The Property Evaluation of Oil-paper Insulation After Long-term Storage by Dielectirc Modulus

In order to reduce the weight of transportation, power transformers are often filled with nitrogen gas at transformer substation or factory, in which their insulating performance will degraded because of moisture intrusion, ambient temperature variation and so on. So how to evaluate the insulating condition became an important issue. In this paper, based on the dielectric response theory, dielectric modulus, derived from frequency domain spectroscopy, is used to evaluate the oil-paper insulating condition during long-term storage. Firstly, a well-sealed tank with oil-paper sample is designed. Secondly, permittivity and dielectric modulus of oil-paper sample are measured under different conditions. The relationships between aforementioned measured parameters and insulting condition are analyzed. The tendency of moisture variation could be gotten. Thirdly, thermal-cold cycling treatment is also studied to simulate ambient temperature variation. Finally, the methods of reducing the moisture content of oil-immersed pressboard are discussed. The study results show that moisture intrusion is the main reason of insulating condition decline for oil-paper sample in long-term storage; the insulation condition of samples with varied temperatures become worse than which in constant temperature.