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On constructing a microgrid, it is essential to balance capacity of photovoltaic power generation (PV) systems and storage batteries in accordance with a control target. In the present study, we constructed a simulation model of energy control system in the microgrid. By using this model, we investigated the acceptable capacity of PV systems according the capacity of NaS battery for keeping the target supply-demand power imbalance over 30-min. within ±3 %. Under keeping the control target of power imbalance over 30 min. within ±3 %, we verified the relation between acceptable capacity of PV systems and NaS battery in a microgrid as follows. (1) the minimum required capacity of NaS battery in relation to the capacity of installed PV systems (330 kW) is of the order of ±20 kW, (2) the acceptable capacity of PV systems in relation to the capacity of installed NaS battery (±500 kW) is of the order of 890 kW. Moreover, we clarified the constrained conditions which impact to the power balancing in the microgrid according to the acceptable capacity of the PV systems.Index Terms--microgrid, photovoltaic power generation, NaS battery, supply-demand power imbalance over 30-min.T. Shimakage is with R&D Headquarters,

A microgrid (MG) is one of the measures for enhancing the high penetration of renewable energy (RE)-based distributed generators (DGs). If a number of MGs are controlled to maintain the predetermined electricity demand including RE-based DGs as negative demand, they would contribute to supply-demand balancing of the whole electric power system. For constructing an MG economically, capacity optimization of controllable DGs against RE-based DGs is essential. Using a numerical simulation model developed on the basis of a demonstrative study on an MG using PAFC and NaS battery as controllable DGs and photovoltaic power generation system (PVS) as an RE-based DG, this study discusses the influence of the forecast accuracy of PVS output on capacity optimization. Three forecast cases with different accuracy are compared. The main results are as follows. Even with no forecast error during every 30-minute period, the ideal forecast method, the required capacity of NaS batteries reaches about 40% of the PVS capacity for mitigating the instantaneous forecast error within 30 minutes. The required capacity to compensate for forecast error is doubled under the actual forecast method. The influence of forecast error can be reduced by adjusting the scheduled power output of controllable DGs according to weather forecasts. Besides, the required capacity can be reduced significantly if the error of balancing control in an MG is acceptable for a few percent of periods, because the total periods of large forecast error are not frequent. balance in supply and demand over 30 minutes.

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