Two-Tier Aggregation of Distributed Energy Storage Units Considering Network Access Purchase

The number of distributed energy storage units (ESUs) within a distribution network is expected to increase because of the rapid deployment of 5G base stations, and they can be aggregated and formulated into a bid model of polytopic shape, which is consistent with market rules. However, it is challenging to design such a polytope that best captures their aggregate flexibility and satisfies distribution network constraints. This paper proposes a two-tier inner approximation aggregation model that aggregates the flexibility of ESUs first to the corresponding network buses and then to the substation. The uncertain power load of the 5G base station affects the minimum reserved energy of its backup battery. Therefore, with the aid of distributionally robust chanceconstrained programming, a customized affine transformationbased method for inner approximation of the flexibility set is proposed to capture the charge/discharge logic and to handle uncertainty. In addition, the acquisition of network access limits is integrated into the optimal flexibility design of the aggregator by deriving the approximate form of the cost function in an auction framework. Case studies demonstrate the effectiveness of the proposed model in terms of economy and uncertainty handling.