedance instead of by the rating plate figures ,3 4 System restrictions insulation co ordination. The direct electrical connection between the primary and secondary three phase systems. implies that they will have a common neutral point and that the three phase connection of the. auto transformer is in star In practice the systems will normally be effectively earthed and the. neutral 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 very. low see 5 5 2 of IEC 60076 3 , It may alternatively be foreseen that not all neutrals of several transformers in a station. will be directly earthed This is in order to reduce the prospective earth fault currents The. unearthed neutrals will however usually be provided with a surge arrester for protection. against transient impulses The specified arrester rated voltage and the insulation level of. the neutral will be co ordinated with the power frequency voltage appearing at the unearthed. neutral during a system earth fault , In extra high voltage systems with long overhead lines the possibility of successful. single pole reclosing may be improved by specially tuned reactor earthing This requires a. relatively high insulation of the transformer neutral which is connected via the tuning reactor. to earth , The series winding of an auto transformer sometimes presents design difficulties for the. insulation 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 line. terminal The stress corresponding to the whole impulse insulation level of the high voltage side. will therefore be distributed along the series winding only This represents a correspondingly. higher turn to turn voltage compared with an overvoltage across the low voltage side distributed. along the common winding , 3 5 Voltage regulation in system interconnection autotransformers. Variation of the voltage ratio in an auto connected transformer may be arranged in different. ways Some of these follow the underlying principles of 5 1 of IEC 60076 1 Others do not. because the number of effective turns is changed in both windings simultaneously . 60076 8 IEC 1997 23 , The tapping turns will be either at the neutral terminal or at the joint between the common and. the series windings common point see figure 6 ,3 5 1 Tapping turns at the neutral. X, IEC 1124 97, Regulation at the neutral simultaneously increases or decreases the number of turns in both the high voltage. and low voltage windings but the ratio between the windings changes This type of regulation will be insufficient. in the sense that it requires many regulating turns for the specified range of variation of ratio Therefore the. volts per turn in the transformer will vary considerably across the tapping range variable flux The phenomenon. gets more pronounced the closer the ratio of the transformer approaches unity low value This has to be. covered by a corresponding over dimensioning of the magnetic circuit It will also result in unequal voltages per. step , The obvious advantage of regulation in the neutral is that the tapping winding and the tap changer will be close. to neutral potential and require only low insulation level to earth . Figure 6 Tapping turns at the common neutral,3 5 2 Tapping turns at the X terminal. Regulation arranged at the auto interconnection in the transformer the low voltage side line. terminal requires the tapping winding and tapchanger to be designed with the insulation level. of the X terminal They will be directly exposed to steep front voltage transients from lightning. or switching surges Figure 7 shows a number of different arrangements . 60076 8 IEC 1997 25 , a b c , X X X, IEC 1125 97, a The number of turns in the common winding remains unchanged This is a logical choice if the low voltage. system 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 remains. constant 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 the. ratio varies more than in case b Case b on the other hand permits plus minus utilization of the tapping. winding by reversing it as indicated in the figure . Figure 7 Tapping turns at the lower voltage terminal. 4 Zero sequence properties neutral load current and earth fault conditions . magnetic saturation and inrush current, This clause outlines the characteristics of three phase transformers and banks of single phase. transformers with regard to asymmetrical three phase service conditions . There are differences depending on the geometry of the magnetic circuit and on the. combination of three phase connections of the windings . The asymmetrical conditions comprise transient disturbances as well as asymmetries during. continuous service giving rise to , temporary loss of symmetry of three phase voltages and consequently of the symmetry. of 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 system. A 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 further. information on this method and its application see textbooks on power system analysis .
Part 1: General (IEC 60076-1, Ed. 2.1 (2000) MOD) Any table, figure or text of the international standard that is struck through is not part of this standard. Any Australian/New Zealand table, figure or text that is added is part of this standard and is identified by shading. 1 Scope and service conditions 1.1 Scope
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