Defect investigation in medium-voltage EPR cable

Given the increasing asset age of the medium voltage distribution network, more failures are being observed. The ability to understand the source or mechanism of a particular defect plays an important role in facilitating a more effective diagnostic replacement strategy. This paper investigates the cause of failure in a medium-voltage EPR-insulated underground cable. The cable was taken out of service after breakdown occurred. After a 40 m section was removed for more accurate lab-based analysis, the fault location was pinpointed to within a few centimeters and confirmed as a shunt resistance fault between the core and sheath. This investigation is concerned with a study of observed particle contamination in the EPR layer as it relates to the observed breakdown in the cable. Geometric details of the surrounding dielectric have been acquired using 3-dimensional X-ray computed tomography techniques. This has revealed details of numerous high-density particles contaminating the dielectric medium. The average particle diameter was around 100 µm. To investigate the effect on cable breakdown, the electric field distribution in the region between the core and sheath has been modeled in the presence of contaminating particles using finite element techniques. The field was also modeled in the presence of an observed eccentricity of the core. Electric field variation due to core eccentricity was less severe than that due to particle contamination but slightly increases electric field intensity. It is hypothesized that the particle contamination resulted in a localised electric field sufficient to initiate partial discharge, leading to localised degradation and eventual breakdown.