Enhancing operational reliability of hybrid microgrids under fault conditions using energy storage–assisted control

Ensuring the operational reliability of converter-based islanded hybrid AC/DC microgrids under fault conditions remains a major challenge due to limited fault-current contribution and complex interactions introduced by power electronic converters interfacing energy storage systems (ESSs) that operate as distributed generation units. In particular, inter-harmonic fault currents generated by mismatched droop characteristics of converter-based distributed generators (CBDGs) realized by ESSs and interlinking converters (ICs) can severely degrade the reliability of conventional current-based protection schemes. Moreover, the inherently low fault current levels of CBDGs based on ESS interfaces often lead to relay miscoordination in AC microgrids. To address these challenges, this paper proposes a novel communication-less protection strategy specifically designed for converter-based islanded hybrid microgrids. The core contribution of the proposed scheme lies in the coordinated activation of virtual impedance fault limiters (VI-FLs) embedded within the control of ESS-based CBDGs and ICs, which simultaneously suppress inter-harmonic fault components and regulate fault current levels in a protection-oriented manner. Building upon this controlled fault response, an inverse-time admittance-based protection framework is developed to achieve optimal protection coordination (OPC) on the AC side of the hybrid microgrid. Simulation results obtained from a modified IEEE 33-bus islanded hybrid microgrid demonstrate that the proposed strategy successfully achieves optimal protection coordination for both bolted and high-resistance faults up to 10 Ω[jls-end-space/], while effectively suppressing inter-harmonic fault currents. The proposed framework reduces relay operating times, improves coordination margins, and enhances fault ride-through performance by maintaining stable voltage and frequency recovery characteristics. Consequently, the proposed communication-less protection framework significantly improves the selectivity, speed, and reliability of protection in hybrid AC/DC microgrids without relying on communication infrastructure.