An investigation of the thermal performance of an oil filled transformer winding

Manufacturers may be required to design transformers to overload specifications. Oil viscosity changes and loss variation with temperature are not accounted for in the industry loading guide. A comprehensive test program was performed using a winding assembly with embedded thermocouples. Steady-state and transient tests were performed for natural and nondirected forced oil calculation. The temperature distribution in the winding was not linear with height as usually assumed. A large number of temperature sensors is required to determine the hot spot location. The location of the hot spot was found to be different and less predictable for natural oil calculation than for nondirected forced oil circulation, especially during overload conditions. Improved loaded guide equations were developed to account for oil viscosity, resistance change, cooling duct oil rise and location of the hot spot