Time-to-breakdown and breakdown voltage for oil-impregnated insulation subjected to thermal aging

Oil-impregnated (OI) insulation is widely used in many types of high-voltage components such as transformers and cables. The most important requirement for the insulation is to withstand operational stresses without breakdown for the whole period of operation. That means insulation should be able to operate for a period of 30-40 years. For this reason, assessment of the insulation parameters such as breakdown voltage and time-to-breakdown is always done by rapid and endurance testing. However, such tests are always performed on new samples of the insulation and the electrical stress is the only aging factor. Therefore, for better life-time estimation of the insulation, it is important to know if the life expectancy might change due to thermal aging. In this paper, the results will be presented of an investigation into time-to-breakdown and breakdown voltage for thermally degraded OI insulation. In particular, for samples of insulation characterized by different levels of thermal aging, a comparison will be made of time-to-breakdown and breakdown voltage.