Parameters Affecting the Increase in Dielectric Loss, Caused by Carbon Black Paper Screens, for Oil Paper Dielectrics

An important factor in the design of power cables is the type of screen used over conductor and insulation. Some designers have used aluminum metallized paper tape for many years, but others have preferred carbon black (CB) paper tapes. It was discovered a few years ago, however, that CB paper screens increased the power loss in the cable dielectric at high voltages and temperatures. From hundreds of tests on model and experimental cables this increase in dielectric loss in the cable is shown to result from an internal or mass effect and a surface effect. A small part of the loss increment occurs within the body or mass of the CB paper itself, but a much larger part represents a surface effect believed to be caused by emission of electrons from the paper. These effects are larger at the outer electrode of the model cables than at the conductor. The loss increment is influenced to only a small extent by changes in the amount and treatment of gas channel CB used in the CB paper. However, a reduction in power loss can be obtained by the use of a special CB paper in which a single sheet of paper is formed with CB on one side and insulating paper on the other.