Short-Term Voltage Stability of Power Systems with High Inverter Penetration under Small Disturbances

The penetration of inverter-based resources (IBR) has reached critically high levels in many power systems. There are growing concerns due to decreased mechanical inertia as well as voltage stability issues in such power systems. Commercial IBRs operate as grid-following (GFL) sources; they track and follow the voltage and frequency at the terminals of the inverter. These inverter controls rely on existing synchronous machines to provide a stiff voltage and frequency and do not actively contribute to supporting the system stability. In contrast, a grid-forming (GFM) inverter actively controls voltage and frequency at its inverter terminals, which can be help support the system during disturbances. In this work, the voltage stability of the IEEE 39 bus system with increasing inverter penetration is investigated for small disturbances. Simulations are performed with increasing penetration of both GFL and GFM inverters and the resulting waveforms are presented. Instabilities are observed with GFL control at higher penetration levels whereas GFM controls exhibited stable operation for small disturbances. GFL controls demonstrated increased sensitivity to nonlinear load models while GFM controls did not exhibit any such issues.