Aggregate Frequency Support Capability Assessment of Active Distribution Network Considering Power Flow Constraints

Utilizing distributed energy resources (DERs) within active distribution networks (ADNs) for primary frequency regulation (PFR) is of great value to power system frequency safety and stability. Nevertheless, the frequency support capability (FSC) of ADN is constrained by both the capacities of DERs and the carrying capacity of network power flow (PF). To ensure PF security, assessing the FSC of ADNs before PFR services is essential. However, the lack of line parameters in low-voltage ADNs results in inaccuracy in physics-based assessment models. Furthermore, the multitude of nodes and DERs contributes to the high-dimensional nonlinearity of PF model in ADNs, rendering optimization problems challenging to solve analytically. Therefore, this article proposes a data-driven online assessment method for FSC of ADNs, which uses the Koopman operator to augment dimension of partial state space, converts the original complex nonlinear PF into a high-dimensional linear model, and then constructs a readily solvable linear assessment optimization model. On this basis, considering the interactive characteristics between the FSC of bulk grid and ADNs, an alternating iterative method is proposed to precisely evaluate the maximum FSC of ADNs. A modified IEEE 82-node system is used to verify the effectiveness and feasibility of the proposed method. Numerical results indicate that the proposed method enhances computational efficiency by an average of 99.34% compared to traditional physics-based methods, achieving nearly 100% PF capacity utilization during the PFR process.