Robust Synchronization of Multiple Converter-Based Weak Microgrids for Smooth Interconnection

Integrating distributed energy resources (DERs) to the conventional distribution networks results in the appearance of the concept of microgrid (MG), i.e. a set of DERs and consumers, which can be operated autonomously or in grid-connected mode. The challenge of weak converter-based MGs is taken into account in the synchronization problem of multiple MGs. Such a synchronization process is fully related to the voltage phase and magnitude of the connection points as well as the MG frequency. Therefore, the voltage/frequency fluctuations of MGs have a considerable impact on the synchronization process. In this paper, a robust $H_\infty$ controller is proposed for the synchronization process of multiple MGs as very weak grids to cope with these fluctuations and to obtain a smooth interconnection. The disturbances and uncertainties of the weak voltage and frequency are considered in the robust control synthesis, and the controller design process is completely described including parametric/unstructured uncertainty modeling, open/closed-loop modeling, and stability/performance weighting coefficient specifying. Moreover, all requirements of the synchronization procedure and control layer functionalities of MG hierarchical control, where they are fulfilled, are discussed including logic commands, control and communication signals. Time-domain simulations show a desirable performance according to design of the proposed robust synchronization control strategy.