Strict fluctuation control required of new mega solar power plants

The influence of solar power and wind power on short-cycle frequency fluctuation has become a concern in the region covered by Hokkaido Electric Power Co Inc (HEPCO) in line with the increase of electricity generated by solar power and wind power. Responding to the issue, mega solar power plants that exceed 2MW in output are required to take measures including installation of a storage battery to relax the output fluctuation for connection to the grid, in respect to plants that are constructed after the total output of solar power plants that applied for connection to the grids operated by HEPCO exceeds 400MW.

The technical requirement is strict and the range of output fluctuation needs to be controlled to 1% per minute or less in all time zones. The system that was introduced this time was designed to satisfy the requirement.

As for the configuration of the system, a PV inverter is attached to the solar panels and the storage battery system, respectively, and the power is converted from DC to AC for transmission to the grid after combination, which is called the "AC link." When the output from solar panels decreases due to the passage of clouds, for example, DC power is converted to AC power by the PV inverter attached to the storage battery and is boosted to 6,600V for discharge.

Conversely, when the output increases, the excess solar power is reduced in voltage and converted to DC power for charging in the storage battery. The charge/discharge operations are repeated frequently depending on the output fluctuation to smooth the output.

The combined AC power is boosted to 33,000V by a transformer inside the facility and is transmitted to a substation located about 6km from the facility. The power is further boosted to 66,000V at the substation and is connected to TEPCO Higashi-Kushiro Substation via the grid.

The inverter for solar panels was manufactured by Nissin Electric Co Ltd and the inverter for the storage battery was manufactured by TMEIC. Mitsubishi Electric Corp handled the entire system and controlled the output fluctuation range to 1% per minute by integrally controlling the two inverters. The charge/discharge control of the storage battery system was decided through discussions with Mitsubishi Electric Corp and utilizing the results of frequency control at the projects on Iki Island and Oki Islands.