ABSTRACT
The energy consumed by transformer presents in the form of heat. It raises temperatures of oil, winding and structural parts. Server high winding hot spot temperatures cause local overheating, generates gases in service and threatens insulation integrity, consequently shortening unit service life. Thermal aging mechanism and loss of life are described. Temperature rise limits of oil, winding and winding hot spot for normal service are listed. The mechanisms of natural oil circulation with natural and forced air flows, forced oil circulation to bring heat from windings to ambient are discussed in detail. Temperature rise calculation equations of oil, winding and winding hot spot, and the difference between average and top oil rises, are presented. Loading beyond rated power rating can be conducted as the price of sacrificing a unit’s life, the temperature rises calculation methods of oil, winding and winding hot spot under overloading and loss of life are provided, accompanied with an example. Empirical equations of winding gradient under natural oil flow are given, and the methods to make efficient oil flow in the winding are discussed.
