Dielectric strength of power cables: a statistical approach

The extruded electric power cables have substituted the paper cables in medium level voltage, giving advantages in manufacturing processes, as well as in cable laying, maintenance, operation and in terminal and splice assembly. In spite of this result, a more accurate research on the breakdown strength distribution is needed, in order to improve the knowledge on the insulation reliability, and its consequence in the design and in the qualification. With this aim, this work shows a methodology to evaluate extruded cables quality by means of sample breakdown tests. Weibull distribution was adopted, and the distribution shape and scale parameters were obtained by means of short time breakdown tests. The confidence limits were obtained by the Monte Carlo technique. The service reliability was estimated by using the geometric transformation and the time equivalence of the applied stress. Considering the experimental results, it was concluded that the tested cable samples insulation has a good performance with electric stress, in the 15 kV voltage class, since the estimated failure rate resulted in acceptable values, as well as the expected number of failures