Three-phase photovoltaic generators modeling in unbalanced short-circuit operating conditions

Photovoltaic systems (PVSs) are among the most diffuse Distributed Generators based on renewable energy sources. PVSs contribute to the short-circuit currents during a fault, modifying the short-circuit capacity of the distribution systems. Then, the contribution of PVSs to the fault current must be adequately modeled to extend the traditional short-circuit analysis to distribution networks with PVSs. In this paper an analytical model based on the phase-coordinates approach is proposed to evaluate the fault contributions of three-phase PVSs connected to unbalanced distribution networks in presence of symmetrical and asymmetrical shunt short-circuits, with and without fault impedance. The model of the PVS takes into account the environmental conditions, the inverter control system, the maximum current that can flow through the inverter switching devices, the filter, the interface transformer and the self-protections imposed by the Standard IEEE 1547. Both a first cycles and a steady-state model of the PVS are developed accounting for inverter control systems equipped with VAr or LVRT control schemes or inverter control system without reactive power regulation. The model of the PVS is applied to a typical North American MV network and the results of the proposed approach are compared with time-domain simulations.