Ancillary Service for Frequency Support - Design of a Battery Storage Based Ancillary Service for Frequency Support in the Nordic Power System

Abstract

In recent years, the frequency in the Nordic power system has been experiencing an increasing number of minutes outside the normal frequency band of ±100mHz determined by ENTSO-E. It could be argued, that this trend is expected to continue, specially with the current tendency to replace conventional generation with intermittent, renewable energy sources that are decoupled from the grid. This in turn, has given rise to new opportunities in the Ancillary Service (AS) market, that can take advantage of network connected devices such as Energy Storage Systems (ESS), to increase the grid’s flexibility. This thesis deals with the design and implementation of a Battery Energy Storage System (BESS) that aims to provide the grid with frequency support in the frame of the Nordic reserve product Frequency Containment Reserve for Disturbance operation (FCR-D). The proposed Ancillary Service (AS) consists of a dynamic battery model that is controlled with an energy-curtailment strategy dependant on its State of Charge (SOC). The control system for the Voltage Source Converter (VSC) that interfaces the battery with the grid is conformed of a two-degree-of-freedom current controller implemented in the synchronous reference frame that receives the active and reactive power references from the outer frequency and voltage controllers, respectively. The proposed AS was implemented using the simulation tool DIgSILENT Power- Factory. The models were tested in CIGRÉ’s Nordic32A bus system with three different study cases: the loss of a generating unit that caused the frequency to drop below 49 Hz, where it was proven that a 50MW, 300MWh BESS could reduce the Nadir enough to avoid the activation of more drastic frequency-controlled actions. The second scenario explored the effect of converter rating versus battery capacity when the BESS is providing FCR-D, where it was concluded that the BESS capacity is not the limiting factor for providing the service but rather the converter’s rating. Finally, the impact of the BESS was studied in a weaker grid where conventional generation was replaced with static generation. Here, it was shown that the proposed AS effect was more noticeable in the weaker grid, where the same BESS was able to reduce the Nadir by an additional 8.11% for the same frequency event, compared to the base case where no conventional generation was phased-out.