The asymmetric dq-axis structure or parameters of the controller in the grid-connected inverter will cause the coupling between two disturbed frequency components of the system in the static frame. This coupling factor will affect the stability of the power system. This paper analyzes the mechanism of frequency coupling

and derives the equivalent admittance of inverter under the two coupling frequency components. By using the two equivalent admittances

the stability of the grid-connected inverter system can be judged by the Nyquist criterion. Compared with the approaches using the generalized Nyquist criterion

the stability judgement process of the proposed method is simpler. By using this method

it is convenient to obtain the phase margin or gain margin of the system and guide the controller design. Finally

the correctness of the stability judgement results is verified through experiments.