Abstract：Under the island operation mode, unbalanced load in the micro-grid will not only cause the voltage distortion of the inverter, but also affect the power sharing of the parallel inverters. In this paper, the fundamental reason of the output voltage distortion is firstly analyzed. Then, based on the three-phase four-wire inverter topology of the split capacitor and 3D-SVPWM, a zero-axis control strategy is proposed to suppress the zero-sequence component of the output voltage and control the voltage balance of the split capacitors. Meanwhile, the quasi-resonant control is used to suppress the negative sequence component of the output voltage. Subsequently, the influence of the unbalanced load on output power and their basic relationship are analyzed in detail for the parallel operation of multi-inverters based on droop control. An improved sequential droop control method is proposed. The virtual impedance based on the second-order generalized integral is used to reduce difference among each sequence components of output impedance, which effectively solves the imbalance sharing problem of the negative sequence output power and zero sequence current caused by unbalanced load. Simulation and experimental results verify the effectiveness of the proposed scheme.
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2018, Vol. 33 
