Harmonic Virtual Impedance Design for Parallel-Connected Grid-Tied Synchronverters

In recent years, synchronverters have become a preferred control solution for the integration of renewable energy sources and battery systems to power networks. In this control strategy, the converter acts as a fundamental frequency voltage source and, as such, is unable to deal with harmonic power quality issues. In this paper, the harmonic control is frequently addressed using virtual impedances; however, they have been seldom studied for synchronverters. This paper presents a detailed design procedure for harmonic virtual impedances applied to parallel-connected synchronverters. The proposed method offers a tailor-made design of the converter output impedance and large stability margins. The method is used to decouple the dynamics of the synchronverter control, and the proposed virtual-impedance is explained in detail. The obtained results showed the output impedance of each synchronverter can be designed to achieve a tradeoff in power quality between current and voltage. In addition, the effect of frequency variations was explored and two solutions were provided and compared. All the control system improvements were experimentally validated using two 15-kVA battery-supported synchronverters connected to a distorted weak grid.