Cavity formation in mass-impregnated high voltage direct current cable insulation

A test cell has been designed in which we were able to visually observe formation, growth and collapse of cavities in mass-impregnated insulation during thermal cycling, both with and without DC voltage applied. Formation of cavities in mass-impregnated paper insulated cables is assumed to cause the reduction in dielectric strength experienced during the cooling period of a thermal cycle. Insulation-paper with simulated butt gaps was dried and impregnated in the cell. The cell was sealed at 55°C and cavities were observed by cycling the cell between this temperature and room temperature. The influence of an applied DC voltage, high and low interfacial. Pressure and butt gaps have been studied. The first cavity was generally formed in a butt gap. Initially it grew along the butt gap, but growth between butt gaps was observed at lower temperatures. The growth from one butt gap into an adjacent butt gap reduced the extent of cavities between the butt gaps. Growth of cavities between the butt gaps is probably the reason for the decrease in dielectric strength of the cable insulation during the first hours of the cooling period