IEEE Std C57.116-2014

IEEE Guide for Transformers Directly Connected to Generators IEEE Power and Energy Society Sponsored by the Transformers Committee IEEE Park Avenue New York, NY 10016-5997 USA IEEE Std C57.116™-2014 (Revision of IEEE Std C57.116-1989) IEEE Std C57.116™-2014 (Revision of IEEE Std C57.116-1989) IEEE Guide for Transformers Directly Connected to Generators Sponsor Transformers Committee of the IEEE Power and Energy Society Approved 27 March 2014 IEEE-SA Standards Board Abstract: Information on the selection and application considerations for the unit power transformer and unit auxiliaries power transformer is provided in this guide Consideration is given to connections that include direct connection and connections through generator circuit breakers and load-break switches The considerations referred to in this guide apply to hydroelectric and thermal electric generating stations Various power transformer connections and possible operating problems under normal and abnormal conditions are treated Phasing procedures, basic impulse insulation level selection, and loading practices are not covered Keywords: back-feed, electrical parameters, generator bus, IEEE C57.116™, load tap changing, overcurrent, overexcitation, transformer connections, transmission system, unit auxiliaries, unit power transformer • The Institute of Electrical and Electronics Engineers, Inc Park Avenue, New York, NY 10016-5997, USA Copyright © 2014 by The Institute of Electrical and Electronics Engineers, Inc All rights reserved Published 27 May 2014 Printed in the United States of America IEEE is a registered trademark in the U.S Patent & Trademark Office, owned by The Institute of Electrical and Electronics Engineers, Incorporated PDF: Print: ISBN 978-0-7381-9098-3 ISBN 978-0-7381-9099-0 STD98640 STDPD98640 IEEE prohibits discrimination, harassment, and bullying For more information, visit http://www.ieee.org/web/aboutus/whatis/policies/p9-26.html No part of this publication may be reproduced in any 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determination of the validity of any patent rights, and the risk of infringement of such rights, is entirely their own responsibility Further information may be obtained from the IEEE Standards Association Participants At the time this IEEE guide was completed, the Power Transformers-Directly Connected GSU Working Group had the following membership: Gary Hoffman, Chair William Griesacker, Vice Chair Randall Kyle, Secretary Mark Baldwin Peter Balma Wallace Binder Michael Botti James Campbell John Crouse Ramsis Girgis David Harris Jose Izquierdo John John Wayne Johnson Krzystof Kulasek Jeffrey LaMarca John Lackey Emilio Morales Cruz Jeffrey Nazarko Bipin Patel Jeffrey Ray Dinesh Sankarakurup Roderick Sauls Stephen Schroeder Ewald Schweiger Hemchandra Shertukde Jin H Sim Eduardo Tolachir David Wallach Joe Watson Kipp Yule Richard vonGemmingen The following members of the individual balloting committee voted on this guide Balloters may have voted for approval, disapproval, or abstention Mohamed Abdel Khalek William Ackerman Emmanuel Agamloh Saleman Alibhay Stephen Antosz Carlo Arpino Senthil Kumar Asok Kumar Peter Balma Thomas Barnes Barry Beaster Robert Beavers Jeffrey Benach W J (Bill) Bergman Steven Bezner Wallace Binder Thomas Blackburn W Boettger Paul Boman Kenneth Bow Derek Brown Gustavo Brunello Zeeky Bukhala Carl Bush William Byrd Paul Cardinal Robert Christman Craig Colopy Stephen Conrad Randall Crellin John Crouse Willaim Darovny Alan Darwin Glenn Davis Matthew Davis Davide De Luca Scott Digby Gary Donner Ahmed Elneweihi Ahmed ElSerafi Dan Evans Jorge Fernandez Daher Sudath Fernando Joseph Foldi Marcel Fortin Rostyslaw Fostiak Fredric Friend Doaa Galal Robert Ganser Frank Gerleve Ali Ghafourian Alexander Glaninger-Katschnig James Graham Stephen Grier William Griesacker J Travis Griffith Randall Groves Ajit Gwal David Harris Roger Hayes Lee Herron Gary Heuston Werner Hoelzl Robert Hoerauf Gary Hoffman Jill Holmes Philip Hopkinson vi Copyright © 2014 IEEE All rights reserved David Horvath Andrew Jones Laszlo Kadar C Kalra Gael Kennedy Sheldon Kennedy Yuri Khersonsky James Kinney Joseph L Koepfinger Boris Kogan Jim Kulchisky Saumen Kundu John Lackey Chung-Yiu Lam Jeffrey LaMarca Thomas La Rose Raluca Lascu Aleksandr Levin Albert Livshitz Thomas Lundquist Greg Luri Richard Marek J Dennis Marlow William McBride William McCown Michael Mcdonald James Mciver Joseph Melanson James Michalec John Miller T David Mills Daniel Mulkey Jerry Murphy R Murphy Ryan Musgrove K R M Nair Michael Newman Charles Ngethe Lorraine Padden Mirko Palazzo Bansi Patel Dhiru Patel Shawn Patterson Paulette Payne Powell Brian Penny Christopher Petrola Alvaro Portillo Iulian Profir John Roach Michael Roberts Charles Rogers Oleg Roizman John Rossetti Thomas Rozek Dinesh Sankarakurup Steven Sano Roderick Sauls Bartien Sayogo Stephen Schroeder Robert Seitz Devki Sharma Veselin Skendzic James Smith Jerry Smith John Spare Brian Sparling Gary Stoedter James Swank Michael Thompson Robert Thornton-Jones James Timperley Eduardo Tolcachir Demetrios Tziouvaras Joe Uchiyama Eric Udren Gerald Vaughn John Vergis Jane Verner David Wallach William Walter Barry Ward Daniel Ward Joe Watson Kenneth White James Wilson Larry Yonce Richard Young Jian Yu Kipp Yule