Maximizing the potential of energy storage to provide fast frequency control

Due to their fast responsiveness, energy storage units such as batteries can provide fast frequency control to power systems. However, when the control signal is biased or substantially autocorrelated, they cannot provide services for extended periods of time because they have limited energy capacities. To improve the ability of batteries to provide frequency control, we can offset their frequency response so that they only respond to fast and zero-mean frequency deviations, while passing slower and biased deviations to other resources. We propose two new heuristics to offset frequency control signals, and a method to compare these heuristics to previously developed heuristics. This method allows us to quantify the heuristics from the point of view of both the storage unit operator (return on investment, ROI) and the transmission system operator (impact on the power system). Considering battery degradation, the new heuristics yield better ROIs, and we find that our results are not very sensitive to the battery's energy-to-power ratio around its optimum. We also find that the best choice of heuristic depends on what impact matters most to the transmission system operator.