Dielectric properties of oil-soaked pressboard as affected by water

The effect of water, in amounts capable of being absorbed from a normal atmosphere, upon the dielectric properties of oil-soaked pressboard has been investigated over the temperature range from 20° to 90° C. Curves relating loss tangent, permittivity and electric breakdown strength of 0.2-in thick material with absorbed moisture and temperature have been plotted, and from these a final relationship between dielectric loss and breakdown strength has been obtained. With the data provided it is possible to make a fair estimate of the one-minute breakdown value of the material for any temperature between 20° and 90° C from a dielectric-loss measurement made at a small fraction of the breakdown voltage. Abnormalities in the loss-tangent/temperature characteristics resulting from the unequal distribution and subsequent migration of the absorbed water¿effects likely to be experienced when moisture is absorbed after oil impregnation¿are described. Brief reference is made to the implication of these findings in connection with the testing of the applied insulation.