Storage System Design for Improved Primary Frequency Control From Hydropower Units

The upcoming modifications to the criteria for providing Frequency Containment Reserves (FCR) in the Nordic synchronous area present severe challenges for some hydropower units. By quantifying the difference between the required FCR power response and the actual response provided by the unit, an additional power response from an energy storage system can be specified to augment the hydropower unit, thus securing its participation in FCR in the future. A practical method to control the storage system, by applying a band-pass filter on the negative frequency deviation, is presented in this paper. The method is verified using a set of field measurements from Swedish hydropower units currently providing FCR. It is found that all tested units are able to satisfy the upcoming criteria with the specified storage systems. The optimality of the method is discussed by comparing the storage system power and energy ratings to the theoretical minimum. Obtained power ratings between 3% and 24% of the FCR power, combined with a short storage system duration of up to 2.5 minutes, indicate that the method presents a possible application for high-power and low-energy storage technologies.