Semi-Distributed Optimal Secondary Control Based on Decoupled Linearized Power Flow for Large-Area Droop-Controlled Microgrids

Power-flow-based optimal secondary control of microgrids has been an active research subject in recent years. However, the standard optimal control's computational burden will likely become intractable in large-area microgrids with a large number of fast-dynamic distributed energy resources (DERs). This work proposes a semi-distributed optimal control scheme to overcome the scalability issue. First, the proposed semi-distributed control strategy segregates the large-area microgrid into multiple sub-microgrids. Then, modified decoupled linearized power flow with Q-V droop is adopted to model the behavior of each sub-microgrid cluster. It is followed by forming a novel quadratic cost function that captures the interaction of both intra-and inter-cluster microgrids, in terms of load-bus voltages and DERs' reactive powers. This is achieved by exploiting the graph theory of cooperative control method. The viability and performance of the proposed semi-distributed control scheme has been verified in conjunction with three distinctive cases: reactive power sharing correction without voltage regulation, single-load bus voltage regulation without reactive power sharing correction, and optimal reactive power sharing and voltage regulation. Despite having separated instances of secondary control, it is proven that the proposed semi-distributed control adopting linear power flow algorithm can realize single-objective (i.e. optimal reactive power sharing or single bus voltage regulation) and multi-objective (e.g. load bus voltage regulation with optimal trade-offs in reactive power sharing) control in the microgrid.