Research On Power Quality Conditioner For Electric Railway

Single-phase 25kV 50Hz power supply system is widely adopted for electrified railway in China, and the load is dominately rectifier electric locomotive, which brings about reactive power, negative sequence and harmonic current to the utility grid. Some measures have been taken at present, however the result is unsatisfactory.According to the characteristics of the balanced traction transformer, negative sequence current and voltage fluctuation in three-phase-system caused by traction load are comprehensively analyzed. Then, the conditions of suppression the negative sequence and voltage fluctuation are obtained. A new power quality conditioner based on balanced traction transformer is presented in this thesis, which can compensate reactive power, negative sequence and harmonic current simultaneously.The main circuit topology and operating principle of the power quality conditioner are introduced. The relationship of voltage and current among the conditioner, traction transformer and taction load are analyzed. And the mathematical model of comprehensive compensation of the conditioner is also given.Based on the steady-state model of the conditioner, its steady-state operation is analyzed. Then the control strategys of the back-to-back converter have been proposed. Stabling DC voltage strategy has been implemented on the M-phase converter, and the other employs constant active power strategy. The design methods of main circuit parameters are also given. A new three-level hysteresis modulation algorithm has been proposed, which can improve response speed, reduce switching frequency and output harmonic current. The principle of the three-level hysteresis modulation and state transition logic of switching is analyzed in detail.In corresponding to the power quality measurement results of traction substation and design methodology of parameters, the control strategy and modulation method of conditioner have been simulated via PSCAD/EMTDC. To further demonstrate the effectiveness of the proposed method, a prototype of the power quality conditioner has been constructed in the laboratory. The proposed method has been applied to the laboratory set-up. Experimental results show the vacancy and feasibility of the proposed system configuration, control strategys and modulation method.