Simulations of open phase conditions on the high voltage side of YNd05-power plant transformers

Simulations of open phase conditions on the high voltage side of YNd05-power plant transformers Disclaimer: All information presented in the report, the results and the related computer program, data, calculation methods, etc. have been created and tested by Amprion GmbH to the best of its knowledge and with utmost care. Nevertheless, errors cannot be completely excluded. Amprion GmbH assumes no liability for the topicality, correctness, completeness or quality of the information provided. It is not allowed to distribute this document without having received permission. Martin Lösing Amprion GmbH, 44139 Dortmund - Germany Workshop: Robust Power, BWR, May 2017

10-kV-grid L1 L2 L3 YNd05-transformer 45 MVA 110 kv/10.5 kv L1 L2 L3 110-kV-line open phase condition ~ ~ ~ 110-kV-grid Petersen coil Contents Introduction to open phase conditions on the high voltage side of YNd05-power plant transformers Simplified simulation model of a typical 1500-MVApower plant Simulation versus measurement Various cases of open phase conditions Conclusion 2 Work progress 15.01.2014

Introduction: Open phase conditions on the high voltage side of YNd05-power plant transformers - Simulations: First simulations were done to investigate the unbalance of the 10-kV-auxiliary asynchronous motors and of the generator during open phase conditions (one phase or two phases open) on the high voltage side of YNd05-transformers of the power plant (interface grid power plant). Simulations were performed for both cases: generator out of operation (maintenance) and generator in power operation. Without generator in operation the power plant auxiliary load is supplied either by means of the 380-kV-grid or from the 110-kV-grid. The simulations are based on a simplified model using a 1500-MVA-Generator power plant. Typical data for the power plant transformers, the generator, the 10-kV-motors and the grid connections are used (see page 6). 3

Introduction: Open phase conditions on the high voltage side of YNd05-power plant transformers The simulations were done with the PSS-Netomac power system analysis software (network torsion machine control, Siemens) High frequency transient voltages or currents during the changeover from normal operation to open phase conditions are not considered in these simulations. The end of the open phase is open (not grounded) in these simulations. - Test and measurement: For a 55-MVA-Transformer (110 kv/10.5 kv) measurements were performed for the noloaded transformer with reduced voltage. The measurements were compared with the results of the simulation model. Also measurements of generator behaviour during open phase conditions (real cases from the practice) were compared with the results of the simulation model. 4

Voltage unbalance during open phase conditions on high voltage side The unbalance of the voltage (auxiliary grid) and the unbalanced load of the induction motors (negative sequence current) during open phase conditions are mainly influenced by: transformer vector group transformer neutral point connection: directly earthed (solidly grounded) or neutral point not grounded (connection open) length of the 380-kV-power transmission line, length of the 110-kV-line transformer core design, transformer short circuit voltage, no-load current and losses, positive and zero sequence impedances. loading of the transformer (high, low or unloaded) Grid earthing conditions (earth fault factor), short circuit power of the grid location of the open phase auxiliary load composition (induction motors and resistance load, mixed load) motor parameters, generator regulation etc. 5

Simplified model of a typical 1500-MVA-power plant and the electrical grid connection (example) Generator: 1500 MVA, 27 kv with generator voltage controller (PSS included) and turbine controller model H = 4.7 s Inertia Constant of the whole turbo shaft (turbines, exciter and generator) 6 x d x q x d " x q " = 2.04 pu = 1.94 pu = 0.35 pu = 0.39 pu For the transformers the zero sequence impedances and the no load current and losses are modelled. For the grid the zero sequence impedance is also modelled. For the induction motors dynamic models are used. Standby transformers: 2*45 MVA, 110 kv/10.5 kv star delta, YNd05 transformer neutral points open (not grounded) u x = 11.6 % Three legged core type Grid: Short circuit power 380-kV-grid S kq = 25 GVA Short circuit power 110-kV-grid S kq = 4.5 GVA Step up transformers: (unit generator transformers) 2*725 MVA 420 kv/ 27 kv u x =16.4 % star delta, YNd05 transformer neutral points solidly grounded Five legged core type Auxiliary transformers: 2*76 MVA 27kV/10.5kV/10.5kV Yy0y0 transformer neutral points open u x12 = 8%, u x13 = 8 %, u x23 = 15.8 %

Measurement versus Simulation (Test 1) Test object: 55-MVA-transformer (standby transformer, not loaded, 110 kv/10.5 kv, neutral point not grounded) For the transformer in this test circuit a clear voltage unbalance is shown on the 10-kVside of the transformer. measurement Simplified scheme (Extract from German protocol) Around 91 % of the value without open phase condition.. The voltage unbalance depends on: the winding capacitances, the cable capacitances, the no-load transformer current, etc. 10-kV-phase-phase voltages (feeding voltage on high voltage side reduced for the test) 7

