Resolving Sub-Cycle Overvoltage Issue in Solar Farms

In the last five years, multiple losses of solar farms have been reported due to tripping of solar photovoltaic inverters following a grid disturbance. These inverters employed momentary cessation (MC) as a mechanism to ride through disturbances, where they remained electrically connected, but stopped contributing currents. The tripping occurred while recovering from the disturbance following the MC mode. Sub-cycle overvoltage was identified as the cause of tripping, where the instantaneous terminal voltage of inverters rose above a hard threshold for a fraction of a cycle. This paper first describes simulation of a scaled model of a real solar farm in PSCAD to recreate these events. Gaining insights from the results and from the NERC reports of field events, the paper proposes and validates through further simulations a method to alleviate sub-cycle overvoltage.