Luis Zambrano Waldemar Ziomek When the IEEE-SA Standards Board approved this guide on 27 March 2014, it had the following membership: John Kulick, Chair Jon Walter Rosdahl, Vice Chair Richard H Hulett, Past Chair Konstantinos Karachalios, Secretary Peter Balma Farooq Bari Ted Burse Clint Chaplin Stephen Dukes Jean-Philippe Faure Gary Hoffman Michael Janezic Jeffrey Katz Joseph L Koepfinger* David J Law Hung Ling Oleg Logvinov Ted Olsen Glenn Parsons *Member Emeritus Also included are the following nonvoting IEEE-SA Standards Board liaisons: Richard DeBlasio, DOE Representative Michael Janezic, NIST Representative Michelle Turner IEEE Standards Program Manager, Document Development Erin Spiewak IEEE Standards Program Manager, Technical Program Manager vii Copyright © 2014 IEEE All rights reserved Ron Petersen Adrian Stephens Peter Sutherland Yatin Trivedi Phil Winston Don Wright Yu Yuan Introduction This introduction is not part of IEEE Std C57.116™-2014, IEEE Guide for Transformers Directly Connected to Generators Transformers, directly connected to generators, experience excitation and short-circuit duties beyond those covered in other power transformer standards Therefore, in 1979, the IEEE Transformers Committee decided that an application guide for such transformers was needed In 1989, the IEEE Std C57.116 was approved and describes the selection, application, and specification considerations for the unit and unit auxiliaries transformers taking into account their connections, voltage and kilovolt ampere ratings; and excitation and through-fault capabilities during possible operating conditions, both normal and abnormal Also included are load-tap changing and isolated phase-bus duct heating considerations This guide does not address phasing procedures, basic impulse level selection, or loading practices In 2011, the IEEE Transformers Committee decided to revise this guide Therefore, this guide is based on the latest knowledge, design, and application experience of the industry including:  An updated Equation (1)  New material on unit auxiliaries transformer faults and calculations  Expanded guidance on use of isolated phase-bus  A new clause on unit transformer operation in back-feed configuration  A new bibliography  General updates to match the current IEEE Style Manual This standard is intended to provide guidance to application and specification engineers and therefore its use is strictly voluntary Its use may become mandatory only when required by a duly constituted legal authority or when specified in a contractual relationship viii Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators 7.5 Unit auxiliaries transformer loading Although the generator itself can operate satisfactorily anywhere from 95% to 105% of its rated voltage, the unit auxiliaries load (pump motors, fan motors, etc.), because of other variables, may not be able to tolerate the resultant swing in auxiliaries bus voltage To overcome this problem, UAT with load-tap changers or buses with voltage regulating devices may be required Load-tap changing may be used to compensate for the effects of voltage variations However, evaluation of this application should consider its delayed response, effect on frequency of maintenance and possible effects on availability of the turbine generator unit In industrial applications, capacitors are sometimes used to provide needed voltage boosts during low source voltage conditions or power-factor corrections Utilities seldom use capacitors, voltage regulators, or load-tap changing transformers for voltage regulation of auxiliaries Without such voltage regulation help, the operating voltage range of the generator may be restricted so as to provide adequate voltage levels for the auxiliaries system The restricted generator operating voltage may prevent use of the full range of the generator’s var capability Transformer overcurrent considerations 8.1 Unit transformer faults The standard mechanical force and the thermal short-circuit requirements described in IEEE Std C57.12.00 are normally satisfactory for UTs For a fault on the system, location (1) as indicated by the X in Figure 1, the fault current through the UT will consist of the fault contribution from the generator and the UAT(s) if present and in service The maximum fault contribution from the generator is determined by the generator subtransient reactance The value of this reactance varies from 15% to 25% on the generator base The contributions from the UAT(s) will be determined by the extensive motor load typically present on the auxiliary buses they service Typically, the impedance of the UAT limits this contribution to a fairly small value as compared to the generator contribution, however, when present, their contribution should be accounted for when determining the maximum available fault current Since the UT MVA rating approaches the generator MVA rating, it is apparent that the fault current through a UT designed and built in accordance with IEEE Std C57.12.00 will be considerably less than its through-fault capability For a fault on the generator side of the UT, the maximum fault contribution through the UT is determined by its own impedance and system equivalent impedance 8.2 Unit auxiliaries transformer faults The standard mechanical force and the thermal short-circuit