Study On Capacity Analysis And MMC Simulation Of Combined Co-phase Power Supply System

In order to meet the rapid development of electrified railway in China, it is necessary to study the traction power supply system. Co-phase traction power system can not only cancel the electric phase separation, which has reduced speed of traction load and affect reliability of traction system, but also solve the power quality problem such as negative sequence,harmonics, reactive power and so on. Combined co-phase traction power supply system is the latest concept. It stands out in the existing co-phase power supply system because structure of the compensation device is more flexible, the capacity can be less than the traction transformer capacity, and investment cost of the system is cheaper, and so on. But according to the current test operation, how to further reduce the capacity of the compensation device when national standard can be achieved, and improve the possibility about compensating high frequency harmonics that AC-DC-AC locomotives has produced become an urgent problem to be solved. In view of the above situation on combined co-phase power supply system, the following researchs are carried out in this thesis.(1) Based on the analysis of the combined co-phase power supply system working principle and under the condition of full compensation and satisfaction compensation, the relationship between the primary and secondary current transformations of the combined transformer is calculated. The parameters of the power quality after compensation are further determined, and the three-dimensional graphs are drawn for different traction loads. Based on the above calculation, two optimization models are established. Their constraints both are to meet national standard of the power quality. One of the objective functions is the minimum capacity of the single-phase back-to-back (BTB) compensation device, the other is the minimum capacity of traction transformer and BTB compensation device capacity. Then,using the actual parameters of traction substation, the optimize calculation is made by the optimizing toolbox, and the minimum capacity of the two models under satisfactory compensation are obtained.(2) In order to ensure the capacity of compensator conform to the traction power supply system capacity level, and high frequency harmonics can be compensated at the same time, a combined co-phase power supply system based on modular multilevel converter (MMC) is proposed. Based on the single - phase MMC, the mathematical model of the system is established. Besides, the calculation formula of the number of sub module (SM), the value of buffer inductance and DC side suspension capacitor of SM in main circuit are all given.(3) The control strategy of combined co-phase device (CPD) based on MMC is researched. According to the principle of non-harmonic current detection on general active power filter (APF), the scheme of no traction load current of combined CPD is designed. In order to compensate the extensive harmonic spectrum of the traction load producting, vector proportional integral (VPI) controller is selected for current tracking. On this basis, an improved VPI control strategy with no load current detection is proposed. The Matlab/simulink model of the AC-DC locomotive, AC-DC-AC locomotive and mixed-run locomotives are established. The correctness of the improved control strategy and the effectiveness of compensating the high frequency harmonics are verified by simulation.Combining the improved VPI controller with the MMC capacitor voltage balance control, the final control scheme is obtained. Besides, the applicability of the improved control method to the combined CPD based on MMC is verified by simulation.