A New Principle of Cable Design

Study of cable behavior on loadcycle tests led to the conclusion that instability of the insulation results from its deformation, leaving ionizable voids in the inner layers, where deterioration starts. This led to the conception of a cable in which the inner and outer layers of paper would have different moduli of elasticity, the outer papers having the higher modulus so that during the cooling period of a load cycle, the outer papers would compress the inner ones and so reduce or eliminate voids therein. The tension of application of the tapes is the same for all layers and does not depart from ordinary practice. It was found that cables made on this principle maintained a remarkable stability of power factor under load-cycle conditions. Finally, a study of cable impregnants showed that an essential characteristic is stability under electrical discharges. An impregnant was developed consisting of a paraffin base oil with a definite proportion of a new synthetic derivative of wood rosin, which possesses this characteristic in a marked degree. This, used in combination with the above-described structure gives the maximum stability we have been able to attain.