A Pre-synchronization Strategy for Grid-forming Virtual Oscillator Controlled Inverters

Voltage controlled inverters in ac systems are susceptible to damage if the controller is not properly initialized before startup. Since currents are not controlled explicitly, the voltage reference of the controller must be closely aligned with the point of common coupling voltage to prevent large current transients when power delivery begins. In this paper, we are focused on a particular control strategy called virtual oscillator control and propose a pre-synchronization method that guarantees graceful addition of units into an existing ac system. The proposed method is generalized and can be used to add oscillator-controlled inverters to a stiff grid or an islanded microgrid with other inverters. An equivalent circuit model of the pre-synchronization control is derived along with its dynamical properties, design guidelines are given, and experimental results are shown for a 1.5kW inverter.