BESS as a primary and secondary grid service on a sub-transmission network
What is the result of applying a Battery Energy Storage System (BESS) as a Non-Wires Alternative (NWA) to provide primary and secondary grid services? This article attempts to find the optimal use and best application to support primary and secondary grid service requirements.
South Africa’s power utility Eskom conducted interconnection studies across the nine sites where it intends to install 270MW, 800MWh BESS as a Non-Wires Alternative (NWA)[1]. The contents of this paper are based on a verifying interconnection analysis to check the efficacy of the BESS size for peak shaving, line overload mitigation, and primary frequency response at a sub-transmission substation.
At the Melkhout site, the BESS has two key functions, including providing primary and secondary services. A 320MWh BESS is intended to be installed and connected at Melkhout substation with the primary aim of peak shaving. A secondary service required from the BESS is reducing overloads on Grassridge-Sunnyside 132kV lines. With the existing wind farms within the Melkhout area, during off-peak periods (early mornings with high winds), the national transmission system operator often requests some wind farm operators to curtail their outputs. The Eskom operating unit frequently receives requests for generation curtailment from the system operator due to the grid’s incapability to absorb the generation from the operating unit [2].
The deemed energy costs that Eskom needs to pay the wind farm operators (IPPs) represent an unnecessary expenditure considering its financial burden. Reducing these costs is an excellent opportunity for the operating unit to reduce part of Eskom’s operating costs. Using the BESS as a non-wires alternative is envisaged to operate for a minimum of a four-hour discharge cycle during the system peak period. A standardised and acceptable methodology is expected to be applied for all the identified sites for Eskom to apply in the BESS rollout across its footprint.
The Melkhout 132/66/22 kV distribution substation has 2 x 40MVA, 132/66kV and 2 x 20MVA, 132/22kV transformers and is connected to Grassridge 400/132kV transmission substation through 2 x 132kV lines and to Wittekleibosch collector station and Kareedouw substation through 1 x 132kV and 1 x 66kV lines respectively. Currently, the substation is interconnected with 423MW wind farms[3]. Further, there is a plan to connect an approved 140MW wind farm to the Melkhout substation. The decision to locate a 320MWh BESS at Melkhout substation was based on space availability in the area, sub-transmission constraints, congestion during high wind generation, avoidance of wind generation curtailment and deemed energy cost.
