Subcycle Overvoltage Dynamics in Solar PVs

Abstract

In 2017 and 2018, bulk power system (BPS) connected solar photovoltaic (PV) inverters tripped after grid disturbances in South California, causing large-scale power loss. One cause of PV tripping is subcycle overvoltage experienced by PV inverters when the grid suffers voltage dip and PVs enter into momentary cessation. This paper examines the underlying mechanism of the subcycle overvoltage dynamics. A dq-frame analytical model is built for a PV grid-integration system with a focus on the grid side converter (GSC) and grid interaction. PV's dc circuit dynamics and controls are ignored. Eigenvalue analysis indicates that two modes cause the subcycle overvoltage. The modes are associated with shunt compensation, grid electromagnetic dynamics, and PV inverter controls. Furthermore, a more insightful explanation of subcycle overvoltage is offered relying on s-domain admittance models. The dq-frame admittance is derived for the PV viewed from the point of common coupling (PCC). A circuit model relying on the derived admittance can demonstrate subcycle overvoltage due to momentary cessation. Impacts of grid strength, phase-locked-loop (PLL) parameters and converter current control parameters are all examined. It is found that the inherent LC dynamics due to shunt compensation and grid inductance are the main dynamics that can cause overvoltage, and momentary cessation excites overvoltage dynamics.