Considerations about the application of the enlargement law to dielectric strength data relevant to power cables

Theoretical considerations about the “enlargement law”, that were developed by one of the authors in a previous work, are employed here for the comparison between two different compounds candidate for the realization of full-size cables, on the basis of dielectric strength test results relevant to small-size cable models. In particular, compound performances are investigated as a function of cable length, searching for the existence of the so-called crossing length, i.e. a crossover point between the performances of the two compounds, so that one of the two performs better above such length, and conversely the other below such length. The practical application of this theory is then illustrated via a benchmark between two Ethylene-Propylene Rubber (EPR) compounds, based on lightning impulse strength tests relevant to cable models. Compound performances are investigated for different full-size cable voltage ratings, i.e. 20 kV and 150 kV. The application shows that the above-mentioned crossing length does exist, but it depends strongly on cable size and voltage rating, so that in practice the choice of the best compound requires a comparison between the value of crossing length and the typical installation lengths of real cables.