requirements described in IEEE Std C57.12.00 are normally not satisfactory for certain types of through faults on the UAT The through faults that are of interest are three-phase faults on the secondary of the UAT, location (3) as indicated by the X in Figure For those cases where a generator breaker is not used, the UAT may be subjected to a more severe through-fault duty than similar transformers in network applications because:  The direct-current decrement is less than in network applications  The short-circuit duration is not limited to 10 cycles to 20 cycles, as in the case for network transformers, but may be as long as 10 s to 20 s 22 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators  Depending on the generator loading prior to the fault, the UAT may have a voltage of up to 125% of its rated voltage imposed on it during the short circuit The UAT shall be designed to mechanically and thermally withstand the environment in which it operates The energy available under the fault decrement curve can be a major factor in the determination of the mechanical and thermal requirements of the UAT The short-circuit current on the UAT consists of two parts Up to the point of system breaker and generator exciter field tripping, the root mean square (rms) symmetrical value is relatively constant, any alternatecurrent decrement being more or less compensated for by increased field current through the action of the voltage regulator Once the protective system operates, the system breaker opens and the field of the generator exciter is tripped Opening of the system breaker removes the load from the generator and interrupts the fault contribution from the system The generator terminal voltage will rise to a value determined by the shape of the generator saturation curve, the magnitude of the field current before the exciter was tripped, the frequency and the short-circuit current, that flows in the UAT after the trip For rated pre-fault load, the field current would yield a generator terminal voltage of about 135% of rated voltage under no-load conditions; in general, the armature reaction of the UAT through-fault current will reduce this terminal voltage to about 125% of rated voltage Once the field of the exciter is tripped, this current will now start decaying at a rate largely determined by T'do, the generator direct axis transient open-circuit time constant This is shown in Figure 13(a) T'do can range from 3.5 s for two-pole (3600 rpm) generators to s for four-pole (1800 rpm) generators Special attention should be given to hydro units with frequency dependent excitation systems and which are increasing their speed after a load rejection For such units, the maximum rise in flux is responsible for the current Figure 13(a) shows a symmetrical rms fault current that flows through the UAT during a three-phase fault at location (3) as indicated by the X in Figure F1 is the initial symmetrical rms fault current magnitude at the instant the fault occurs The system breaker and the generator field excitation trip at tr second(s) The fault current rises to F2, corresponding to the increased generator voltage due to full-load rejection at time tr seconds, the sudden jump in current to F2 when load rejection occurs and the subsequent non-exponential decays, as shown in Figure 13(a) This nonlinear decay curve can be replaced by the straight-line decay shown in Figure 13(a) if the integrated I2t under the straight-line curve equals that under the nonlinear curve This can be approximately achieved if a straight line is drawn to intersect the time axis at time tr + 2T'do seconds Thus the total fault duration (TF) in seconds is approximately: TF ≅ t r + 2Tdo' (3) where tr = the total elapsed time between fault inception and the opening of generator exciter field Typically, tr is in the range of cycles–60 cycles The cycles represent a high speed system breaker opening and the 60 cycles represent backup clearing time for the system breaker for a failed station auxiliaries bus breaker during a three-phase fault on the station auxiliary bus The maximum current asymmetry occurs at the inception of the fault and it depends upon the fault X/R ratio Therefore, the highest asymmetrical rms current results at the instant the fault occurs This means the highest mechanical force is exerted on the UAT at this time However, the maximum symmetrical fault occurs at the time of full-load rejection and decays to zero over the several seconds that follow A relatively smaller change in the symmetrical fault current at full-load rejection results in a considerably lower asymmetry factor Thus, the magnitude F2 as shown becomes a major factor in determining the thermal duty required for the UAT An integrated I2t value of the curve of Figure 13 (a) represents the thermal energy the UAT will be subjected to under a three-phase fault condition 23 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators Figure 13 (b) shows a plot of asymmetrical-fault current versus time, typical of a transformer designed to meet the requirements of IEEE Std C57.12.00 Since, in general, the generator direct axis transient opencircuit time constant of all large modern generators exceeds s, the short-circuit withstand capabilities provided by IEEE Std C57.12.00 are not adequate for UAT applications This can