Two-stage robust voltage/var control strategy of active distribution networks with hybrid distribution transformers

This paper addresses the challenge of integrating Hybrid Distribution Transformers (HDTs) into the unbalanced Active Distribution Networks (ADNs). A two-stage robust VVC strategy is proposed to coordinate the operation of HDTs with a full spectrum of existing grid assets, including legacy devices such as on-load tap changers (OLTCs) and capacitor banks (CBs), and other flexible resources like Energy Storage Systems (ESS) and Demand Response (DR). The first stage determines a robust day-ahead schedule for slow-acting resources (OLTCs, CBs, ESS, DR), while the second stage performs real-time adjustments of HDTs to counteract uncertainties from PV generation and load demand. An accurate HDT power injection model, based on the auxiliary branch method, is developed to ensure modeling fidelity by incorporating internal impedance and series converter dynamics. The framework is built upon the coupled-phase second-order cone programming branch flow model (CP-SOCP-BFM) to accurately manage the network's three-phase unbalance. To solve the resulting large-scale, non-convex problem, a computationally efficient solution framework combining the Inexact Column-and-Constraint Generation (i-CCG) algorithm and the Alternating Optimization Procedure (AOP) is proposed, which guarantees robust global optimality. The effectiveness of the proposed strategy is validated on a modified IEEE 33-bus system. Comprehensive comparative studies demonstrate its superiority over deterministic, heuristic-based, and standard robust optimization methods in terms of both robustness and computational efficiency.