Integrated control of mega-solar facility, storage battery
In the project in Shiriuchicho, to achieve the requirement for a "1% fluctuation per minute," a storage battery with an output of 12.5MW and a capacity of 7.2MWh was set up for the PV inverters with a total output of 17.5MW for the whole mega-solar plant. The circuit was designed in the "AC link" method, in which PV inverters are separately set up for each PV and storage battery systems and the output power is transmitted to the power grid after being converted from direct current (DC) to alternating current (AC) and combined with generated power (Fig. 6).
The "TMEIC Battery Control System (TMBCS)" manufactured by TMEIC was adopted for charge-discharge control in conjunction with the mega-solar output. With the main site controller (MSC) that collectively controls multiple PV inverters set up in the mega-solar site also working in conjunction with the inverters for the storage battery, the TMBCS controls battery charge-discharge so sharp fluctuations in PV output can be smoothed while monitoring the amount of power generation at the grid point of the overall site in real time.
Given that the PV inverter's output is larger than the storage battery's maximum output, however, it is possible that the TMBCS cannot completely smooth sharp fluctuations in PV output and fail to keep a "1% fluctuation per minute," and in such case, the PV output should consequently be limited.
"If the capacity of a storage battery is further boosted, the output restriction amount will decrease while initial investment increases accordingly," Orix said. "We decided on the optimum battery capacity from the viewpoint of return on investment."
According to TMEIC, the TMBCS enables smoothing of sharp fluctuations in output of a mega-solar plant even with a relatively small battery capacity. To achieve the grid requirement for a "1% fluctuation per minute," a storage battery with a capacity equivalent to about 80% of the PV inverter output is usually set up while the capacity was only about 70% compared with the PV inverters this time.