Resynchronization of Islanded Virtual Synchronous Machines by Cascaded Phase and Frequency Controllers Acting on the Internal Power Reference

Virtual Synchronous Machines (VSMs) have the capability to operate in grid connected or in islanded mode in the same way as synchronous machines. Furthermore, VSMs can inherently support seamless transition from grid connected to islanded mode. However, the reconnection of a VSM-based islanded to the grid requires a dedicated synchronization controller. The synchronization process should align the frequency and phase angle of the islanded system to the grid voltage to avoid abrupt transients in the currents when closing the breaker connection to the grid. This paper presents a simple and flexible control structure for grid resynchronization of a VSM operating in islanded mode. The scheme is based on two cascaded regulators operating on the phase and frequency difference between the VSM and the grid to generate an offset in the power reference. Starting from a simplified analytical model of the islanded system, a tuning procedure ensuring a controlled resynchronization transient is derived. Numerical results confirm the intended operation of the synchronization scheme with the associated tuning algorithm and validate their robustness against variations in the initial conditions of the synchronization process and to the virtual inertia value.