An improved hysteresis current control scheme during grid voltage zero-crossing for grid-connected three-level inverters

Abstract

For three-level inverters, hysteresis current control (HCC) has been widely used in applications such as active power filters due to its fast-dynamic response, and it does not need to control each frequency separately with multiple controllers, which makes it simpler. However, the large fluctuation range of switching frequency is a serious disadvantage of hysteresis current control. Especially, the switching frequency decreases significantly during grid voltage zero-crossing. This frequency fluctuation will cause low-frequency harmonics. The purpose of this paper is to propose a sampling compensation scheme, which can solve the problem of switching frequency fluctuation near the zero-crossing of grid voltage. Based on the variable hysteresis band strategy, the virtual sampling and switching time prediction algorithm is designed, which does not need to increase the sampling frequency or add additional controller. Therefore, nearly fixed switching frequency can be achieved while maintaining the control performance. Both simulation and experimental tests have verified the efficacy of the proposed scheme.