Circulating Current Mitigation and Harmonic Current Compensation for Multifunction Parallel Three-Level Four-Leg Converters

The multifunction parallel three-level four-leg converters are proposed for high-power applications, which combine grid-connected renewable generation and active power filter (APF). However, the challenge of zero-sequence circulating current (ZSCC) is inevitable which degrades the output currents’ quality and reduces the stability of the system. To overcome this limitation, a proportional integral (PI) + feedforward control strategy and space vector modulation based on the nonaxial redundant vectors (NARVs) are proposed in this article. First, the model of ZSCC is established and analyzed in detail. The analysis reveals that ZSCC is determined by zero-sequence duty-cycle difference and zero-sequence benchmark function. Then, by analyzing the space vector of the three-level four-leg converter, it is concluded that ZSCC is affected by different NARVs. Consequently, a PI + feedforward control strategy is proposed to mitigate ZSCC by adjusting the duty cycle of NARVs in each pulse width modulation (PWM) sampling period. The simulation and experimental results verify the effectiveness of the proposed control strategy.