Measurement versus Simulation (Test 2) Test object: 55-MVA-transformer (standby transformer, not loaded, 110 kv/10.5 kv, neutral point grounded) Simplified scheme (Extract from German protocol) Around 100 % of the value without open phase condition.. In this case with transformer neutral point grounded no voltage unbalance is shown on the 10-kV-side Simulation Leiter-Leiter- Spannung 10-kV-Seite measurement 10-kV-phase-phase voltages (feeding voltage on high voltage side reduced for the test) 8

Simulation Cases: Case 1: One phase on high voltage side (110 kv) of the transformers in open phase condition. Transformer loaded with motor in operation, neutral points not grounded.) Case 2: One phase on the 380-kV-power transmission line in open phase condition. Case 2a) With generator in power operation, transformer neutral points grounded. Case 2b) With generator in power operation, transformer neutral points not grounded. Case 3: One phase on the 380-kV-power transmission line in open phase condition. Case 3a) Generator not in operation, transformer neutral points grounded. Case 3b) Generator not in operation, transformer neutral points not grounded. Case 4: Two phases on the 380-kV-power transmission line in open phase condition. Case 4a) With generator in power operation, transformer neutral points grounded. Case 4b) With generator in power operation, transformer neutral points not grounded. Case 5: Two phases on the 380-kV-power transmission line in open phase condition. Case 5a) Generator not in operation, transformer neutral points grounded. Case 5b) Generator not in operation, transformer neutral points not grounded. 9

Case 1: One phase on high voltage side (110 kv) of the transformers in open phase condition (transformer loaded with motor in operation, neutral points not grounded.) 10

Case 1: One phase on high voltage side (110 kv) of the transformers in open phase condition (transformer loaded with motor in operation, neutral points not grounded.) In this case a relative high voltage unbalance occurs. In this simulation the negative sequence voltage is around 19 % (based on rated voltage Phase Earth) Motor busbar 11

Case 1: One phase on high voltage side (110 kv) of the transformers in open phase condition (transformer loaded with motor in operation, neutral points not grounded.) In this simulation the motor is not pulling out (stalling).if the motor is higher loaded, motor pulling out of step is possible (electrical torque falls below breakdown torque). Motor speed would then break down. 12 In this simulation the negative sequence current of the motor is around 100 % based on motor rated current. A 100-Hz-component occurs in the electrical motor torque

Case 2a: One phase on the 380-kV-power transmission line in open phase condition with generator in power operation, step up transformer neutral points grounded. 13

Case 2a: One phase on the 380-kV-power transmission line in open phase condition with generator in power operation, step up transformer neutral points grounded. 14

Case 2a: One phase on the 380-kV-power transmission line in open phase condition Output from generator protection. Simulated voltages/currents were fed into the protection device via Comtrade interface Negative sequence current of generator in % (based on rated generator current) (RMS) In the simulated case the protection operates after around 52 s. (depending on the protection settings) 15

Case 2a: One phase on the 380-kV-power transmission line in open phase condition with generator in power operation, step up transformer neutral points grounded. In this simulation the negative sequence voltage is around 7 % - 8 % (based on rated voltage Phase Earth) 16

Case 2a: One phase on the 380-kV-power transmission line in open phase condition with generator in power operation, step up transformer neutral points grounded In this simulation the negative sequence current of the motor is around 40 % - 45 % (based on rated motor current) 17

Case 2a: One phase on the 380-kV-power transmission line in open phase condition with generator in power operation, step up transformer neutral points grounded. Without PSS (Power System Stabilizer) in the AVR, poorly damped or undamped generator power oscillations can occur in this situation. The behavior depends on: the grid short circuit power, the zero sequence reactance of the grid, length of the power transmission line, excitation of the generator (underexcited operation is more critical with respect to undamped power oscillations) etc. In the worst case the generator loses synchronism. 18

Case 2a: Measurement versus Simulation Synchrongenerator 780 MVA Prinzipbild ~ P G = 600 MW Q G = 200 MVAr 21 kv Eigenbedarf 800 MVA 21 kv/420 kv 380-kV-power line 10,7 km 380-kV- Netz Open phase condition L2 with generator in power operation, step up transformer neutral point grounded Real case from practice: (780 MVA power plant) i(t) Gen-current L1 t/s 20 ka Measurement generator current (output from generator protection) Gen-current L2 Beginning of the fault not measured Gen-current L3 Change in scaling! 38 ka (Amplitude) t/s 20 ka 40 ka 20 ka Negative sequence current of the generator is around 28 % (based on rated generator current) i(t) 30 30 ka Gen-STROM L1 in ka 0 t/s Simulation generator current Gen-STROM L2 in ka -30 30 30 ka 0-30 30 30 ka Gen-STROM L3 in ka 0 40 ka (Amplitude) 19-30 0 0.25 0.50 0.75 1.00 [s]