be seen easily by comparing the current versus time curves of Figure 13 The user specifications should include a current plot similar to Figure 13 (a) for the transformer designer to adequately account for the mechanical (F1) and thermal (I2t under the curve) duties The nonlinear decay curve can be approximated as discussed above After the DC component has decayed, the magnitude of the mechanical and thermal stresses during the short circuit are proportional to the I2t delivered to the UAT by the symmetrical rms current The I2t factor is calculated by integrating incremental I2dt values over the period of the short circuit (0 < t < t ) I (t ) = F1 (4) r  t − tr I (t ) = F2 1 − 2Tdo'     (t r < t < t r + 2Tdo' ) (5) Total thermal energy the UAT is subjected to during the total fault time is calculated by squaring both I(t) functions and integrating from t = up to (tr + 2T´do) seconds: tr tr + 2T 'do tr I 2t = ∫ F12 dt + ∫  t −t  F22 1 − ' r  dt  2T  (6) Solving this yield to: I t = F12 * t r + F2 * 2T ' (7) The following parameters are defined for Equation (3), Equation (4), Equation (5), Equation (6), and Equation (7) where t tr Tf T’do dt F1 F2 I2t I1(t) I2(t) = time elapsed since fault initiation at t = seconds = time elapsed between fault inception and system breaker and generator field excitation trip (s) = total fault duration (s) = generator direct axis transient open circuit time constant (s) = differential of t = initial symmetrical rms fault current at fault occurrence (A) = symmetrical fault current at t = tr (A) = thermal energy produced during the fault (A2-s) = function describing fault current versus time from fault initiation until the system breaker and generator field excitation trip; < t < tr = function describing fault current versus time from system breaker and generator field excitation trip until twice the generator direct axis transient open circuit time constant; tr < t < tr+2T´do 24 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators Figure 13—Unit auxiliaries transformer fault decrement curve at constant frequency 8.3 Fast load transfer—mechanical considerations A fast transfer of load from the UAT to the SST may under certain conditions result in high-circulating currents flowing through the two transformers that could exceed the mechanical design capability of one of them If the two source voltages as shown in Figure are sufficiently out-of-phase and the circuit breakers (5) are closed at the same instant of time, a circulating current will flow The magnitude of the circulating current will be a function of the difference in source voltages and the impedances of the UAT and the SST The calculations for the circulating current can be refined by adding system impedances in series with the impedances for the UAT and the SST Generally, system impedances are relatively small and have no significant effect From the vector relationship of Et and Es in Figure 14, the expression for Ic in Figure 14 can be represented as follows: IC = For Et − ES EC = ZU + Z S ZU + Z S Et = E S = E and (8) ZU + Z S = Z EC = E − cos Φ (9) 25 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators IC = E − cos Φ Z (10) The following parameters are defined for the above expression and the parameters shown in Figure 14, Figure 15, and Figure 16 where Et = generator voltage Es = system voltage Ec = vectorial difference between the two source voltages (Et and Es) = phase angle difference between two source voltages Φ Ic = circulating current Zu = impedance of UAT Zs = impedance of SST EUAT = voltage across Zu or UAT = | Ic| | Zu | ESST = voltage across Zu or SST = | Ic| | Zs | Zm = motor short-circuit impedance Em = internal voltage of motor R = ratio of Zu to Zs For example, consider the following parameter values The transformer impedances are expressed in per unit (pu) on the same MVA base: Zu Zs Z Φ E = 0.04 pu (reactive) = 0.08 pu (reactive) = 0.12 pu = 180° = 1.0 pu Substitution of these values into the above expression yields Ic = 16.66 pu The fault-current capability of the UAT is 1/0.04 = 25 pu, while for the SST it is only 1/0.08 = 12.5 pu As can be seen, the maximum allowable current for the SST is exceeded by (16.66/12.5–1) or 0.33 pu This translates into a transformer design that requires (1.33)2 or 1.77 times the mechanical strength of a standard transformer design The assumption that the phase angle difference between the two source voltages is 180° provides the maximum value for the circulating current, assuming that each source voltage is equal to 1.0 pu Current values could be higher if the voltage were higher or lower in the case of a lesser angular difference It is possible to experience large phase-angle differences if the Es bus, Location (6), is electrically distant from the system bus of location (1) in Figure If one includes the effect of motors connected on the auxiliaries bus, Location (4) in Figure 1, in the previous analysis, it can be shown that their presence increases the circulating current discussed in the previous paragraphs The effect of the motors can be shown by assuming that when circuit breakers (3) and (5) in Figure are closed, the motors act as a parallel source to both the generator and the system, as shown in Figure 15 By assuming that the voltage, Em, produced by the motors is in phase with one or the