The influence of residual air and moisture: In impregnated paper insulation

The paper describes experiments in study of the separate influence of residual air and moisture in impregnated paper as used for the insulation of high voltage cables. Some sixty similar samples are prepared, dried, evacuated, and impregnated under the same program, except as regards the pressure of evacuation and impregnation. In groups of three the samples were evacuated at various absolute pressures between 2 mm. and 76 cm. Hg. The samples were brass tubes 2.54 cm. in diameter, 122 cm. long, with 25 layers of wood pulp paper applied in the usual lapping spirals. Each sample was equipped with outside test and guard electrodes. Throughout their entire history, i. e., before and after impregnation observations were made on the samples of their dielectric absorption and their final conductivity. These studies have led to the conclusions as to the influence of moisture. After impregnation, the samples are studied as to power factor and dielectric loss over the range of electric stress 20 to 300 volts per mil, and of temperature 20 deg. cent, to 80 deg. cent. These studies have led, principally, to the conclusions of the influence of residual air. They show clearly the importance of thorough impregnation, and the conditions under which it may be accomplished. The causes of rising power-factor — voltage curves and methods of avoidance are clearly indicated. The principal results and conclusions are given at the end of the paper.