On the Passivity of Grid-Forming Converters - Role of Virtual Impedance

Grid-forming inverters are the key enablers for the integration of inverter-based resources at scale. A critical component of grid-forming control architecture is the emulation of virtual impedance. However, virtual impedance implementation along with the inner current loops can affect the converter stability. To ensure harmonic stability under widely varying operating conditions, this paper analyzes the passivity of grid-forming inverter with virtual impedance and converter-side current loop. Effective converter admittance is derived from the control architecture and is then used to analyze the passivity properties of grid-forming inverters. Furthermore, a model-based delay-compensation method is proposed to increase the frequency range of passive behavior. The analysis is verified through Matlab/Simulink and Hardware-in-the-Loop real-time simulations.