other source, the maximum effect can be shown If the motor is represented by its short-circuit impedance, e.g., Zm = 0.2 pu (reactive) on the same MVA base as Zu or Zs, and if this is in parallel with the smaller of the 26 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators two transformer impedances (Zu in the example), then the maximum increase in the circulating current would occur In the above example, then: IC = 1.0 − cos180° (0.04)(0.2) + 0.08 = 17.65 pu (0.04 + 0.2) (11) This indicates that the maximum allowable current for the SST is exceeded by (17.65/12.5–1) or 0.41 pu This corresponds to a transformer design that requires (1.41)2 or 1.99 times the mechanical strength of a standard transformer design If this type of operation is expected, requirements for operation in this environment shall be specified To eliminate the potential of this occurring on a fast transfer, one should ensure that breaker (5) closes only after breaker (3) has opened Figure 16 may be used to determine the overvoltage of a transformer that is fast transferred In the example cited above, ratio R of the UAT impedance to the SST impedance is 0.5, and a 1.33 pu voltage appears across the SST during bus transfer (effect of motors neglected) Figure 16 shows the effect of varying the value of R in the same example A higher Zs unit is in a shutdown status The voltage results in a higher overvoltage of the SST Note that these curves are valid only if Zs is greater than or equal to Zu,, for R ≤ For R greater than 1, the voltage across the UAT becomes the larger voltage, and the same curves (Figure 16) can be used by replacing ESST with EUAT and redefining R as the ratio of the SST impedance to the UAT impedance If completeness is desired, the effects of the motor load, generator impedances, and station impedances should be included in the analysis Motor impedance is in parallel with the impedance of the UAT, and thus its effect is to lower the impedance in series with the SST For more information, see Figure 14 Figure 14—Equivalent circuit neglecting motor load 27 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators Figure 15—Equivalent circuit with motor load 28 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators Figure 16— Ec as a function of the phase angle (è) Load tap changing considerations The range of voltage on UAT buses from the generator and the switchyard may be great enough to consider the application of load tap changing capability for transformers that supply the unit auxiliaries system load Particular attention should be paid to voltage conditions where generator breakers permit the back-feed of station auxiliaries buses when the range criteria for connected load should be evaluated under all conditions Controlling the secondary voltage with the use of load tap changing may be a consideration where the UAT loading alters bus voltages to the extent that auxiliaries would lie outside their acceptable operating voltage range without this control Consideration should be given to manual operation or prevention of load tap changer (LTC) operation during motor starting 9.1 Load tap changing equipment Various types of load tap changing equipment and circuits are used, depending upon circuit parameters LTCs are built with 17, 21, and 33 positions, with the trend in recent years being toward the larger number of steps so as to give a finer degree of regulation The usual range of regulation is ±10% of the rated line voltage; however, 5% above and 15% below the rated primary voltage is often more suitable for UAT application In addition to automatic load-tap changing capability, consideration should be given to remote manual operation The 33 position, ±10% LTCs, have wide acceptance and are considered standard for many applications 29 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators 9.2 Duty considerations The duties that load tap changing transformers will experience for plant operation should be considered in defining operating requirements, including mechanical and thermal withstand capabilities These duties include delayed clearing of generator decrement type short-circuits, motor starting inrush, and evaluation of inadvertent out-of-phase paralleling transients when transferring load from the UATs or the SSTs Inclusion of these duties in the performance specifications should enable the manufacturer to design the equipment for the service intended 9.3 Specifications Load tap changing equipment specifications should be consistent with the requirements of IEEE Std C57.12.10 and IEEE Std C57.131 10 Transformers with isolated phase-bus duct connections Transformers that utilize a high current connection thorough an isolated phase-bus connection can have accompanying strong magnetic fields that may cause unanticipated circulating currents in transformer tanks and covers, bushing pockets, the isolated phase-bus duct and support structures The effects from these unanticipated currents may result in overheating if the transformer design does not consider these conditions as part of the design criteria Non-uniform currents and the consequent flux need to be carefully calculated for the bus enclosure and their effects on the material in the transformer tank The potential for overheating the various transformer components can also depend upon the