Adaptive frequency regulation in a low-inertia low-voltage AC microgrid with multiple hybrid storage systems and photovoltaics

The frequency regulation in isolated low-inertia AC microgrids (MGs) is challenging, as even small power imbalances between supply and demand can compromise dynamic frequency stability. A commonly proposed solution involves using a standalone battery to mitigate power mismatches. However, it cannot provide multiple frequency regulation services simultaneously and suffers from accelerated lifetime reduction. This study considers a low-inertia low-voltage AC MG with distributed residential photovoltaics (PVs) and AC-coupled hybrid storage systems, which regulate frequency on behalf of PVs without modifying PV inverters. An adaptive controller is proposed for each hybrid storage system, comprising a power-type superconducting magnetic energy storage (SMES) and an energy-type battery. To minimize battery stress, SMES manages short-term power fluctuations by providing adaptive virtual inertia, while battery handles long-term demands by providing fixed droop regulation. When frequency deviates from nominal, the proposed controller rapidly increases inertia to suppress both the rate of change of frequency (RoCoF) and the frequency deviation. During frequency restoration, emulated inertia is reduced to zero, preserving MG inherent low inertia and minimizing recovery time. Thus, the benefits of both high and low inertia are leveraged, ensuring rapid response during frequency deviation and bounded adaptation. Additionally, guidelines for key control parameter selection are provided. Finally, time-domain simulations demonstrate the superiority of the proposed controller over a preceding adaptive approach in terms of RoCoF, frequency deviation, and recovery time. • Novel adaptive controller improves frequency stability in low-inertia MGs. • SMES provides adaptive inertia, while battery ensures fixed droop regulation. • Inertia coefficient scales dynamically according to system state. • Benefits of both high and low inertia are exploited.