Comparison of breakdown performances of extruded cables via the enlargement law

In this paper, previous theoretical developments by one of the authors - relevant to the “enlargement law” - are applied for a benchmark between two alternative compounds candidate for the realization of the solid-extruded insulation of power cables. By resorting to the results of dielectric strength tests relevant to cable models, breakdown performances of full-size cables are investigated as a function of cable length. In particular, the focus is on the existence of the so-called crossing length, such that one of the two compounds performs better for cables longer than crossing length, and conversely the other for cables shorter than this. As a practical case-study, the breakdown performances of two cross-Linked Polyethylene (XLPE) compounds are compared on the basis of lightning impulse strength tests relevant to cable models, by analyzing various power cable voltage ratings, i.e. 6 kV, 20 kV, 150 kV and 400 kV. The application shows that, though the crossing length in fact exists, it is a strong function of cable size and voltage rating. As a general consequence, the choice of the best compound should be based on a comparison between the value of crossing length and typical installation lengths of real cables.