The Study On Modular Multi-level Converter Of Combined Co-phase Traction Power Supply System

With the development of power supply technology of traction power supply system, the existing railway power supply system mode has a large number of reactive power current,harmonics and negative sequence current. These problems will do great harm to the power grid. The co-phase power supply system have comprehensively achieved reactive power compensation, harmonic filtering, eliminate negative sequence current and cancel phase split at the exit of substation. The combined co-phase power supply system realize decoupling between traction transformer and compensation device, achieving structure independent and functional combination. Modular multi-level converter is suitable for high voltage and high power applications, when applied to the combined co-phase power supply system compensation device, it can directly hanged on the traction bus, reduce the matching transformer, reduce investment. In this paper, combined co-phase supply system with a single-phase and three-phase modular and modular multi-level converter will be researched.Aiming at the single-phase and two-arm modular multi-level converter structure, the deadbeat current control method is introduced to realize the current control in advance, so that the output has good dynamic performance and harmonic characteristics. In the short sampling period, the next cycle reference current is predicted by the linear interpolation method. The average output voltage of the converter is obtained by the discretization state equation in the sampling period. Combined with the nearest level modulation method can achieve module put in or cut off from the bridge arm, so that the converter output current tracking reference current value. By increasing the capacitance voltage sequencing method can realize the module capacitance voltage balance control.Aiming at the system structure and the introduced control method adopted in this paper,a simulation model is established in MATLAB / Simulink. The simulation results of the current deadbeat control method of the converter are obtained by simulating the traction condition, the regenerative braking condition and the load mutation condition respectively.The results show that the current control of the single-phase and two-arm modular multi-level converter can be used to compensate the negative sequence current along with the harmonic and reactive power compensation. At the same time, the dynamic performance and harmonic characteristics of the system can be improved.In order to meet the design requirements of traction substation and increase the redundant configuration of the converter sub-module, the reliability of the converter is greatly improved under the condition of redundant sub-module.