Impact of Fault Clearing Times and Pre-Fault Converter Setpoints on Microgrid Stability During Islanding

Abstract:
The power system is evolving, shifting from centralized, fossil-fueled generation to renewable converter-based sources. Alongside rising demand, greater flexibility needs, and external threats like climate change and cyberattacks, microgrids emerge as a key scale for improving system resilience. By operating as islanded grids during disruptions, they ensure continuity, redundancy, and enhanced reliability. This work analyses the stability of a medium voltage network with a grid-forming battery converter during the transition from grid-connected to islanded operation in a real-time simulation framework. The microgrid is subjected to intentional islanding under varying fault clearing times. A short-circuit in the high voltage grid is detected by the protection scheme triggering a circuit breaker and isolating the microgrid. In this islanded grid, the droop-controlled grid-forming converter sets the frequency and voltage references. Besides simulating different fault clearing times, we evaluate the influence of the grid-forming battery converter’s pre-fault power setpoints on the microgrid’s stability. The results show that both fault clearing time and battery converter pre-fault active power setpoint significantly impact islanding stability. Charging the battery with active power levels superior to 0.6 MW when the fault occurs leads to frequency related instabilities, which raises a discussion about frequency requirements in islanded microgrid operation. Also, fault clearing times superior to 200 ms should be avoided for a stable islanding. Hence, this paper analyzes crucial parameter that influence the islanding stability of a specific microgrid and proposes a framework for the stability assessment of a converter-based microgrid being subjected to intentional islanding.