When a three-phase grid is unbalanced

the grid-connected inverter based on the traditional double-loop control strategy will generate even-numbered and odd-numbered harmonics on the DC side and AC side

respectively

which will seriously affect the performance of the grid-connected inverter system. For this reason

the control strategy of grid connection is proposed to realize under the positive-negative sequence rotational coordinate system. With the control strategy

different constraint conditions are obtained from different control objectives (inner current reference) and the corresponding control method is designed. To effectively and rapidly separate positive and negative sequences

an instantaneous positive-negative sequence separating method is presented. The forgetting integration algorithm is adopted for filtering. Experimental results under various control objectives are compared to show the effectiveness and simplicity of the algorithm. This work is supported by National Natural Science Foundation of China (No. 51177037).