This work presents a method for designing an energy storage system (ESS) operating in an islanded power system formed by a single synchronous machine. The method utilizes stability and performance criteria to define the island operation. The Nyquist stability criterion is used to ensure a stable response and a performance criterion is used to evaluate the island system's ability to contain the frequency within specified limits during island loading steps.

These criteria are derived using the same framework as Frequency Containment Reserve (FCR) criteria in the Nordic Synchronous Area. A closed-loop model of the island system, including a standard hydropower unit model and an ESS model, is used in MATLAB/Simulink. The models' response is optimized together with the defined criteria to obtain the size and control parameters for the ESS.

A case study with a Swedish hydropower unit with known power response dynamics operating in island mode is presented. Turbine test data is used to parametrize the linear hydropower model and an ESS is designed separately for FCR provision and island operation. The results of the case study show that 86 % more power from the storage system is needed for island operation than FCR provision according to the two defined sets of criteria. It is also shown that while island operation is the expected dimensioning mode for the ESS, the resulting EES size is very sensitive to the criteria and dynamics defined for the island system, with the load step size having the strongest influence on the ESS size.