use of nonmagnetic material in the high current areas and the method of terminating the isolated phase-bus duct at the transformer end This problem could occur on large UTs, either three phase or single phase and unit auxiliaries transformers (UAT) Thermal coordination of materials is important Internal metallic parts (other than those adjacent to conductor insulation and immersed in transformer oil) should not attain a temperature rise in excess of 140 °C High temperature gaskets (e.g., 125 °C) should be utilized for bushing flanges, tank cover, etc where elevated temperatures have the potential to exist The transformer design also shall consider the elevated temperatures that will exist in the area near a high current bushing (>3000A) located inside an enclosed space The bushing utilized should be applicable for the temperature within the enclosure and use of higher temperature special bushings and high temperature gaskets should be evaluated There should be coordination between the transformer manufacturer and the supplier of the isolated phasebus duct The type and size of the interface enclosure, the location of the shorting plate on the bus and the routing of the grounding cable should be considered and evaluated It is possible that heating of the transformer tank and other components due to induced eddy currents generated from currents flowing in the isolated phase-bus duct can be minimized The effect of any tank heating can be kept below acceptable limits determined by the transformer manufacturer Proper location of the isolated phase-bus duct shorting plate, grounding of bus duct enclosure, and routing of the ground cable per the isolated phase-bus duct manufacturer’s recommendations should be followed Alignment of the bus duct and the bushing terminals shall be designed properly so that no undue stress is applied to the LV bushings terminal causing a bushing oil leak or damage The transformer isolated phase-bus duct flange should be designed so that any accumulated liquid (water or oil leakage) should be self-draining from the flange area If a formal thermal coordination is determined to be required, additional evaluations should be performed using programs such as thermal finite element analysis (FEA) modeling This analysis should be included as part of the transformer design process and included in the transformer design review During the transformer’s temperature rise test, the calculated thermal characteristics of the enclosure internals should 30 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators be adjusted as needed according to temperature rise results to validate the FEA model results One approach to modeling the interaction between the transformer and the isolated phase-bus could include:  FEA model of thermal response of the transformer can be created without the bushing enclosure  Data from the transformer temperature rise test could be used to validate the model without enclosure  FEA model shall be adjusted/modified to include bushings within the enclosure  3-dimensional data may be provided to the purchaser for future reference in service In addition, expected temperatures in the bushings, bus duct, and tank cover may be provided as a reference to future analysis New and/or replacement transformer installations connected via an isolated phase-bus duct could have heating problems if the transformer-bus interface is not evaluated It is suggested for new installations that design coordination meetings be arranged between the isolated phase-bus duct manufacturer, the transformer manufacturer, and the user prior to the design of the transformer and the isolated phase-bus duct For replacement transformer, the transformer manufacturer should be given the details of the existing isolated phase-bus duct and the design should take into account the existing interface/configuration In an extreme case the existing isolated phase-bus duct may need to be modified or replaced 11 UT transformers operated in back-feed configuration Many times during the construction of a generating plant, or during an extended maintenance outage, a plant owner will energize the UT from the high side with the generator disconnected This may be required for testing of the transformer, validating generator synchronizing circuits due to construction, or for feeding station load during a plant shutdown maintenance cycle (see Figure 17) The operation of a UT in this step down configuration is frequently referred to as back-feeding the transformer and requires special consideration [B10], [B13], [B14] In addition, UT(s) having two separate LV windings, feeding two different generators, where they are operated with only one generator operating, also requires special consideration (see Figure 18) GENERATOR BREAKER OR DISCONNECTING LINKS GENERATOR OPEN SYSTEM UNIT TRANSFORMER Ib CIRCUIT BREAKER UNIT AUXILIARIES TRANSFORMERS CLOSED Ib - Transformer back-feed current AUXILIARIES LOAD NOTE 1—UT HV and UAT LV grounding are design dependent NOTE 2—UT and UAT neutral connections are omitted for simplicity Figure 17—Typical transformer back-feed operation 31 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators GENERATOR BREAKER OPEN UNIT TRANSFORMER Ib UNIT AUXILIARIES TRANSFORMERS WHEN PRESENT SYSTEM Ig - Ib GENERATOR BREAKER CLOSED CIRCUIT BREAKER CLOSED AUXILIARIES