A control strategy for enhancing the Fault Ride-Through capability of a microgrid during balanced and unbalanced grid voltage sags

In order to widely integrate high power microgrids within the distribution networks, they should possess Fault Ride-Through (FRT) capabilities, as well as provide ancillary services during abnormal grid operation. This paper proposes a new control strategy for improving the ability of a microgrid to ride through balanced and unbalanced grid voltage sags. The microgrid consists of several inverter-interfaced Distributed Energy Resources (DERs), powered by Wind Turbine Generators (WTGs), each combined with a Supercapacitor Energy Storage System (SCESS). During balanced and unbalanced grid voltage sags, the proposed control strategy maintains the microgrid grid-connected, while provides the local loads with a high quality voltage profile. The developed control method is complemented by properly sized controlled series inductances, placed at the point of common coupling (PCC) with the main grid. Under fault conditions, the DERs operate collectively in order to support the voltage within the microgrid, by injecting additional reactive power, without the necessity of any physical communication among them. During unbalanced grid voltage conditions, the DERs are controlled to compensate the unwanted negative and zero sequence voltage components. Thus, the microgrid loads are supplied with a set of balanced three-phase voltages. The proposed control strategy is verified by detailed simulation results.