This　paper　presents　a　novel　fault　detection　method　for　photovoltaic　(PV)　array　using　an　improved　two-stage　Hampel　identifier,　which　can　quickly　detect　the　occurrence　of　faults　and　locate　them　at　PV　module　level.　The　fault　detection　method　is　implemented　in　a　two-level　wireless　sensor　network　based　distributed　on-line　monitoring　system　which　is　developed　to　monitor　the　voltage　of　each　PV　module　and　current　of　all　parallel　PV　strings　in　real-time.　The　1st　stage　Hampel　identifier　detects　and　locates　the　faulty　PV　strings　by　comparing　the　instantaneous　currents　of　different　PV　strings.　The　2nd　stage　Hampel　identifier　detects　and　locates　the　faulty　PV　modules　by　comparing　the　instantaneous　voltages　of　different　PV　modules　in　the　faulty　strings.　Experimental　results　show　that　the　proposed　method　can　successfully　detect　and　locate　faulty　strings　and　modules　in　several　faulty　cases,　including　the　line-line　faults,　the　degradation　faults,　the　partial　shading　faults　and　the　open-circuit　faults.　©　Published　under　licence　by　IOP　Publishing　Ltd.