Electrical aging of extruded dielectric cables: review of existing theories and data

Despite the huge amount of data on so-called electrical aging of extruded HV cables, the fundamental phenomena responsible for it or evolving with aging time, are still far from well understood. It is therefore not surprising why it is so difficult to predict reliable cable lifetimes in service from accelerated aging experiments in the laboratory. The objective of this paper is to review critically the existing theories of electrical aging of solid dielectric materials. A relatively large number of models and theories exist but none of the most often used is known to yield reliable life predictions. One conclusion is that there is a need for a more comprehensive model of electrical aging of extruded dielectric cables. In order to develop this model, an extensive review of existing literature data was undertaken. This paper summarizes the data collected from more than 200 papers on aging of PE, XLPE and EPR cables. It appears that cable breakdown strength should not be plotted on log field vs. log time graphs to yield long-time (i.e. low-field) values, since results obtained over a long time period do not obey an inverse power law. In fact, high-field results are better described by an exponential relation between time and field. The models of Simoni, Montanari and Crine seem to give the best fit to experimental results obtained under a wide variety of experimental conditions. It is also shown that the lower field limit for the exponential regime with XLPE cable is in the 8 to 15 kV/mm range, which corresponds to the onset of strong charge injection. The influence of environment, insulation nature and morphology, and testing temperature are discussed.