Virtual oscillator-based methods for grid-forming inverter control: A review

In inverter-dominant power systems, grid-forming (GFM) inverters regulate voltage and frequency. To construct GFM inverters, conventionally, various control methods based on synchronous machine emulation or droop characteristics have been employed. However, recently, inverters are regulated by Virtual Oscillator Control (VOC) to emulate the dynamics of a weakly nonlinear oscillators. In contrast to droop control, VOC is a time-domain controller that enables interconnected inverters to stabilize arbitrary initial conditions to a synchronized sinusoidal limit cycle rapidly. The fast synchronization, accurate power sharing in grid-connected and islanded modes, simple and robust implementation make it a promising candidate for inverter-dominant power systems. Moreover, VOC removes the computational burden within machine emulation-based methods and provides an approach to measure the frequency without employing phase-locked loop (PLL). Hence, to leverage the advantages of this method and expand its application in power system control, this paper reviews different VOC implementations. Mainly, this paper focuses on the studies related to the fundamental theory of oscillators and design process. Additionally, there are discussions on the stability analysis of VOC-based systems, harmonics and pre-synchronization. Challenges on the compatibility with heterogeneous controllers, fault ride-through and other limitations of VOC including non-inertial response and coupled power control are investigated and solutions are outlined.