Virtual Oscillator Grid-Forming Inverter Stability Analysis Based on Impedance Model

Grid-forming (GFM) inverters are DC-AC converters that regulate their AC terminal frequency and voltage in response to real-time measurements. Virtual oscillator control (VOC) is an emerging strategy of grid-forming controllers. Unlike conventional phasor-based control methods such as droop control and virtual synchronous machine control, VOC is a time-domain controller that global convergence can be almost guaranteed by adding a nonlinear element. In this article, the state space model of a VOC-based inverter system connected to an infinite bus is established to access the steady-state values. In order to study the effect of nonlinear terms on the system stability, a detailed impedance model of the VOC controller is proposed. Numerical simulations validate the impedance model. Through the impedance-based method, how the nonlinear term can influence system stability is analyzed.