High voltage power transformer insulation design

The large power transformer life span depends heavily on insulation - its condition, materials, composition, geometry, etc. The high voltage transformer insulation requires a very focused analysis in the design stage. The large power transformer is subjected to different overvoltages during factory testing and in operation, therefore all resulting electric stresses have to be modeled in the electric field program and compared to the industry accepted withstand curves. The overvoltages include transient voltages from impulse testing, both lightning and switching, as well as power frequency voltages generated during the induced and applied voltage tests. The distributions of electric stresses under these different electric excitation conditions are different. The transient voltage programs with MLCR components (mutual- and self-inductance, capacitance, resistance) are used to establish exact voltage distribution inside the transformer. The electric field analysis for complex units is performed for all energized components, including turn-to-turn, section-to-section, winding-to-winding, winding to core, phase-to-phase, between leads, from winding and leads to constructional parts, bushing to tank, etc. The electric stresses are checked under two main conditions: (i) strike in oil, (ii) creeping discharge along solid insulation. The paper will discuss the analysis for selected transformers and explain the design process needed to maintain the dielectric stresses under critical values.