Hierarchical Flexibility Aggregation of Heterogeneous Demand-Side Energy Storages for Secondary Frequency Regulation

Demand-side energy storages (DESs), such as household batteries and electric vehicles (EVs), have shown great potential in providing fast frequency regulation services. This paper proposes a three-layer hierarchical flexibility aggregation framework to fully evaluate and reliably realize the aggregate flexibility of heterogeneous DESs for secondary frequency regulation (SFR). At the device layer, the regulation capacity of an individual DES is modeled as a two-dimensional feasible region, with the real-time state-of-charge (SoC) constraint and the statistical features of regulation signals well considered. On this basis, heterogeneous DESs are divided into several clusters according to the geometry of their feasible region. At the cluster layer, regulation capacity, dynamic response model as well as regulation costs are efficiently aggregated within each DES cluster by prototype-based maximum inner approximation (MIA). At the aggregator layer, an analytical expression of the multi-cluster coordinated SFR flexibility can be formulated by convex optimizations at each feasible operating point, which provides a comprehensive model for DES aggregators to economically participate in SFR. Comparative simulations validate that the proposed method can accurately capture the aggregate SFR flexibility of heterogeneous DESs with lower flexibility loss and affordable computational burden, while the disaggregation feasibility of aggregate flexibility is further demonstrated using real-world regulation signals.