The switch function is used to analyze the DC side current characteristics of distributed generation（DG） inverters when short circuit or open circuit of the inner power switch tube and asymmetric short-circuit of the grid-connected AC side outlet happen. Based on the changing regularity of the DC side base wave and second harmonics magnitude under different types of faults， a new inverter protection scheme is proposed. The base wave and second order harmonics over current protections are added to the existing protection configuration to offset the deficiency of existing protection， which is normally unable to reflect all kinds of faults comprehensively and quickly enough. The photovoltaic grid-connected system model is established， and the correctness of faults characteristics and effectiveness of the protection scheme are verified.