Abstract: The DC capacitor of photovoltaic grid-connected inverter is a key component of AC and DC power coupling, and the cause of its overvoltage fault is not unique. In order to identify different fault causes, the transient state of the inverter needs to be studied. Consequently, we discuss the evolution of relevant quantities in the double-closed-loop control, and further investigate the abnormality of duty cycle in each carrier cycle based on the space vector pulse width modulation (SVPWM) when the system is out of control. It is revealed that the variable degradation is the main reason of unexpected changes of three-phase current. The simulation and experiment of the DC overvoltage fault caused by three different factors are performed, and the difference of virtual feature is adopted to locate fault causes. The results show that the fault location method can be adopted to effectively determine the fault causes without adding sensors, which has a certain engineering application value.