Research On Several Issues In Power Flow Controller Based On SPH-MMC In Co-phase Power Supply System

With the development of high-speed and heavy haul of electrified railways in China,the problems of negative sequence,reactive power,harmonics,and the existence of electrical sectioning caused by locomotive loads are more prominent.Adopting the co-phase power supply system eliminates the electrical sectioning,and simultaneously compensates the negative sequence,reactive power and harmonic current for electrified railway.It is an ideal mode of traction power supply.The use of single-phase H-bridge modular multilevel converter for co-phase power flow controller combined with composite balance transformer,can save the matching transformer in traditional co-phase traction power supply system,can reduce volume of compensation device and lower the system cost.This is one of the development direction of the co-phase power supply flow controller.Based on the topic of "Research on co-phase continuous traction power supply and technology application of electrified railway",taking the new co-phase power flow controller and the reciprocal power-fed system for burn-in-test as the research object,this paper have completed the following works:(1)The compensation theory of the co-phase power supply is analyzed.The fourquadrant operating principles of the SPH-MMC topology is introduced.The mathematical model of SPH-MMC is given and the circulating current characteristic in SPH-MMC is studied.(2)The designing principles of 3 key parameters in SPH-MMC main circuit are discussed,namely the selection of the power module,the DC capacitance value of the module unit and the value of each bridge arm inductance.Contrary to SPH-MMC for 450 kVA capacity,the loss of switch devices in SPH-MMC is calculated and the heat dissipation structure of the sub module is further studied.By analysis of examples,the optimal fin size of power unit is given by ANASYS ICEPAK.(3)The integrated control strategy of co-phase power flow controller based on SPH-MMC is proposed which including the extraction algorithm of active and reactive current,harmonic,an optimized method for balancing sub-module voltages,PR circulating current suppressing controllers,nearest level modulation and fault-tolerated control strategy for sub-module faults.The 20 modules of the system model is builded using MATLAB/SIMULINK.The simulation results proved the feasibility of the topology,the correctness and effectiveness of the control strategy.(4)By comparing the two kinds of upper layer control methods of transient current control and single-phase dq0 decoupling control,and combining the SPH-MMC internal control studied in(3),a control scheme of the reciprocal power-fed system for burn-in-test is put forward.Through the above research work,this paper has formed a set of SPH-MMC main circuit parameter design method.The given control strategy of power flow controller has certain engineering application value.Single-phase dq0 decoupling control scheme is more suitable for the test system,so that the system can save a lot of test power.