Impact of Phase-Locked Loop Control on the Stability of a High Inverter Penetration Power System

In inverter-dominated power systems, the grid cannot be modeled as a stiff voltage source with constant frequency. Most existing grid-connected inverters operate as grid-following sources in which they measure the frequency at the inverter terminals using a phase-locked loop (PLL). Recent studies have indicated that the PLL controls can result in instability in weak systems. In this work, a simplified theoretical model is derived to understand the instability that can occur in an inverter-dominated power system. The theoretical model demonstrates the degradation in dynamic response attributable to the PLL controls at higher inverter penetration, particularly for constant-power loads. The model can predict the critical penetration level at which instability is initiated. The model predictions are consistent with the results of small-signal eigenvalue trajectories plotted for the full system model. Time domain simulation waveforms are also presented that build confidence in the model predictions. The results demonstrate that the theoretical model can be helpful when designing the PLL controls for grid-connected inverters in inverter-dominated systems.