Optimizing Battery Energy Storage Systems (BESS) for grid stability in South Africa

BESS can also contribute to voltage support and power system flexibility making it a versatile tool in modern grid operations. In South Africa, primary frequency control is provided by governor response and demand response. There are multiple BESS procurement programmes, with some installations designated for fast frequency response.

This paper focuses on determining optimal BESS fast frequency response settings to prevent frequency overshoot during major disturbances. Particularly when BESS operates alongside primary frequency control mechanisms such as governor response and automatic demand response (DR) in the South African interconnected power system. The investigation was motivated by the need to ensure that BESS activation does not exacerbate system instability during high-impact events, such as the credible single contingency. In South Africa, the credible single contingency is the loss of the largest generating unit in the system. The frequency response simulations analysis was conducted using a detailed 2025 South African power system model. These simulations were conducted to evaluate key performance indicators such as frequency Nadir and rate of change of frequency (RoCoF). Multiple BESS activation alternatives were assessed, each defined by a specific frequency threshold and maximum activation time. The aim was to identify the optimal trigger frequency that effectively supplements primary frequency control without causing frequency overshoot. The optimal

BESS fast frequency response configuration complemented governor response to ensure optimal frequency response. Only one DR block was be modelled to assess contribution to the frequency recovery, it was a 350 MW load block with a 4-second delay. The simultaneous activation of BESS and DR was studied to understand the potential impact on frequency overshoot, and if the coordinated deployment of these resources was feasible and beneficial.

The study results show that the introduction of BESS improved the frequency Nadir from 49.506 Hz in the base to 49.561 Hz. This represents a significant enhancement for the South

African grid, which operates as a relatively small and islanded power system. It will provide the foundation for pilot implementations at selected facilities, enabling real-world validation of recommended BESS settings. The recommended optimal trigger frequency for BESS fast response was 49.70 Hz. In this configuration, BESS operates as a supplementary resource to governing response and DR. However, in the future the trigger conditions can be changed to

RoCoF when the system is exposed to low inertia and high RoCoF. A different study will be required to determine the optimal RoCoF setting.