On Stability of PQ-Controlled Grid-Following and Droop-Control Grid-Forming Inverters

This paper demonstrates unexpected instability mode at low power levels. This instability mode is at low frequencies and can be explained using the root locus of the linearized (small signal) model of PQ-controlled inverters. This paper uses a full-order state-space model to analyze the low-power hidden instability mode. The impact of an operating point (setpoint) on the stability is due to the PQ-control nonlinearity and variation of circuit parameters. Though this paper focuses on PQ-controlled grid-following inverters, the low-frequency instability of droop-control grid-forming inverters is briefly investigated. The findings are experimentally verified using a small-scale two-level 208 V, 5 kV A inverter feeding a 12 kW NHR 9410 power grid emulator and a hardware setup of a microgrid fed by two 5 kV A 208V inverters.