DC fault analysis of photovoltaic generation system by EMTP

A short-circuit and grounding fault characteristics of a single-phase photovoltaic (PV) generation system are clarified by numerical simulations using Electro-Magnetic Transients Program (EMTP). When a short-circuit fault occurs in a PV string, reverse currents flow into other strings depending on the number of short-circuit modules. A spike current also flows into the fault string because charged capacitors in power conditioning system (PCS) are discharged by the fault. At a grounding fault the fault current depends on earth resistance and grounding method of a single-phase alternating current (AC) distribution grid connecting to the PV generation system via PCS. Since the voltage to ground of the direct current (DC) circuit in PV generation system connected with single-phase three-wire (1φ3W) system differs from that with three-phase four-wire (3φ4W) system, there is a difference in the grounding fault current. Although the fault current of 1φ3W system is DC, AC component of the commercial frequency is superimposed on the fault current of 3φ4W system. Therefore, PV generation system with 3φ4W system has a tendency to flow a reverse current into sounded PV strings. The fault current increases with the decrease of the earth impedance. These characteristic can be reproduced by a simplified PCS model. Blocking diodes, which are inserted into the PV strings in series, are an effective countermeasure to prevent the spike current due to the discharge from the PCS as well as the reverse current at the short-circuit and the grounding fault. If fuses are employed for the protection, the fault current has to be correctly estimated. It is, however, difficult to interrupt the spike current by the fuses. The capacitance within the input filter in PCS should be reduced or the inductance within the filter should be increased in order to reduce the peak value of the spike current.