Effects of frequency, temperature, compression, and air pressure on the dielectric properties of a multilayer stack of dry kraft paper

Kraft paper is used extensively in the electricity supply industry for the insulation of HV apparatus, such as transformers, capacitors and cables. Because of the fibrous nature of the paper, many layers are stacked or rolled to obtain the required electric strength. The insulation is thus a combination of cellulose and an impregnating fluid, which may be a liquid, a gas or vacuum. The dielectric properties of such a system depend on the dryness, temperature, frequency and the degree of mechanical compression. Although the effects of moisture, temperature and frequency on the dielectric properties of kraft paper have been studied, very little work has been done on the effect of compressive stress, particularly long-term effects. Results are presented of measurements, in the frequency range 50 Hz to 20 kHz, of the effects of compressive stress and temperature on the real and imaginary parts of the complex relative permittivity of a multilayer stack of dry kraft paper under vacuum. The variations of the dc resistance with compressive stress and temperature also are reported. Results are given of the variation of the dielectric properties of a stack of paper following a step change in either the temperature or the air pressure. The long-term variations are attributed to the slow diffusion of ions through the stack, particularly those held at the interfaces between layers.