An intra-string distributed and inter-string decentralized control method for hybrid series-parallel microgrids

The hybrid series-parallel microgrid attracts more attention by combining the advantages of both the series-stacked voltage and parallel-expanded capacity. Low-voltage distributed generations (DGs) are connected in series to form the intra-string, and then multiple strings are interconnected in parallel. For the existing control strategies, both intra-string and inter-string depend on the centralized or distributed control with high communication reliance. It has limited scalability and redundancy under abnormal conditions. Alternatively, in this study, an intra-string distributed and inter-string decentralized control framework is proposed. Within the string, a few DGs close to the AC bus are the leaders to get the string power information and the rest DGs are the followers to acquire the synchronization information through the droop-based distributed consistency. Specifically, the output of the entire string has the active power−angular frequency ( $\omega -P$) droop characteristic, and the decentralized control among strings can be autonomously guaranteed. Moreover, the secondary control is designed to realize multi-mode objectives, including on/off-grid mode switching, grid-connected power interactive management, and off-grid voltage quality regulation. As a result, the proposed method has the ability of plug-and-play capabilities, single-point failure redundancy, and seamless mode-switching. Experimental results are provided to verify the effectiveness of the proposed practical solution.