Generalized Virtual Synchronous Generator Control Design for Renewable Power Systems

Grid-forming inverters (GFMIs) are recognized as one of the key enablers towards highly renewable energy proliferated grids. One of the pivotal characteristicsof GFMIs is the ability to seamlessly switch between grid-connected (GC) and standalone (SA) modes. In this paper, a novel controller is proposed to accurately follow the power reference commands in the GC mode while providing the required amount of virtual inertia in the SA mode to slow down the rate of change of frequency (RoCoF) following a disturbance. The proposed control design, where straightforward equations are given to calculate the controller gains directly, is based on the frequency response of the open-loop system. Furthermore, based on the frequency response of the controller, a condition for the placement of the poles of the controller is derived to guarantee the RoCoF relay limit compliance in the SA mode. The experimental results show that the proposed controller results in lower overshoots and shorter settling times in step responses in the GC mode than the virtual synchronous generator (VSG) controller while providing more virtual inertia than the VSG controller to slow down RoCoF in the SA mode. Furthermore, the experimental results illustrate that the proposed controller can also be designed to support the grid during frequency transients. The performance of the controller is extensively validated experimentally during GC to SA mode transition and vice versa, in a multi-inverter network, and in a wide-area model.