Frequency response of energy storage systems in grids with high level of wind power penetration - Gotland case study

Gotland is a Swedish island that is connected to the grid only by two high voltage direct current (HVDC) cables. The growth of installed wind power on Gotland has been temporarily stopped due to concerns regarding the reliability of the operations. This study investigates the capability of a centralised energy storage system along with or without wind curtailment to support the growth of installed wind power capacity. The energy storage system is tested for maintaining frequency stability during unintentional islanding through dynamic studies using power system simulator for engineering (PSS/E). The results assess the ability of energy storage to prevent frequency instabilities and provide primary frequency response albeit of the absence of any rotating inertia. The analysis determines the requirements for the power and energy capacity of the energy storage system in relation to the exported power from the HVDC cables at the instant of fault, which eventually relates to the wind power capacity. Moreover, the study examines wind power curtailment as primary and/or secondary frequency response and the impact on the energy capacity. Finally, the study did not explore the effect of fast power ramps on the energy storage to leave the door open for all types of inverter-based energy storage technologies.