An integration of distributed generations (DGs) to the utility grid has raised the need for good power quality, safety operation and islanding protection of the grid interconnection. This paper presents the classification and detection of faults in a distributed ge5neration, particularly photovoltaic (PV) grid-connected system. The initial step in fault detection of PV system is recognition, investigation and classification of all possible faults that maybe occur in the system. The classification, simulation and discussion of all possible faults in both AC and DC side of PV system are presented, where 100 kW array connected to a 25 kV grid via a DC-DC boost converter and a three-phase three-level Voltage Source Converter (VSC).