Adaptive Zonal Cooperative Protection Strategy for Flexible On-Load Tap Changer Considering Multi-Energy Complementarity and EV Charging/Discharging Characteristics

The high penetration of renewable energy and the large-scale development of electric vehicles (EVs) have made voltage violations in distribution networks increasingly prominent. The flexible on-load tap changer (F-OLTC), combining mechanical coarse regulation and power-electronic stepless fine adjustment, offers advantages in both regulation range and speed. However, existing protection strategies fail to account for the impact of EV charging/discharging surges and the multi-physics coupling constraints of the equipment, making it difficult to coordinate voltage quality and equipment lifetime. This paper proposes an adaptive zonal cooperative protection strategy for the F-OLTC that accounts for EV charging/discharging characteristics. An integrated protection architecture is constructed that fuses EV state awareness and equipment health monitoring. A multi-feature fusion fault identification method is proposed to distinguish EV disturbances, external faults, and internal converter faults. A comprehensive margin index is established, and dynamic zoning is achieved based on feature-space clustering. A coordinated stepped and stepless adaptive voltage regulation protection is designed, with dynamic adjustment of thresholds and delays, and fast EV power support is dispatched during mechanical switching transients. Simulations on a modified IEEE 33-node system show that the proposed strategy reduces average voltage deviation by 46.2%, tap operations by 28.6%, voltage sag depth by 81%, recovery time by 70.8%, and network loss by 21.9%, effectively improving voltage quality and reducing mechanical stress on the switching devices, which is expected to contribute to extended maintenance intervals and improved equipment reliability.