Rapid Evaluation of Battery System Rating For Frequency Response Operation

Transmission System Operators (TSOs) are obliged to maintain the system's frequency within statutory limits. In the UK these limits are 49.5-50.5 Hz. There are two categories of frequency response services; dynamic or non-dynamic. The main dynamic frequency response services procured by National Grid plc, the UK's TSO, are enhanced frequency response (EFR) and dynamic firm frequency response (DFR). Historical frequency data has been analysed to demonstrate that a battery energy storage system (BESS) providing either EFR narrow (±0.015 Hz deadband), or DFR low will operate for 64.7% of the time at less than 40% of its rated power, and that only 0.24 pu energy is needed to respond to all single event frequency deviations; this 0.24 pu energy suggests the battery cells must be capable of operating at approximately 4C. An algorithm has been developed to estimate the energy and power for a BESS. The inclusion of losses in the algorithm improved the accuracy of the absorbed energy from 82.54% to 98.41% when compared to data from a commercial BESS. The enhanced algorithm has then been used to show that the BESS availability during the most energy demanding day in terms of low frequency events can be maximised by starting with a state of charge (SoC) which targets the upper SoC limit, and by providing the DFR low service. When the frequency data for all of March 2018 is considered, then if the BESS is continuously operated it achieves 100% availability if the BESS provides the DFR low service, whereas the most unavailable days occur if EFR narrow is used.