Initiatives to Supply 100% Renewable Energy on Hahajima Island Microgrid Project

Hahajima Island is World Natural Heritage site. Therefore, this project constructed a microgrid combining photovoltaics, battery storages, and diesel generators (DG) to strengthen the island’s resilience and promote carbon neutrality.

This microgrid consists of the existing diesel generators, newly installed photovoltaics and battery storages, and a newly developed Energy Management System (EMS) to manage these power sources. During the daytime, electricity is supplied by solar power and surplus energy is stored in the battery storages. At night, the batteries are primarily used as the power source, and if the battery charge is depleted, the DGs are started to supply electricity. This system enabled to establish 100% RE supply both during the day and at night.

As the share of renewable energy increases, two major challenges arise: “frequency fluctuations due to reduced system inertia” and “decreased fault detection capability due to reduced fault current.” To address these challenges, a Virtual Synchronous Generator (VSG) was developed, equipped with pseudo-inertia and fault current supply capabilities. Frequency fluctuations due to reduced inertia are managed by combining the pseudo-inertia of the VSG with the EMS’s medium-term and long-term frequency adjustment functions, and actual operation on Hahajima confirmed sufficient frequency control capability. For decreased fault detection capability, the

VSG’s fault current supply and a newly developed distribution line relay enable fault isolation by disconnecting only the faulted distribution line, as confirmed through simulation. In summary, the two major issues for achieving 100% renewable energy supply were resolved, and a fully automated microgrid power system capable of supplying 100% renewable energy was realized on Hahajima. By applying this technology, technical constraints can be relaxed, making it possible to further introduce renewable energy even in large-scale, high-voltage grids such as those on mainland Japan.