Design and Development of DC Cables

The first problem which arises concerning dc cables is determination of the gradient distribution within the insulation. It is shown that the dielectric absorption phenomena may be neglected, for they give only a space gradient distortion at the scale of a paper fiber, which does not seem to have an effect upon the practical dielectric strength of an insulation. Then, the gradient distribution is investigated as a function of the conductivity of the insulator, which varies with temperature and gradient. Consequently, the heating of the conductor tends to shift the maximum stress from the inner to the outer layers of the insulation, while the voltage increase tends to make the stress uniform. The dc breakdown gradients calculated according to the stated laws lead to a value of dielectric strength practically equal to that obtained during impulse tests, except for high temperatures. A sudden decrease of the dc dielectric strength is then observed, which is attributed to the thermal instability caused by the increase of the losses in the insulator.