Advanced Inverter-Based Volt/Var Control for Three-Phase Unbalanced Distribution Networks

Abstract

The proliferation of distributed energy resources (DERs), particularly photovoltaic (PV) systems, has intensified voltage regulation challenges in three-phase unbalanced distribution networks (DNs). These challenges include rapid voltage fluctuations, unequal phase voltage magnitudes, increased network power losses, and limited amounts of DERs the system can host. Active distribution networks (ADNs) leverage PV inverters to provide responsive Volt/Var control (VVC) to regulate bus voltages. However, existing VVC schemes often assume a single-phase, balanced equivalent network, overlook true three-phase unbalance issues, neglect explicit voltage stability margins and fail to quantify their impact on hosting capacity under uncertainty. To bridge these gaps, this thesis focuses on advanced inverter-based VVC methods tailored for three-phase unbalanced distribution networks.

Firstly, a convex voltage-approximated network model is developed for three-phase distribution networks to incorporate voltage magnitudes for broad voltage control applications, while maintaining the computational efficiency of the optimal power flow (OPF) problems. Secondly, a novel voltage unbalance approximation index and a multi-objective VVC method are proposed to quantify voltage unbalance in the OPF problems. This enables network loss minimization and voltage unbalance mitigation under high PV penetration scenarios. Thirdly, a hierarchically coordinated VVC (HC-VVC) method with effective network voltage stability constraints (VSCs) is proposed. A penalty-based convex–concave procedure (CCP) solution algorithm is developed to efficiently solve the resulting nonconvex problem under PV generation and load uncertainties, ensuring robust voltage stability and power loss minimization. Finally, robust and risk-averse hosting capacity (HC) maximization methods with inverter-based VVC are proposed to determine the maximum allowable PV power generation in three-phase unbalanced distribution networks.