I. Introduction
Recently, a large number of distributed generation (DG) systems based on renewable energy sources (RES) are connected to the grid through power electronic interfacing converters, which has brought new opportunities to solve global energy problems, but also poses severe challenges to grid operation and control [1]. To guarantee the safety operation and provide grid voltage and frequency support, virtual synchronous generator (VSG) technology is applied for RES to simulate the rotor inertia and the droop characteristics of synchronous generators [2]. Moreover, as a means to facilitate the use of environment friendly RES, VSG should also play a more important role in injecting high-quality power into the grid.