Development of PCS for battery system installed in megawatt photovoltaic system

Population growth, global warming, depletion of fossil fuels, and ensuring nuclear power safety are serious energy issues, and solutions to these issues have been becoming increasingly important globally. One solution that has been focused on is the use of renewable energy. The introduction of photovoltaic (PV) and wind turbine (WT) systems has been accelerating around the word. However, the power generation efficiency of these renewable energy systems is low, and the power generation level is unstable. At present, therefore, the ratio occupied by renewable energy in the overall power generation system cannot be considerably increased from the viewpoint of grid stability. Nevertheless, renewable energy is one of the most important energy sources for the future, and various studies and field demonstrations are underway. This paper gives an overview of a MW PV plant equipped with a large-capacity battery system (NAS battery), together with the results of grid stability enhancement achieved with the battery system, in trials conducted by Okinawa Electric Power Company Co., at a research facility on Miyako Island. In particular, this paper reports on power stability control and frequency compensation using a power conditioning system (PCS) for energy storage. This MW PV plant and power storage system were installed in an independent power grid on Miyako Island. The total grid capacity is about 50,000 kW. The MW PV system and an existing WT system make up about 16% of the total system capacity, and the battery system makes up about 8%. Power supply instability and frequency fluctuations become larger with an increasing proportion of power generated by renewable energy. The developed PCS using the battery system showed good performance in solving this problem.