Design of Termination for an AC Disruptive Voltage Test on a 35 kV Cable

High voltage cables have been employed in industry for underground and underwater power transmission. The reliable operation of a high voltage cable directly depends on the performance of its insulating material. Thus, acceptable dielectric performance must be assured for cables subjected to both alternate current or impulsive overvoltage. In Brazil, one of the standards that aims to evaluate the internal insulating material of high-voltage cables is the standard NBR 10299. This standard provides overvoltage measurements and statistical techniques for evaluating the cable performance. However, there is a difficulty to manage the electric fields at the cable ends when laboratory measurements are made. The problem is intensified in the case of overvoltage tests which can submit the cable to overvoltages of the order of 5 to 10 times greater than the normal operation voltage. During an overvoltage test which aims to evaluate the internal insulating material, surface and external discharges in the air must be avoided. This paper presents simulated results of a termination cable model developed for such overvoltage tests. In order to reduce electric field at the cable termination components and air surrounding the test sample, computer simulations of electric field distribution employing the finite element method were realized for a 35-kV cable. The applied rms voltage used as an overestimate of breakdown voltage was 300 kV. The proposed termination consists in a toroid electrode to conform electric field and a rubber made stress relief cone. The model allows to reduce electric field in air and, thereby, it avoids external discharges and allows evaluate the disruptive voltage of the internal insulation of the cable.