Dielectric measurements of semi-insulating liquids and solids

Sinusoidal steady state and transient decay measurement techniques were used to monitor the dielectric properties of electrical insulation. Impedance measurements of an air-gap capacitor, immersed in Shell Diala A transformer oil and driven with a 1 V peak sinusoidal signal (5 mHz-10 kHz), showed that the oil conductivity increased with temperature (15-70 degrees C) and aging in air but was independent of the oil moisture content (2-30 ppm). Interfacial double layer effects were also observed. Surface charge effects were studied in a cylindrical electrode apparatus with EHV-Weidmann Hi-Val pressboard on the inner cylinder and transformer oil filling the gap. The pressboard dielectric properties were obtained from time transient decay measurements of the open-circuit voltage after disconnecting a DC voltage source from across the cylinders. These measurements indicate that the conduction mechanism through the pressboard follows a drift dominated unipolar conduction law rather than ohmic conduction. Further frequency sweep dielectrometry measurements using a three-wavelength interdigital electrode sensor confirm the dispersive properties of oil-impregnated pressboard. This sensor was also used to measure the time and space distributions of moisture diffusion into oil-impregnated pressboard.