Topology And Operation Analysis Of Power Electronic Transformer For Combined Co-phase Power Supply System

With China’s high-speed railway developing rapidly,the mode of traditional traction power supply has begun to impose great constraints on heavy-duty freight and high-speed passenger transport.Co-phase power supply system is an ideal solution to traction power supply system.It changes traditional traction power supply into co-phase power supply,which can implement negative sequence,harmonic and reactive power compensation at the same time.It can not only solve the electric phase separation of electrified railway,but also improve the power quality,which is meeting the requirements of high-speed and heavy-duty situation.As a kind of economical co-phase power supply system,combined co-phase power supply system has better practicability,and the output effect is greatly affected by the connection mode and the topology of the compensation device.Therefore,it is of certain significance to study the compensation device applied to the combined co-phase power supply to improve the working efficiency of the electrified railway co-phase power supply system.And following theoretical research has been carried out in this thesis.(1)Based on the analysis of the characteristics of combined co-phase power supply system,and the topology of power electronic transformer(PET)and modular multilevel converter(MMC),a MMC based PET which is suitable for combined co-phase power supply system is proposed.Then based on the compensation principle of combined co-phase power supply system,the relationship between voltage and current of traction transformer and compensation device is analyzed,and the system capacity is analyzed on the basis of port compensation theory.Based on the research above,the capacity relationship between traction transformer,MMC-PET and load while compensating comprehensively is analyzed.Finally,the relationship between the topology and capacity utilization of the MMC system and submodules is analyzed and series topology improvement measures is explained.(2)The basic parameter ranges such as the buffer inductance and submodule’s capacitance of the MMC system are analyzed.And the output waveform of the no-load operation is simulated by the NLM method in MATLAB/Simulink.In order to further improve the output power quality,the circulation current of the system dominated by the second harmonic is analyzed,and the circulation suppression effect is verified in the simulation.The ability of compensation of the MMC-PET is analyzed by a VPI control method which is suitable for combined co-phase power supply system.Finally,based on the method of PWM carrier phase shift control,with the equalization and voltage regulation of the MMC submodule adding,the compensated waveform of power factor and negative sequence imbalance are simulated.(3)According to two MMC submodules with different topologies,the improved dual capacitor-clamped submodule(IDCSM)is proposed,and its normal operation and blocking characteristics are simulated and analyzed.Finally,the energy loss of MMC-PET operation is analyzed,and the possibility of MMC-PET participating in the operation of the direct-coupled co-phase power supply system is discussed.