LOAD Ig UNIT AUXILIARIES TRANSFORMERS Ib - Transformer back-feed current Ig – Generator load current AUXILIARIES LOAD NOTE 1—UT HV and UAT LV grounding are design dependent NOTE 2—UT and UAT neutral connections are omitted for simplicity Figure 18—Typical two generator three winding UT back-feed operation 11.1 Special transformer considerations When the UT is operated in a back-fed mode, the low-voltage delta connected winding may only be connected to an isolated phase-bus, or perhaps generator arresters or PT’s, or some minimal load, but the low-voltage systems reference to ground that is normally through the generator is lost (see Figure 19) In the worst case, all that may be present on the low-voltage system is the stray capacitance to ground of the connected bus This presents several concerns to the transformer The phase-to-ground potentials can shift to as high as the phase-to-phase voltage due to the lack of a neutral reference Nearby switching transients, or energization of the transformer itself, will inject high frequency transients in the – 20 kHz range The lower frequency transients would typically excite the LV winding resonance and the high frequency transients could excite full and partial HV winding resonances Under low load conditions in back-feed mode disconnected from the generator, losses are low The result is low damping of high-voltage oscillations associated with winding resonances resulting in transient voltages in the windings in excess of what the transformer is designed for or tested at the factory with the low-voltage winding grounded possibly resulting in winding failures [B15] Additionally, the low capacitance of the secondary winding and its coupling with the high-voltage winding can transfer transient voltages developing in the low-voltage to the HV winding [B10] and [B1] 32 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators SYSTEM UNIT TRANSFORMER Ib CIRCUIT BREAKER OPEN or DISCONNECTED CLOSED GENERATOR Ib - Transformer back-feed current NOTE 1—UT HV grounding is design dependent NOTE 2—UT neutral connection is omitted for simplicity Figure 19—Back-feed configuration with only generator and bus Operation under these conditions is further exacerbated by the disconnection of the generator, through the removal of links or open breakers, which leaves the transformer and generator bus without protective relaying Similarly, with disconnection of the generator, the generator surge protection (arresters and/or surge capacitors or both) are removed from the circuit leaving the LV winding of the transformer unprotected All of the above should be considered when back-feeding is an anticipated mode of operation Such operation should be discussed with the transformer manufacturer as part of the specification and design of a UT for a new generating facility, or when an existing facility’s UT will be back-fed due to operational needs Some plant owners have provided for the back-feed configuration by:  Installing a carefully designed temporary capacitor bank to ground  Utilizing properly sized generator circuit breaker capacitors  Providing extra transient protection at the low-voltage side of the UT  Not allowing back-feed operation under any circumstances Protective relaying and surge protection should be designed to adequately protect the transformer and all associated equipment for normal, startup, shutdown, commissioning, or plant overhaul situations Additional information on these transformer considerations can be found in [B10], [B15], [B1], and [B11] 11.2 Protective relaying considerations The protective relaying for the back-feed of a UT should be reviewed for all operating configurations, including both providing normal plant startup and shutdown power as well as infrequent commissioning or plant rebuild or overhaul situations For example, the back-feeding of a UT is many times facilitated by disconnecting the generator by the removal of isolated phase-bus links or generator terminal links, since generator breakers were not installed As a result, the normal protective relaying schemes, which may utilize current transformers within the generator, are rendered inoperative and the low-voltage bus system and the transformer may become unprotected Prior to operating in this mode, or when designing a plant to operate in this mode, it shall be verified that the protective relaying system has been designed to protect the transformer and bus under all operating configurations, fault conditions, and aforementioned special transformer considerations At some plants, this may require the addition of protective relay schemes to provide adequate protection under back-feed conditions 33 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators 11.3 Surge protection considerations Just as protective relaying may be compromised in back-feed configurations, so can surge protection The disconnection of the generator through the removal of links, may remove the generator surge protection (arresters and/or surge capacitors, or both) from the circuit leaving the low-voltage winding of the transformer completely unprotected As in many typical generator installations, the low-voltage side of the transformer is not protected by typical station class arresters, because the bus is typically enclosed and frequently within a building and