In October 2020, the Japanese government declared the goal of achieving carbon neutrality by 2050 and, with the aim of realizing a decarbonized society, is promoting the use of solar power, wind power, and other forms of renewable energy as main power sources. The Sixth Strategic Energy Plan, approved by the Cabinet on October 22, 2021, states that “to utilize distributed energy resources such as renewable energy and cogeneration in local communities, we hope to see the creation of microgrids and other self-reliant and distributed energy systems, which will also contribute to efficient energy use through local production for local consumption, strengthen resilience, etc.,” indicating growing expectations for microgrids (Figure 1) that can provide power self-sufficiency on outages due to disasters, etc. Overseas, in addition to addressing environmental issues, there have been multiple projects in Asian and African countries to build microgrids utilizing renewable energy and storage batteries that will supply electricity to areas where electric power networks are undeveloped (off-grid areas). As of 2015, the worldwide microgrid capacity exceeded 12,000 megawatts(*2), and further expansion is expected in the future.
In a conventional bulk power system, even in the event of fluctuations in demand or renewable energy output, the inertial (the property that tries to maintain a state) of rotating bodies such as turbines used for thermal power generation suppress rapid changes in system frequency, thus maintaining a stable power supply. However, if renewable energy becomes the main power source in the future and there is a decreased ratio of power sources such as thermal power generation that use large turbines, then there will be less inertial force from rotating bodies, which may affect the stability of electric power supply. Estimated costs for measures to deal with such a shortage of inertia range from 5.1 to 12.9 billion yen per year if the ratio of renewable energy in the bulk power system becomes 50%–60%(*3).
Solar and wind power are assumed to be the main power sources in microgrids, which are small-scale energy systems as compared to bulk power system. The amount of power generated fluctuates depending on the weather, and there is no connection to thermal power plants that use large turbines. As a result, power supply instability due to a lack of inertia will be even more pronounced. To ensure microgrid stability, it will therefore be essential to develop technologies to compensate for lack of inertia and stabilize power supplies, and to demonstrate these technologies and put them into practical use as soon as possible.