Gas-Filled Cavities in Solid-Type Cables

Shape, size, gas pressure, and distribution of cavities are inivestigated in the oil-impregnated insulation of solidtype cables for the purpose of estimating dielectric strength. Possible cavity locations and the influence which acting forces and gas solution in the oil have on the cavities, are discussed first. Gas solution is treated as a diffusion process and the time variation of pressure in the cavities calculated. Cavity properties vary continually and the changes are discussed from manufacture to first operation, when the cable is put on load, and when it is cooling. Cavitation on cooling is treated, growing rate of new cavities and pressure drop across the insulation calculated, and the influence of cavitation upon the temperature rise limit considered. Corona occurrence is estimated with the help of this analysis of cavity properties. Means which diminish corona are discussed and compared with current practice in cable design. It is thought that cavity analysis might be usefully applied also to other apparatus with impregnated insulation, e. g., transformers and capacitors.