ABSTRACT
Insulation is the bones of a transformer. Any type of insulation failure causes a transformer to go out of service. A robust insulation structure is essential for a unit to operate well and in the long term. In this chapter, power frequency over voltages during short-circuit are discussed. The types of voltage exerting on transformer during service and tests are presented, with analysis emphasized on impulse voltage development in winding by directly hit, or by transferred voltage, in barrier between two windings which has electrical connection, and an example is offered. The concept of DIL (design insulation level) and its application in design are introduced. Commonly used materials such as mineral oil, natural ester liquid, Kraft paper, pressboard and laminated wood are discussed on their dielectric and thermal performances, as well as on their moisture absorption and generation, gas in oil, aging, etc. The stresses in the structure such as in oil duct, at corner of conductor and along solid insulation surface are analyzed. The ways to design major, minor, end and lead insulation structures to achieve partial discharge-free structure are presented in detail. Finally, stress calculations of some of very basic field are given to help with practical issues.
