Investigation of Moisture Accumulation Induced by Thermal-Fluid Interaction in the High-Voltage Lead Exit of a 750 kV Transformer

As key components of power systems, transformers are significantly affected by moisture migration in their insulation, impacting performance and reliability. This study combines numerical simulations and experiments to investigate moisture migration in the insulating oil of a 750 kV transformer high-voltage lead exit and its interaction with the thermal flow field. The results indicate that as the operational load and ambient temperature increase, the temperatures of the lead exit and bushing rise, with the hotspot concentrated at the base of the bushing’s capacitor core. However, the small contact area and high thermal resistance between the bushing and the lead exit minimize the impact of the bushing’s temperature rise on the lead exit. The lead exit remains cool, with a slow oil flow, though the temperature gradually increases along the axial direction. Influenced by the local structure and thermal convection, two adjacent, oppositely directed closed oil flow loops form at the lower end of the lead exit and the elevated seat base. Further research finds that under high temperature and high flow conditions, moisture diffuses effectively in the lead exit, but a higher initial moisture concentration inhibits this process. Consequently, moisture accumulates in the low-temperature area of the lead exit’s lower section and the low-flow region at the elevated seat base. When conditions for free water formation exist, free water forms in these regions, and this effect intensifies with higher initial moisture, decreased load, and lower ambient temperature. This study provides theoretical support for optimizing the design and operation of transformer lead exits, improving long-term reliability.