Applied Mechanics and Materials Submitted: 2014-07-20 ISSN: 1662-7482, Vols. 644-650, pp 3861-3865 Accepted: 2014-07-22 doi:10.4028/www.scientific.net/amm.644-650.3861 Online: 2014-09-22 2014 Trans Tech Publications, Switzerland Simulation Research on Starting Resistor Parameter Selection of 10kV Chain Static Var Generator Shaogang Hu 1, Shuhan Wang 2, Jun Liu 1, Jihao Wang 3, Sicong Wu 3 1 State Grid Liaoning Electric Power Supply Co., Ltd. Ansan Power Supply Branch, Ansan Liaoning 114001 China 2 Cornell University, Ithaca, NY14853,USA 3 Rongxin Power Electronic Co., Ltd., Anshan, Liaoning 114051, China Keywords: SVG, starting resistor, PSCAD Abstract. Before commissioning, SVG first requires a separated excitation process: network voltage charges the capacitor on DC side in each H-bridge power module via the starting resistor (IGBT in all modules shall lock impulse). After charging, SVG has enough power for dynamic var compensation of the system. This paper introduces fundamental functions of the starting resistor in SVG, analyzes the influence of starting resistor selection on safe operation of the device, raises the principle of selection and conducts simulation verification by using of PSCAD. Introduction Before each time of operation, SVG needs to charge the device. The charging process is essentially to the alternative current of the grid to the H-bridge under the rectifier, and to convert it into direct current for energy storage for each capacitor on DC side. Actually, this is an uncontrollable rectification process for each power unit of the chain SVG. To prevent over-current or over-voltage in charging process from damaging IGBT or other devices, people introduce the starting resistor to keep the charging current in a safe and reasonable range. Fig. 1 Chain SVG Electrical Diagram Generally, the starting resistor of the chain SVG is arranged at the front end of the connection reactor, with the circuit breakers connected at both ends in parallel. When the grid current has finished charging of DC side capacitor in the power module via the starting resistor, the circuit breaker is closed. The charging resistor enters bypass status. When the equipment stops running charge again, the circuit breaker is open and the starting resistor is put into operation again. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, USA-11/05/16,19:32:38)
3862 Machine Tool Technology, Mechatronics and Information Engineering Chain SVG charging process simulation First, let me introduce the simulation model. Its parameters are as follows: SVG voltage level: 10kV, capacity:±10mvar, connected to upper level 35kV system via the step-down transformer. SVG adopts chain topological structure and three-phase star connection. Each phase is connected with 12 H-bridge power unit modules. SVG starting process: 1. t=0s, H-bridge power unit module PWM locks impulse, breaker BRK and BRK1 are disconnected. The entire device is isolated from the grid. 2. SVG DC side capacitor charging. 3. t=0.7s, circuit breaker BRK1 is closed, starting resistor is bypassed, SVG is connected into the grid via the reactor, H-bridge power unit module PWM unlocks, SVG runs according to the set control mode. The DC side capacitor of the power module charges and discharges automatically. Fig. 2 Chain SVG global DC side capacitor average voltage curve We can see from Figure 2 that at 0.1s, the grid current starts charging SVG power units DC side capacitors via the starting resistor. The entire charging process lasts for about 0.15s. DC side capacitor voltage of each power unit module finally reaches 1000V, ready for SVG starting and operation. Analysis shows that the charging period and the charging current are related to the starting resistance. The starting process of charging resistors of different resistances undergoes simulated analysis as follows. Analysis of starting process of starting resistors with different resistances
Applied Mechanics and Materials Vols. 644-650 3863 Fig.3. Charging current and voltage of the starting resistor at 5Ω and power unit DC side average voltage curve Fig. 4. Charging current and voltage of the starting resistor at 50Ω and power unit DC side average voltage curve
3864 Machine Tool Technology, Mechatronics and Information Engineering By comparing Figure 3 with Figure 4 can we get that the smaller the starting resistor, the shorter the charging time and the larger the charging current. On the contrary, the larger the starting resistor, the longer the charging time and the smaller the charging current. When the starting resistor takes 5Ω, the charging current peak reaches 800A rapidly, which has great impact on the anti-parallel diode in the power unit. When the starting resistor takes 50Ω, the charging current peak is reduced to 150A, but the charging time is extended to 0.5s. Assuming that the equipment charging process is long (due to large starting resistance) and the starting resistor operation device is cut before charging has been finished, what impact will be incurred to the device? Now simulation research is conducted for the work condition. Cut off starting resistor and start device simulation before charging is finished Fig.5. Power unit DC side average voltage curve for cutting off starting resistor (20Ω) after charging has been finished Fig. 6.Power unit DC side average voltage curve for cutting off starting resistor (50Ω) during charging From Figure 5 can we see that before cutting of the charging resistor, if power unit DC side capacitor charging is completed smoothly, then the entire power unit DC side average voltage curve is ascending smoothly. However, in Figure 6, the resistor is cut off during charging. Although power unit DC side capacitor finally reaches the set value, the charging curve has a sudden increase at the end ( U = 100V ), i.e., overshoot voltage comes out. The overshoot voltage is actually caused by the impact current due to big gap between IGBT and the absorbing circuit diode end voltage after the resistor is cut off and the voltage on both ends of the DC side capacitor becomes low, causing harm to devices inside the power unit, affecting the service life of the device and even the normal operation of SVG. Conclusion As a part of SVG, the starting resistor doesn t participate in reactive compensation directly, but it provides initial power for the start and operation of the device, and affects safety and service life of devices inside the equipment. When selecting the starting resistor, both the charging time and the impact on SVG elements shall be considered to provide guarantee for the stable operation of the entire device.
Applied Mechanics and Materials Vols. 644-650 3865 References [1] Ma Youjie, Gong Jinxia, Zhou Xuesong. STATCOM Parameter Selection and Impact on Voltage Stability, journal of Tianjin University of Technology,2007,23(5):46-48; [2] Du, S. X., J. J. Liu, et al. A Novel DC Voltage Control Method for STATCOM Based on Hybrid Multilevel H-Bridge Converter[J].IEEE Transactions on Power Electronics,2013,28(1):101-111. [3] Han C.,Z.O.Yang,Bin Chen,et al.evaluation of cascade multilevel converter based STATCOM for arc furnace flicker mitigation[j].ieee Transactions on Industy Applications,2007,43(2):378-385 [4] Yang Yihan. Fundamental of Electrical Power System [M]. Beijing: China Water & Power Press, 1986.
Machine Tool Technology, Mechatronics and Information Engineering 10.4028/www.scientific.net/AMM.644-650 Simulation Research on Starting Resistor Parameter Selection of 10kV Chain Static Var Generator 10.4028/www.scientific.net/AMM.644-650.3861 DOI References [2] Du, S. X., J. J. Liu, et al. A Novel DC Voltage Control Method for STATCOM Based on Hybrid Multilevel H-Bridge Converter[J]. IEEE Transactions on Power Electronics, 2013, 28(1): 101-111. 10.1109/TPEL.2012.2195508 [3] Han C.,Z.O. Yang, Bin Chen, et al. Evaluation of cascade multilevel converter based STATCOM for arc furnace flicker mitigation[j]. IEEE Transactions on Industy Applications, 2007, 43(2): 378-385. 10.1109/TIA.2006.889896