Case 2b: One phase on the 380-kV-power transmission line in open phase condition with generator in power operation, step up transformer neutral points not grounded. In this case the generator becomes asynchronous (also with PSS) against the grid. The out of step protection (pole slipping protection) can not detect this asymmetrical operation of the generator against the grid in this simulation. 20

Case 3a: One phase on the 380-kV-power transmission line in open phase condition generator not in operation, step up transformer neutral points grounded 21

Case 3a: One phase on the 380-kV-power transmission line in open phase condition generator not in operation, step up transformer neutral points grounded. 22 Open phase In this simulation only a very small voltage unbalance occurs

Case 3a: One phase on the 380-kV-power transmission line in open phase condition generator not in operation, step up transformer neutral points grounded In this simulation only a relative small unbalance is shown 23

Case 3b: One phase on the 380-kV-power transmission line in open phase condition generator not in operation, step up transformer neutral points not grounded. Motor currents in pu, 1 pu = 100 % = rated motor current (amplitude) In this simulation the motor is not pulling out (stalling).if the motor is higher loaded, motor pulling out of step is possible (electrical torque falls below breakdown torque). Motor speed would then break down. 24 Negative sequence current of the motor is in this simulation around 100 % based on motor rated current. A 100-Hz-component occurs in the electrical motor torque

Case 3b: One phase on the 380-kV-power transmission line in open phase condition generator not in operation, step up transformer neutral points not grounded. In this simulation then negative sequence voltage is around 18 % (based on rated voltage Phase Earth) 25 In this case a relative high voltage unbalance occurs

Case 4a: Two phases on the 380-kV-power transmission line in open phase condition with generator in power operation, step up transformer neutral points grounded. In this simulation the generator becomes asynchronous against the grid (out of step operation, generator speed increases). For this situation further investigations of the 380-kV-line protection are necessary. 26

Case 4a: Two phases on the 380-kV-power transmission line in open phase condition with generator in power operation, step up transformer neutral points grounded. 10-kV-voltage during asynchronous generator operation. 28

Case 4a: Two phases on the 380-kV-power transmission line in open phase condition (Protection) Output from generator protection. Simulated voltages/currents were fed into the generator protection device via Comtrade interface 27kV 3 positive sequence generator voltage (RMS) Generator Impedance locus diagram (positive sequence) [Ohm] negative sequence generator voltage (RMS t/s positive sequence generator current (RMS) Function Out of step protection negative sequence generator current (RMS) t/s The out of step protection (pole slipping protection) does not detect this asynchronous generator operation in this simulation, because the impedance locus is outside of the rectangular window. The adaptation/extension of the rectangular window can result in unselective settings of the out of step protection. The impedance characteristic would be outside the desired protection zone and can also trip in case of network faults. A possible criterion to detect such unsymmetrical out of step operation could be the negative sequence generator voltage (smoothed). More investigations are necessary. Special attention has to paid, if in the out of step protection a negative sequence blocking is activated. This has to be examined. However, an unselective unneeded tripping has to be avoided. Moreover the burden for the damper windings and the shaft during the unsymmetrical out of step operation must be examined. 30

Current secondary in A Voltage secondary in V Case 4a: Two phases on the 380-kV-power transmission line in open phase condition (Measurement) Real case from practice: (190 MVA power plant) 5000 with generator in operation, step up transformer neutral point grounded u(t) generator voltage (Phase-Earth) Two phases open due to a malfunction in the circuit breaker (220 kv power line of the power plant). Step up Transformer neutral point grounded. Generator becomes asynchronous against the grid. Generator voltage Phase-Earth (RMS) 40 Generator current (RMS) Output from generator protection 2 31 No reaction from generator protection occurs. The out of step protection (pole slipping protection) did not detect this asynchronous operation.

Case 4b: Two phases on the 380-kV-power transmission line in open phase condition. with generator in power operation, step up transformer neutral points not grounded. With two open phases on the 380-kV-power transmission line, very high voltages for the step up transformers can occur. No power transfer about the step up transformers (generator speed accelerates). For these cases the behavior of the surge arresters has to be examined. 32

Case 5a: Two phases on the 380-kV-power transmission line in open phase condition generator not in operation, step up transformer neutral points grounded. 33

Case 5a: Two phases on the 380-kV-power transmission line in open phase condition generator not in operation, step up transformer neutral points grounded. In this simulation the negative sequence voltage is around 18 % (based on rated voltage Phase Earth) 34

