An important characteristic of the microgrid inverter is that the three-phase output voltage remains balanced under unbalanced load conditions when operating in the off-grid mode to provide the microgrid with stable voltage support. The unbalanced mechanism of inverter under unbalanced load conditions is analyzed first. A simple and effective control strategy is proposed

in which the resonant controller is introduced into the voltage control loop to suppress the pulse signal(whose frequency is twice that of the inverter output voltage)existing in the control loop under the unbalanced load condition. The positive and negative sequence separation of voltage and current can be removed and the control structure is greatly simplified. A control loop model based on the proposed control strategy is developed

and the control loop parameters and the resonant controller design method are given. Finally

simulation and experimental results have verified the effectiveness of the proposed control strategy in suppressing the output voltage distortion caused by the unbalanced load. This work is supported by National High Technology Research and Development Program of China(863 Program)(No. 2015AA050603)and State Grid Corporation of China.