Effect of Moisture on Low Frequency Relaxation of Oil-Paper Insulation

The insulation performance of oil-paper insulation power equipment is seriously affected by moisture in oil and pressboard, but the current dielectric response diagnosis technologies are difficult to distinguish the effect of moisture and aging. In order to seek an effective moisture assessment method based on dielectric response technologies, the low frequency relaxation of oil-impregnated pressboard is tested and analyzed. Firstly, oil-impregnated pressboard samples of different moisture contents were prepared; then the frequency dielectric responses were tested, and the low frequency relaxation curves were obtained; finally, the effect of moisture on low frequency relaxation was discussed. The test results show that, the relaxation peak increases by moisture content, while the characteristic frequency of the peak remains the same. Besides, a new relaxation peak appeared at higher frequency range when moisture content is above 4%, which is considered to be caused by free water in oil-impregnated pressboard.