Fault Ride-Through Capability of a Microgrid With WTGs and Supercapacitor Storage During Balanced and Unbalanced Utility Voltage Sags

The concept of microgrids is recently attracting considerable interest. However, in order to widely integrate microgrids within the distribution networks, a shift in the philosophy of interconnecting them with the utility grid seems necessary. A grid-connected microgrid is required to possess Faults Ride-Through (FRT) capabilities, as well as provide ancillary services during abnormal grid operation. In this paper, a control strategy for improving the ability of an inverter-based microgrid to ride though symmetrical and asymmetrical grid faults is proposed. The microgrid is formed of several Distributed Energy Resources (DERs), which utilize Wind Turbine Generators (WTGs) as primary renewable energy source, each combined with a Supercapacitor Energy Storage System (SCESS). During balanced and unbalanced grid voltage sags, aim of the proposed control strategy is to keep the microgrid connected to the grid, according to the FRT requirements, while maintaining an acceptable voltage profile within the common ac bus. Each DER is controlled to support the voltage within the microgrid by injecting reactive power, without any physical communication. During unbalanced utility voltage conditions, the DERs operate collectively in order to compensate the undesirable negative and zero sequence voltage components. Thus, a set of balanced three-phase voltages is provided within the common ac bus. Simulation results demonstrate that the microgrid can ride through heavily balanced and unbalanced utility voltage sags, while supplying its loads with a high quality voltage profile.