Research On Experimental System Of CO-Phase Power Supply Based On MMC

The power supply system of the electrified railways in our country has problems like phase separation, negative sequence, reactive power and harmonic, which restrict the development of high speed railway and heavy haul railway. Co-phase power supply system can improve power quality of traction power supply system and remove neutral-section passing device. Co-phase power supply device based on the modular multilevel converter can realize high voltage、great power requirements, and hang on traction feeder directly. It will save matching traction transformers, reduce the volume of co-phase power supply unit, in order to cut down expenses. This paper will research key technologies of co-phase power supply system based on MMC and its test platform.The structure of co-phase power supply system is studied, which consisted of MMC and the combined connection transformer. Considering technical application, the MMC with two bridge arms and four bridge arms are compared. The minimum DC voltage of the latter is only about half of the former, and its DC side is not in need of the support capacitor, therefore the four bridge arms structure is superior to the two bridge arms structure in general.Researching on the topology and working principle of the MMC. Characteristics of the circulating current in MMC are considered. Among parts of the circulating current, the active and reactive component, which related to the system transmission power, can’t be eliminated by capacitor voltage balancing control method. However the control can be used to suppress the averaging voltage and phase to phase parts.Study of the modulation mode and control strategies of the MMC. The sub module capacitor balancing control strategy and the PR regulator are used to control the circulating current, and the effectiveness of the control strategy is verified by simulation. The active power, reactive power and harmonic separation methods are used to detect and compensate the current. The CPS-SPWM modulation mode and transient current control strategy suitable for the MMC phase compensation device are adopted.Simulation of co-phase power supply system based on MMC. According to the technical requirements of the experimental platform, the parameters of the main circuit of the MMC converter are calculated. By making use of the MATLAB/simulink, the simulation model of the traction substation of the co-phase power supply is built. The simulation experiments are carried out on traction condition, load change condition and regenerative braking of AC-DC-AC electric locomotive, and on AC-DC electric locomotive with reactive power and harmonic. Simulation results show the validity of the control algorithm and the correctness of the design of the parameters.The design of co-phase power supply test platform. This platform will achieve reciprocal power feed. The three level control of the experimental platform is designed, and the content of signal detection and communication transmission is studied. For the MMC float module, the isolated power supply is designed, using high frequency inverter power supply. Self-excited charging method design of sub module capacitor is designed. The feasibility of the pre-charging scheme is verified by the simulation. The function of reciprocal power feed is also confirmed by simulation.