Approaches to the forensic failure investigation of medium voltage polymeric cables

Medium voltage (MV) polymeric cables are critical assets in modern power systems, since their introduction, approximately five decades ago. More recently, there has been a growing emphasis on the maintenance testing of polymeric cables, with some of them being prone to premature failures. This paper covers the various techniques available for the forensic failure investigation of polymeric cables through practical examples. Some of these include diagnostic electrical tests such as Insulation Resistance, AC Hipot & PD, and nonelectrical procedures such as visual inspection, optical microscopy, and differential scanning calorimetry (DSC). The recommended order of performing these techniques is also presented, based on the suspected failure mode. This serves as a practical guideline for utilities, towards identifying the type of cable failure sustained in the field. Diagnostic electrical testing (post-failure) helps establish the most probable cause of failure associated with polymeric cables and/or their accessories. A very useful, but less explored technique in polymer morphology is the DSC, which can be used to reveal the thermal history of a polymeric cable since its processing. In this regard, a detailed case study is presented to highlight the use of a DSC thermogram for determining the maximum temperature experienced by a polymeric cable and the potential source of heat.