Hidden Modes of Instability for Inverters in Weak Grids
Tareq Hossen, Behrooz Mirafzal
Abstract
This article introduces a hidden instability mode of grid-following inverters at low power levels. It is known that grid-following inverters become unstable due to weak grid conditions and improper control parameters. However, this article elaborates on a hidden instability mode, which can occur at a power level below the inverter's nominal value. The nonlinearity of PQ control loops explains how the inverter becomes unstable for a narrow range of active power setpoints below the inverter's nominal value. This phenomenon is explained using a full-order dynamic model of PQ-controlled inverters. This work also presents a stability criterion, identifying a margin for this hidden instability mode. This phenomenon and the stability criterion are verified by a set of experiments using a 3 kVA inverter feeding a programmable 208 V, 12 kW power grid emulator.