A synchronization controller for grid reconnection of islanded virtual synchronous machines

The control of power electronic grid interfaces as Virtual Synchronous Machines (VSMs) is emerging as a suitable approach for converters in distributed power systems. VSM implementations including an internal model of a virtual inertia can also inherently allow for both grid-integrated and standalone applications. Thus, such control systems are suitable for Microgrids and other power system configurations designed for operation in both islanded and grid-connected modes. However, a temporarily islanded system must be able to reconnect to the main grid through a smooth and seamless process. This paper presents a control strategy for seamless reconnection of islanded VSM-based grids. The proposed controller is acting on the frequency reference for the VSMs until the frequency and phase differences between the local grid and the main grid are eliminated. The operation is similar to recently proposed algorithms for automatic grid connection of synchronous generators, and is therefore compatible with local grids containing both VSMs and rotating machines. The operation of the proposed control loop is demonstrated by time domain simulations of the resynchronization process for an islanded system with two VSMs operating in parallel, and is verified by laboratory experiments on a single VSM.