DOUBLY-FED ASYNCHRONOUS MACHINE WITH 3-LEVEL VSI FOR VARIABLE SPEED PUMP STORAGE

Transcription

1 DOUBLY-FED ASYNCHRONOUS MACHINE WITH 3-LEVEL VSI FOR VARIABLE SPEED PUMP STORAGE A. Sapin, A. Hodder, J.-J. Simond Swiss Federal Institute of Technology Electromechanics and Electrical Machines Laboratory (LEME) CH-1015 Lausanne, Switzerland Tel: Fax: D. Schafer Alstom Power Generation Ltd R&D / Technology Director CH-5242 Birr, Switzerland Tel: Fax: daniel.schafer@ch.abb.com ABSTRACT This paper deals with a fixed-frequency variable-speed motor/generator with 3-level VSI (Voltage Source Inverter) cascade. The 3-level VSI is applied to a DASM (Doubly-fed ASynchronous Machine) for large pump storage plants. The extension to 3 levels is the original part of this paper. The whole system behavior has been simulated using the SIMSEN simulation software. This paper focuses on the simulation results and also on the comparison with the 12-pulse cycloconverter topology. Keywords: Varspeed, Generator, High Power 3-level VSI, Doubly-fed ASynchronous Machine DASM, Adjustable Speed Drive, Vector Control, pump storage, Hydro Power. 1 INTRODUCTION Recently, doubly-fed adjustable speed systems have been applied to pumped storage power plants, because of many advantages such as adjustable pump input, improved turbine efficiency and power system stabilization [1]. The studied drive topology is represented in Fig. 1. The usual topology for this kind of drives is the cyclo-converter [2]. Due to the recent improvement of semiconductors, like IGCT (Integrated Gate Control Thyristor), the 3-level VSI topology can now be used in large adjustable speed drives like DASM for instance. The use of a 3-level VSI leads to many improvements. The 3-level VSI only need 1 transformer, 24 hard-driven GTO s with diodes and 12 NPC (Neutral Point Clamp) diodes, whereas the cyclo-converter needs 3 transformers of 3-winding type and 72 semiconductors. Another point is that the 3-level VSI can be use like a STATCOM (STATic COMpensator) on the network side, and work with a power factor equal to 1. Depending on the apparent power of the converter transformer, it can even supply reactive power. In comparison, the cyclo-converter total reactive power has to be supplied by the AC grid. To maintain the required global power factor of the power plant, the machine has to produce the cyclo-converter reactive power. This leads to an increased rotor current. The 2-level VSI cascade control has been presented in [3]. In comparison with it, the 3-level VSI leads to a lower THD (Total Harmonic Distortion). A large capacity GTO converter for DASM has been presented in [4]. The aim of this paper is to present the performance of the 3-level VSI cascade and to compare it with the conventional 12-pulse cyclo-converter cascade. The DASM used in this paper has the following rated values: Apparent power: 230 MVA Line-to-line voltage: KV Frequency: 50 Hz Number of poles: 2p = 18 DC-link voltage: 4 kv Fig. 1. VARSPEED : Doubly-fed induction motor/generator with 3-level VSI cascade

2 2 SYSTEM MODELING AND CONTROL The studied power circuit and the regulation block diagram are given in Fig Power part The Varspeed is made up of a wound rotor asynchronous machine (1). The fixed frequency stator of this machine is connected to the AC grid through the main transformer (2). The cascade converter is connected to the AC grid through the converter transformer (3). The first 3-level VSI (4) supplies the DC-link (6). The wound rotor of the machine is fed by another VSI (4). 2.2 Regulation part The PLL block (7) gives the electrical stator angle θ s, which corresponds to the position of the stator voltage phasor of the machine. The position sensor (8) gives the mechanical rotor angle θ m. This mechanical angle, subtracted to θ s gives the electrical rotor angle θ r. The control of the machine is based on 4 degrees of freedom: the speed set value n set, the stator reactive current set value i sqset, the converter transformer reactive current set value i tqset and the DC-link voltage set value u dcset. The speed regulator (9) calculates the stator active current set value i sdset used by the stator current regulator (11). The coordinates transform block (10) calculates the stator active i sd and reactive i sq currents of the machine, using the θ s angle. The stator current regulator (11) calculates the required active i rdset and reactive i rqset rotor current set values. The coordinates transform block (13) calculates the rotor active i rd and reactive i rq currents of the machine using the θ r angle. The rotor current regulator (12) calculates the required rotor output voltages u cmd and u cmq. After a reverse coordinates transform (14), the command signals tune the gates control (15) of the VSI (5). The DC-link voltage regulator (16) calculates the active current i tdset set value used by the transformer current regulator (17). The coordinates transform block (18) calculates the transformer primary side active i td and reactive i tq currents using the θ s angle. The transformer current regulator (17) calculates the required transformer output voltages u cmd and u cmq. After a reverse coordinates transform (19), the command signals are tuning the gates control (20) of the VSI (4). i tqset 17 Rit i td i tq i tdset 12 Rir i rd i rq 18 u 19 cmd u cmt 20 Gates Control u cmq Ru i t 16 u cmd 14 u cmr 15 Gates Control u cmq i r 13 u dcset 3 C1 6 C2 Converter Transformer θ s CT 4 CM 5 7 PLL θ s 1 2 ASM Network Transformer i s n Position Sensor θ m 8 10 i sd i sq i rqset Ris Rn 11 i rdset 9 i sdset i sqset n set 1. Asynchronous machine with wound rotor. 2. Main transformer. 3. Converter transformer level VSI on network side level VSI on rotor side. 6. DC-link. 7. Phase Locked Loop (PLL). 8. Rotor position sensor. 9. Speed regulator. 10. Stator coordinates 11. Stator current regulator. 12. Rotor current regulator. 13. Rotor coordinates 14. Rotor reverse coordinates 15. Rotor VSI gates control. 16. DC-link voltage regulator. 17. Transformer current regulator. 18. Transformer coordinates 19. Transformer reverse coordinates 20. Transformer VSI gates control. θ r Fig. 2. Studied power circuit and regulation block diagram. 2.3 Modeling and implementation into SIMSEN The SIMSEN simulation software has been developed to simulate power systems in details [5]. The whole power circuit (see Fig. 1.) has been modeled including the mechanical system. SIMSEN appeared to be a powerful simulation system, especially when reconnecting the cascade transformer to the AC grid. This allows to estimate correctly the global power plant current wave in the main transformer and to take into account the influence of the VSI commutations on the stator voltages. As SIMSEN is based on a modular structure, it is very easy to define the regulation block diagram in details. For further information:

3 3 SIMULATION RESULTS IN STEADY-STATE The studied case is a steady-state operating point in supersynchronous motor conditions (speed = 1.1 p.u., mechanical torque = -0.8 p.u., machine power factor = 1, transformer power factor = 1 for the 3-level VSI cascade). This means that the power factor is set to 1 on the secondary side of the main transformer with the 3-level VSI cascade. The left column presents the results of the 12-pulse cyclo-converter cascade and the right column presents the results of the 3-level VSI cascade. Simulations with other operating points have also been performed, but are not included in the present paper. Fig. 4. Varspeed with 3-level VSI cascade Fig. 3. Varspeed with 12-pulse cyclo-converter cascade Fig. 5. Motor phase voltage and current Fig. 6. Motor phase voltage and current Fig. 7. Spectrum of motor phase current Fig. 8. Spectrum of motor phase current Fig. 9. Motor air-gap torque and speed Fig. 10. Motor air-gap torque and speed - 3 -

4 Fig. 11. Fig. 12. Fig. 13. Spectrum of main transformer phase current Fig. 14. Spectrum of main transformer phase current Fig. 15. Cyclo-converter transformer currents Fig. 16. Converter current at network side Fig. 17. Voltage and current of a thyristor valve Fig. 18. DC-link voltages 4 STEADY-STATE RESULTS ANALYSIS As expected, the 3-level VSI cascade seems to be a very suitable solution. In comparison with the 12-pulse cycloconverter, the main transformer currents as well as the machine currents contain higher harmonics orders. The 3-level VSI cascade has however lower THD (Total Harmonics Distortion) as the 12-pulse cyclo-converter: THD (machine current with cyclo cascade): 0.582% THD (machine current with VSI cascade): 0.449% THD (main transfo. current with cyclo cascade): 0.937% THD (main transfo. current with VSI cascade): 0.622% To reach the above THD values, improved PWM control has been applied to the modulation shape of the 3-level VSI on the network side. This control requires 250 Hz switching frequency of each GTO valve. On the machine side, a 500 Hz constant switching frequency has been selected. This switching frequency could even be reduced without so much influence on the rotor current. Such switching frequencies fulfill the requirements of the new hard-driven GTO Thyristors. It is interesting to observe that the 3-level VSI cascade leads to a reduced main transformer current (see Fig. 11 and 12) due to the fact that the power factor of the 12-pulse cyclo-converter cascade is depending on its operating point and cannot be directly controlled (line-commutation)

5 5 SIMULATION RESULTS IN TRANSIENTS The simulated case corresponds to a single phase short circuit on the high voltage side of the main transformer, occurring during 60 ms with an initial operating point equal to the one specified in point 3. The next figures show that the DASM motor/generator with 3-level VSI cascade stabilizes very quickly after a voltage drop. The NP (Neutral Point) unbalanced voltages control has also been implemented. This control equalizes the two DC-link voltages. The 3-level VSI on the network side helps maintaining the voltage on the AC side. This is a helpful feature of this VSI, which works like a STATCOM (STATtic COMpensator). Fig. 23. DC-link voltages Fig. 19. Line phase voltages of the machine Fig. 24. Fig. 20. Stator phase currents of the machine 6 CONCLUSIONS The 3-level VSI cascade is well suited for the Doubly-fed ASynchronous Machine (DASM). An example of large power DASM for variable speed pump storage has been simulated and compared with the conventional 12-pulse cyclo-converter cascade. Both simulations have been performed using the SIMSEN simulation software, which proved to be a powerful tool to simulate such complex power systems topologies. The 3-level VSI cascade allows reaching very low values of THD for the machine current as well as for the main transformer current. These values are even lower as those obtained with the 12-pulse cyclo-converter cascade. The 3-level VSI cascade has the advantage that not only the machine operates with a high power factor, but also the converter transformer. It also demonstrates a stable behavior under ride-through conditions. Fig. 21. Fig. 22. Rotor phase currents of the machine Air-gap torque and speed of the machine REFERENCES [1] J.-J. SIMOND, D. SCHAFER, Expected benefits of adjustable speed pumped storage in the European network, HYDROPOWER into the next century, October 1999, Gmunden, Austria. [2] D. SCHAFER, J.-J. SIMOND, Adjustable Speed Asynchronous Machine in Hydro Power Plants and its Advantages for the Electric Grid Stability, CIGRE Report, Paris [3] H. STEMMLER, A. OMLIN, Converter Controlled Fixed- Frequency Variable-Speed Motor/Generator, IPEC 95, Japan. [4] S. FURUYA, F. WADA, K. HACHIYA, K. KUDO, Large Capacity GTO Inverter-Converter for Double-Fed Adjustable Speed System, Symposium Tokyo, CIGRE [5] A. SAPIN, J.-J. SIMOND, SIMSEN : A modular software package for the analysis of power networks and electrical machines, CICEM 95, Hangzhou, China