Abstract
The permittivity reduction of insulating paper is expected to improve a dielectric strength in oilimmersed large power transformers. Using several kinds of newly developed synthetic paper with low permittivity, insulation characteristics of them are investigated under lightning impulse and AC voltage conditions by making simple coil models. In the case of AC voltage, AC partial discharge inception voltages are improved by using low permittivity synthetic paper. However in the case of lightning impulse voltages, some of the low permittivity insulating paper show the characteristic that partial discharge inception voltages are not improved. According to the still camera observation of the lightning impulse test and SEM observation of the insulating paper surface, it is found that the half lapped part of insulating paper becomes weak points of the partial discharge inception and low permittivity insulation papers with long fibers on the surface or the edge of paper give lower partial discharge inception voltages. It is cleared that not only the permittivity value of paper, but also the surface condition of the paper has an important role to get good insulation performance effectively for low permittivity paper.
