Multi-string Photovoltaic Grid-Connected Inverter Based on Alternate Single-phase PWM Control

This paper presents a multi-string three-phase photovoltaic(PV) grid-connected inverter using a novel alternate single-phase current hysteresis PWM control method. In the multi-string PV grid-connected inverter, the maximum power point tracking(MPPT) of each PV array is independent, which leads to higher energy conversion efficiency. In the alternate single-phase PWM method, one line frequency cycle is averagely divided into six periods. Only one of the three-phase legs is switching based on PWM control and the devices in the other two-phase legs keep the switching state in each period. Therefore, the average switching frequency of the power devices is reduced to about 1/3 of that in a conventional three-phase PWM method, and the corresponding switching loss is reduced effectively. The efficiency of the whole system is improved further. Current hysteresis PWM tracking control can easily realize sinusoidal output current of inverter with fast dynamic response and good tracking performance. Unity power factor of grid-connected current can be obtained, and the system has good stabilization. The current total harmonic distortion (THD) is low and acceptable. The simulation results of Matlab prove the multi-string PV grid-connected inverter with the alternate single-phase PWM control is feasible and effective.