Hybrid hysteresis current control and low-frequency current harmonics mitigation based on proportional resonant in dc/ac inverter

In the two-stage photovoltaic (PV) micro-inverters, the bipolar hysteresis current control with soft switching for dc/ac inverter can achieve fast dynamic response, robust current regulation and high efficiency even with higher switching frequency. To further reduce the switching losses, unipolar hysteresis current control is desirable. However, it suffers from zero-crossing distortions and low frequency harmonics in the grid current. To solve those problems, this paper proposes a combined proportional resonant (PR) and hybrid hysteresis current control strategy. By investigating the reasons for zero-crossing distortions, hybrid hysteresis current control is applied to enable bipolar modulation around the zero-crossing zone and unipolar modulation in all other zones of the line period. At the same time, due to the wide hysteresis bands for soft switching, low output inductance and non-negligible sampling intervals from the digital controller, there will be many low frequency harmonics in the spectrum of output inductor current which deteriorates the quality of power absorbed by the grid. Therefore, the PR control may be used as well to mitigate the low frequency harmonics. The theoretical analysis of the proposed solutions are explained in this paper. Detailed simulation and experimental results are presented to verify the feasibility of the proposed control strategy.