A secondary control method for voltage unbalance compensation and accurate load sharing in networked microgrids

Interconnection of microgrids (MGs) provides the opportunity of increasing loadability and enhancing reliability by allowing power exchange among individual MGs. Control of networked MGs requires coordination of distributed energy resources (DERs) within each MG such that the sum of the MG's load and exported power is shared proportionally among DERs while maintaining power quality. This paper proposes a new hierarchical control scheme for accurate load sharing and power quality enhancement in networked MGs with unbalanced load currents. The proposed scheme is comprised of local DER controllers, MG control units, and Networked MG control center. The MG control units adjust the power exchange at the value specified by the control center and regulate the voltage at the point of common coupling (PCC) by means of a secondary control signal. At the DER level, droop control is adopted to enable load sharing with fast dynamic response. The droop controller output is combined with the secondary control signal and a correction voltage which eliminates the effect of line impedances on the current sharing accuracy. This way, accurate sharing of individual phase currents is realized without requirement of data exchange among DERs. Simulation and experimental results showcase the effectiveness of the proposed scheme.