Distribution of moisture in oil immersed pressboard under transient temperature condition

Moisture has strong influence on insulation performance of oil pressboard system in power transformer. Moisture in oil immersed pressboard is uniform under constant temperature condition. Moisture in pressboard can be calculated according Oommen paper oil equilibrium curves. Temperature of power transformer is related to the atmospheric temperature and the load. If atmospheric temperature or load changed, oil and pressboard insulation will work under transient temperature condition. If temperature increased, moisture will move from the pressboard to the oil; if temperature decreased, moisture will move from the oil to the pressboard. In this case, the distribution of moisture in oil immersed pressboard is un-uniform. The ununiform distribution of moisture can be ignored in very thin pressboard. However, it is important to understand the distribution in thick pressboard for assessment of insulation condition. In this paper, a test system has been set up which consists of programmable oven, Karl Fischer moisture meter and oil-pressboard samples. Transient condition of temperature decrease from 80 °C to 20°C in 5 hours has been simulated. Moisture in various pressboards from 0.5mm to 5mm has been measured. The results have been presented and analyzed.