Case 5a: Two phases on the 380-kV-power transmission line in open phase condition generator not in operation, step up transformer neutral points grounded. Motor currents in pu, 1 pu = 100 % = rated motor current (amplitude) In this simulation the negative sequence current of the motor is around 100 % based on motor rated current. A 100-Hz-component occurs in the electrical motor torque In this simulation the motor is not pulling out (stalling). If the motor is higher loaded, motor pulling out of step is possible (electrical torque falls below breakdown torque). Motor speed would then break down. 35

Case 5a: Two phases on the 380-kV-power transmission line in open phase condition generator not in operation, step up transformer neutral points grounded. Small auxiliary load In this simulation the negative sequence voltage is around 18 % (based on rated voltage Phase Earth) In this simulation (generator not in operation, small auxiliary load), the current on the transformer neutral points is very small. In practice it is difficult to distinguish this current from the current on the transformer neutral points during normal operation. 36

Case 5b: Two phases on the 380-kV-power transmission line in open phase condition generator not in operation, step up transformer neutral points not grounded. With two open phases on the 380-kV-power transmission line, very high voltages for the step up transformers can occur. For these cases the behavior of the surge arresters has to be examined. 37

Summary of open phase conditions on the high voltage side of YNd05-power plant transformers Among other things the neutral point connection for the investigated power plant transformers are very important for the unbalance of the voltage on the low voltage side of the transformer. Also the grid connection has an important influence. The no-load situation (transformer standby) is completely different from situations where the transformer is loaded with induction motors in operation. For no-loaded YNd05-transformers (standby) with open neutral point the no-load current (excitation current), the no-load losses, the transformer winding capacitances (stray capacitances) and cable capacitances of feeding lines or cables have an important influence of the unbalance. In some shown simulation cases the motors obtain a high negative sequence current (up to 100 % and more, based on motor rated current) and a high 100-Hz-component occurs in the electrical motor torque. 38

Summary of open phase conditions on the high voltage side of YNd05-power plant transformers These first simulations for the considered configuration show, that the open phase conditions in most cases may be detectable with special new dedicated measurement devices. - Possibilities of detection: For example on the low voltage side of the transformer by measuring of: negative sequence voltage of the 10-kV-auxiliary bus bar voltages or the 27-kV-voltage negative sequence voltage of the 10-kV-side of the standby transformers (open transformer neutral point) negative sequence current of selected motors negative sequence current of outgoing 10-kV-cables in the auxiliary related to the positive sequence current measuring of the three phase to phase voltages (unbalance between the three phase to phase voltages) 39

Summary of open phase conditions on the high voltage side of YNd05-power plant transformers For example on the high voltage side of the transformer by measuring of: voltage of the neutral point to earth (transformer neutral point connection is open) current of the transformer neutral point to earth (transformer neutral point is solidly grounded, transformer must be loaded). Unbalance of the line currents. measuring of the three phase to phase voltages (unbalance between the three phase to phase voltages) and of the negative sequence voltage. etc. Each individual configuration has to be carefully considered. 40

Summary of open phase conditions on the high voltage side of YNd05-power plant transformers In case of transformer grounded neutral point and one phase on the high voltage side in open phase condition, the unbalance is very small, specially if the transformer is lowly loaded. Detection of the open phase is very difficult. Further investigations are necessary. It is important to separate open phase conditions from voltage unbalance during normal grid operation (for example during transformer inrush currents and for motor start-up) and short time unsymmetrical grid faults. In the case of open phase conditions on the 380-kV-power transmission line with generator in operation, the performance of the generator protection is very important, because the generator power is swinging or an out of step operation of the generator can occur. For this asynchronous operation the out of step protection has to be examined. The shown simulation results are only examples for the considered model. The results can be different for other grid connection configurations and other transformer or generator data. For all cases further investigations (simulations and measurements) are necessary, together with motor-, generator-, line protection- and measurement-experts. It would be helpful to exchange experience and simulation results between the international partners. 41

Literature: - PSS Netomac: Simulation program (network torsion machine control) - A practical guide for detecting single-phasing of three phase power systems by J. Horak, G.F.Johnson, Basler Electric - Alstom: Open Phase detection of unloaded power transformers (2014) - EPRI, Analysis of Station Auxiliary Transformer, Response to Open Phase Conditions, IEEE SC4, July 30, 2013 EPRI: Reports 2013 und 2015 (WANO) - Cigre 2016, Performance of generator protection during power system failures selected protection functions and new experiences with unsymmetrical faults, Herrmann, Lösing, Hötzel - Cigre C4.307 Resonance and Ferro Resonance in Power Networks 2013 - Cigre C4.307, Tutorial 2013 - Cigre A2/C4.39, Electrical Transient Interaction between Transformers and the power System April 2014 - Cigre C4.307, Transformer Energization in Power Systems 2013 - IAEA Report: Impact of Open phase Conditions on Electrical Power Systems 42