Bi-Level Active-Reactive Voltage Regulation Strategy for Distribution Networks with Hybrid Grid-Forming and Grid-Following PV Inverters

The high penetration of distributed photovoltaic (DPV) systems poses significant challenges to voltage stability in distribution networks due to their intermittency and volatility. The diversification of inverter control strategies, particularly the coexistence of grid-forming (GFM) and grid-following (GFL) inverters, creates complex hybrid integration scenarios. Coordinating these inverter types with traditional devices like on-load tap changers (OLTCs) and shunt capacitor banks (SCBs) for effective voltage regulation is thus a crucial issue. This study develops a novel bi-level active-reactive coordinated voltage regulation strategy. A comprehensive voltage regulation model for hybrid inverter systems is established, followed by the formulation of a bi-level optimization framework. This framework decouples the active and reactive power optimization: the upper layer minimizes the total daily operational cost, and the lower layer minimizes the average node voltage deviation, with iterative coupling between them. To handle the discrete nature of OLTCs and SCBs, the Fisher optimal segmentation algorithm is applied to determine their optimal action timings and settings, minimizing mechanical operations. Case studies on an IEEE 33-node system confirm that the proposed strategy achieves superior performance compared to conventional methods, offering enhanced voltage control accuracy, improved economic efficiency, and a significant reduction in switching operations.