Gotland as a microgrid -  Energy storage systems frequency response in grids with high level of renewable energy penetration

Sammanfattning: The Swedish island of Gotland , situated about 100 km from mainland Sweden in the Baltic Sea, represents a power system with a high wind power penetration. The island is connected to the mainland Sweden exclusively via two HVDC cables that provide the only source of active power and frequency control. The two cables can operate in different configurations, i.e. import or export power from or to mainland. However, in order to ensure the N-1 criterion, one of the cables currently always must import power from the mainland. This means that the available power exporting capacity is limited to the rated power of one of the cables. Therefore, in the case of having a fault on the exporting HVDC cable during low load demand and high wind power production, the power system will suffer from high active power transients that will increase the frequency above the acceptable threshold. Consequently, the protection system will trip the over-frequency relays, triggering cascading outages on the island that might eventually lead to blackout if the problem is not addressed correctly. Thus, increasing the renewable energy production on Gotland is currently considered as a risk that will increase the probability of instable over-frequency contingencies. This has led the local grid operator to cap the installed wind power capacity to its current level. Therefore, the ability to preserve the stability of the power system during islanded operations until the HVDC cables fault is cleared or the emergency reserves are online is essential for the growth of installed wind power capacity. The main objective of the thesis is to examine the capability of a centralized energy storage along with or without wind curtailment. The ESS is tested for maintaining the frequency stability during the unintentional islanding through dynamic studies using the software PSS/E. The results show that an ESS prevents frequency instabilities and provide frequency response during HVDC cables fault albeit of the absence of any form of rotating inertia. The results show that for today’s 185 MW of installed wind power capacity, an energy storage of 50 MW power capacity will reduce over-frequency instabilities in the case of HVDC cables fault from 13% to 1%. The analysis finds that the power capacity of the energy storage depends on the exported power from the HVDC cables at the instant of fault, which eventually relates to the installed wind power capacity. finally, the study shows that using wind power curtailment will significantly decrease the energy capacity of the energy storage.