therefore not exposed to lightning Further, unless a generator breaker is present, the low-voltage winding is not exposed to switching transients generated by low-voltage side switching, but certainly can be exposed to surges transferred from the high-voltage winding There could be a UAT that is present; however, it is typically switched on its low-voltage side, making switching transients one bus removed from the generator UT Station class arresters could be installed on the low-voltage winding of a UT, but like all arresters would have to be adequately sized for the available fault current and any temporary overvoltage conditions that may exist For instance, full load rejection would expose the arresters to a temporary overvoltage until the field of the generator decays, or in the back-feed mode, such arresters could potentially see long duration rise in phase potential to phase-to-phase as the voltage system may now be without a ground reference Moreover, the arrester would probably have to be installed in a metal enclosure, as it will be inside the power plant building All of these conditions would have to be considered in selecting an arrester Further, it is critical to note that while the installation of low side to ground arresters will help protect the UT in a back-fed configuration for phase-to-ground transients, it will not adequately protect for phase-to-phase transients [B12]; nor are the arresters alone sufficient to address the special transformer considerations described in 11.1 34 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators Annex A (informative) Bibliography Bibliographical references are resources that provide additional or helpful material but not need to be understood or used to implement this standard Reference to these resources is made for informational use only [B1] IEC Std 60071-2-1996, Insulation Coordination–Part 2: Application Guide, Annex–E–Transferred Over-Voltages in Transformers [B2] IEEE Power System Relay Committee, Rotating Machinery Subcommittee, J-5 Working Group Report, “Coordination of Generator Protection with Generator Excitation Control and Generator Capability,” 2007 [B3] IEEE Power System Relay Committee, Rotating Machinery Protection Subcommittee, J-9 Working Group Report, “Motor Bus Transfer Applications Issues and Considerations,” 2012 [B4] IEEE Std 505™-1977, IEEE Standard Nomenclature for Generating Station Electric Power Systems [B5] IEEE Std 1312™, IEEE Standard Preferred Voltage Ratings for Alternating-Current Electrical Systems and Equipment Operating at Voltages Above 230 kV Nominal [B6] IEEE Std C37.013™, IEEE Standard for AC High-Voltage Generator Circuit Breakers Rated on a Symmetrical Current Basis [B7] IEEE Std C37.102™, IEEE Guide for AC Generator Protection [B8] IEEE Std C57.12.01™, IEEE Standard General Requirements for Dry-Type Distribution and Power Transformers, Including those with Solid-Cast and/or Resin Encapsulated Windings [B9] IEEE Std C57.12.80™, IEEE Standard Terminology for Power and Distribution Transformers [B10] Lackey, J G., A S Morched, L Marti, and R H Brierley, “Analysis of Internal Winding Stresses in EHV Generator Step-Up Transformer Failures,” IEEE Transactions on Power Delivery, vol 11, no 2, pp 888− 894, Apr 1996 [B11] Lee, K H and J M Schneider, “Rockport Transient Voltage Monitoring System: Analysis and Simulation of Recorded Waveforms,” IEEE Transactions on Power Delivery, vol 4, no 3, pp 1794−1805, Jul 1989 [B12] Keri, A J., Y I Musa, and J A Halladay, “Insulation Coordination for Delta Connected Transformers,” IEEE Transactions on Power Delivery, vol 9, no 2, pp 772−780, Apr 1994 [B13] “Main Transformer Failures at the North Anna Nuclear Power Station, NRC Report on North Anna Nuclear Power Station,” Dec 22, 1982 [B14] McNutt, W J., “Failures of Generator Step-Up Transformers Under Back-Feeding Conditions,” CIGRE SC12 Colloquium, Florence, Italy, Oct 1987 IEC publications are available from the Sales Department of the International Electrotechnical Commission, rue de Varembé, PO Box 131, CH-1211, Geneva 20, Switzerland (http://www.iec.ch/) IEC publications are also available in the United States from the Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor, New York, NY 10036, USA (http://www.ansi.org) IEEE publications are available from The Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/) 35 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators [B15] Preininger, G., R J Musil, E Schopper, and S Wenger, “Voltage Stresses Produced by Aperiodic and Oscillating System Over-voltages in Transformer Windings,” IEEE Transactions on Power Apparatus and Systems, vol PAS-100, no 1, pp 431− 441, Jan 1981 36 Copyright © 2014 IEEE All rights reserved ... IEEE Std C37.013™, IEEE Standard for AC High-Voltage Generator Circuit Breakers Rated on a Symmetrical Current Basis [B7] IEEE Std C37.102™, IEEE Guide for AC Generator Protection [B8] IEEE Std. .. connection may be obtained using a standard transformer (see IEEE Std C57.12.70) 21 Copyright © 2014 IEEE All rights reserved IEEE Std C57.116-2014 IEEE Guide for Transformers Directly Connected to Generators... for Electric Power Systems and Equipment IEEE Std 666™, IEEE Design Guide for Electric Power Service Systems for Generating Stations 2, IEEE Std C50.13™, IEEE Standard for Cylindrical-Rotor 50