A Novel Hybrid Capacity Regulating Transformer and Its Dual-Mode Flexible Switching Strategy

Distribution systems with high penetration of distributed renewable energy sources (DRES) encounter challenges such as inefficient operation under light-load conditions, and power quality disturbances. To address these issues, this article proposes a novel hybrid capacity regulating transformer (HCRT) integrated with an auxiliary multifunctional converter (MFC) and a dual-mode flexible switching strategy. An equivalent circuit model of the HCRT is developed to derive the arc-free tap-changing mechanism for both large- and small-capacity operations. Optimal energy-saving operating is achieved through coordinated control of the auxiliary MFC. During mode transitions, the auxiliary MFC fully supports the load and decouples tap-changing operations on both windings via solid-state switches, enabling zero-current, arc-free switching and simplifying the on-load tap changers design. A four-stage capacity regulation (FSCR) process is introduced to reduce control complexity. Furthermore, a dual-mode flexible switching strategy is proposed for FSCR, employing a weighting coefficient to coordinate grid-following and grid-forming control, enabling smooth mode transitions without inducing load voltage disturbances. Simulation and experimental results validate the effectiveness and feasibility of the proposed topology and control scheme.