Reducing SoC-Management and losses of battery energy storage systems during provision of frequency containment reserve

Battery energy storage systems (BESS) are increasingly being used to provide Frequency Containment Reserve (FCR) due to the gradual decline in prices of Li-ion cells. Compared to conventional generators providing FCR, the limited energy reservoir of BESS requires suitable management of the state of charge (SoC-management). The energy to restore the SoC of BESS can either be traded on the energy market or balanced by other generation units inside the same balance group. In both cases costs arise, which can be minimized by reducing the energy for SoC-management. The possibility to use degrees of freedom (DoF) during provision of FCR allows such a reduction of SoC-management. This paper analyses the influence of these DoF on the SoC-management via simulation in MATLAB/SIMULINK and presents measurements of an implementation of these DoF at a BESS test site. The second topic in this paper investigates the losses that arise during provision of FCR and SoC-management. Since the frequency deviations in the European power grid are usually very low, the power demand of FCR is carried out in the partial load range of the BESS and its converters. As the efficiency of converters in the partial load range is rather small, the losses increase. In case of BESS consisting of several subunits it is possible to reduce the overall losses by only activating the number of subunits that is needed to cover the actual power demand. This paper presents an algorithm to implement this idea and investigates its effectiveness by simulation in MATLAB/SIMULINK. The results proof that the use of DoF reduces the energy needed for SoC-management. But the consideration of losses reduces the effect of the DoF. The presented algorithm to minimize losses can be applied to reduce losses and also makes it possible to fully exploit the potential of the DoF.