6 - Battery energy storage contribution to system frequency for grids with high renewable energy sources penetration

Abstract

The chapter aims to present a novel methodology to assess the impact on system frequency of frequency control (FC) and fast frequency response (FFR) by limited energy reservoirs (LERs). LERs, and specifically battery energy storage systems (BESS), are expected to provide a relevant share of regulating power in the next future. When analyzing the operation of LERs, the impact of the possible depletion over time of energy content during the service provision should be considered. Therefore, the classical approach of assessing the frequency for tens of seconds after an incident (e.g., a step response or a load ramp) can fail to properly evaluate all the possible issues. A dynamic simulation encompassing a longer period should be performed, and the energy content evolution in the LERs should be checked. A proper BESS model is hence implemented in a tool capable of modeling the steady-state and transient response of power systems. This tool is used to perform dynamic simulations with FFR provision by LERs over an extended period. The proposed methodology for adapting an empirical BESS model to be adopted in dynamic simulations of power systems is described. Then, a methodology is proposed to assess the system frequency in the real-world operation over an extended time interval (tens of minutes to hours). Gathering frequency data, network data, and electricity market data, the power imbalance over a period can be reconstructed and fed as input to the model, sharing it equally to all the loads. The proposed methodology is hence tested in a case study of an FFR service according to the control strategy of the Italian Fast Reserve. The estimated benefits and the possible drawbacks of fast regulation are checked by carefully analyzing the frequency profile in different nodes. The results are given in terms of the improvement of the frequency nadir after an event. Additional results can deal with the system cost to ensure the same quality of supply, the needed amount of fast-responding units, and the residual energy content in the LERs after the frequency event.