Impedance-Based Stability Analysis of Grid-Forming Inverters with Virtual Impedance or Angle Droop for Improved Robustness

Droop control of grid-forming inverters typically links the active power with the frequency and the reactive power with the amplitude of the output voltage to achieve synchronization and power sharing. This leads to power os ciliations between multiple inverters. Adding a virtual inductance or an active power-angle droop path to the control helps to dampen these oscillations and improves the control dynamics. An impedance model of an inverter including these control strategies is developed in this paper. The model is used to analyze the small-signal stability of a microgrid with two grid-forming inverters. Laboratory measurements validate the results of the impedance-based stability analysis.