Operation Strategy of Multi-Energy Storage System for Ancillary Services

Energy storage systems (ESSs) used for ancillary purposes in power systems have different capacities and output characteristics, and so need to be scheduled and operated together based on their state of charge rather than individually. This paper proposes a simple but effective method to allocate the energy required for spinning reserve or frequency regulation, properly to each ESS within a multi-ESS with the aim of improving system reliability. For this, an ESS reliability model is first proposed, and the reliability of the entire system is then evaluated using reliability models for other utilities such as conventional generators and renewable energy sources. To better demonstrate the improvement in reliability, two new reliability indices are introduced besides the existing indices: loss of frequency probability and loss of duration probability. Through case studies, it is shown that the required power is more effectively distributed across each ESS by the allocation method and thus the method can contribute to securing more spinning reserve and alleviating more frequency fluctuation, rather than the conventional method.