Impact of Grid Strength on Sub-cycle Dynamics in Solar PV Systems

The increasing penetration of inverter-based resources (IBRs) is changing grid dynamics and challenging grid planning, operation, and protection. Particularly, the increasing integration of IBRs may drive the power grid towards weak grid conditions, where potential dynamic stability issues may become significant. Recently, it was reported that the unintended loss of solar generation occurred in Southern California over a large geographic area. One of the major reasons for this generation loss is the tripping of solar generators due to the overvoltage in a less than one cycle time frame (i.e., sub-cycle overvoltage) experienced by solar photovoltaic (PV) inverters, especially when solar PV inverters enter the momentary cessation operation mode in response to abnormal grid disturbances. In this paper, the impact of grid strength on sub-cycle dynamics resulting from momentary cessation is investigated in a power system with distributed solar PV integration. In this investigation, distributed solar PVs are modeled with detailed grid-following inverter models considering inverter switching dynamics and momentary cessation function. It is found that undesired sub-cycle overvoltage has a positive relation to grid strength at points of integration (POIs) of solar PVs. At the weak POIs, server sub-cycle overvoltage occurs not only when momentary cessation starts to cease energy injection but also when momentary cessation restarts to inject energy into the system during the restoration process. Furhtermore, the interaction between solar PVs making POIs weak exacerbates the severity of the sub-cycle overvoltage at the POIs. Thus, in the power system with high penetration of solar PVs, it is important to improve grid strength in power system planning and quickly recover grid strength in power system operation following disturbances.