Crossing quantities: How to compare electrical strength performances of insulation compounds for power cables

An innovative theory—derived from the ‘‘enlargement law’’—about how to compare the breakdown performances of power cable insulation compounds is illustrated and applied herein. This theory enables the selection between two different compounds candidate for the insulation of power cables via dielectric strength tests performed on cable models. In particular, compound performances are investigated vs. cable length looking for the so-called crossing length, i.e. a crossover point between the performances of the two compounds, such that one of the two performs better above this length, and conversely the other below it. The application of this theory consists in a comparison between two EPR compounds, based on lightning impulse breakdown tests realized on mini-cables, and shows that the crossing length depends strongly on cable size and voltage rating. Therefore, the practical selection of the best compound must consider the typical installation lengths of real cables. Finally, the relationship between crossing quantities and failure probability is analyzed, highlighting that while crossing length does depend on failure probability, crossing strength and breakdown voltage do not. Thus, since crossing strength depends mainly on compounds Weibull parameters, it is the key for comparing the intrinsic breakdown characteristics of competing compounds.