Impact of Controller Saturation on Instability Behavior of Grid-Connected Inverters

For the power-electronics-based power system, a kind of stable operation with constant-amplitude oscillation was often witnessed in the reported instability incidents. In some cases, the root is proved to be negative damping by the theoretical analysis based on the linear model and the impedance-based stability theory. However, the negative damped resonances should diverge. This leaves a void why the analysis result is not in accord with the field measurement. This article aims to provide a potential answer for the disaccord, i.e., the controller saturation which is commonly ignored in the modeling process due to its nonlinearity. The controller saturation is linearized by the describing function and the improved linear model is developed. By applying the generalized Nyquist stability criterion, the impact of the controller saturation on the instability behavior of grid-connected inverters is identified, which reveals the underlying mechanism of the constant-amplitude oscillation. The theoretical expectations are finally validated by the experimental results.