Aggregated frequency support feasible region ofDER cluster: A physics-informed state space transformation method

Aggregating distributed energy resource (DER) cluster to participate in primary frequency regulation (PFR) can effectively improve the frequency stability of power systems. To ensure the dynamic power flow (PF) security, the maximum frequency support capability (FSC) of clusters should be characterized prior to the PFR process. However, PF model of DER clusters exhibits strong non-convexity and nonlinearity, and the droop characteristic of DERs further increases the complexity of analytical solution, especially in low-voltage cluster without accurate parameters. Moreover, the interdependence of FSCs among clusters contributes to the complexity of PFR dynamics, rendering the accurate FSC evaluations for clusters challenging. Therefore, this paper proposes a cluster FSC assessment method based on physics-informed PF state space transformation and optimization relaxation. Using the incomplete state space dimension-lifting based on the Koopman operator, the original nonlinear PF is significantly simplified in the transformed space. By combining the piecewise McCormick technique with the boundary contraction algorithm, the remaining bilinear terms are relaxed in the reconstructed model. An alternating iteration method is presented to consider the co-expansion characteristics between the bulk grid and DER clusters. The simulation results indicate that the proposed method can achieve fast and accurate FSC assessment without relying on physical parameters.