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At present, the fault diagnosis method for the grid-connected inverter is mainly applied to the open-loop system, and the actual grid-connected inverter is mostly a closed-loop control system. Therefore, for the grid-connected inverter in the closed-loop state, this paper proposes a fault diagnosis method based on phase current. By analyzing the phase current of the grid-connected inverter in the closed loop state under normal and open fault types, the fault characteristics of the average value of the phase current and the average absolute residual value under the open circuit fault are obtained. Firstly, the average value of the phase current generated by the Park transform in the closed loop control loop is compared with the measured phase current, and then averaged according to the normalized phase current. The value is combined with the average absolute residual to diagnose the fault. Finally, it is verified by experiments that the diagnostic method can quickly detect and identify single-tube and multi-tube open-circuit faults under closed-loop control.