Fault development upon water ingress in damaged low voltage underground power cables with polymer insulation

Developments in the electricity sector, related to integration of sustainable energy, not only affects the medium voltage but also the low voltage infrastructure. Condition monitoring of Low Voltage (LV) underground power cables becomes of increasing interest to safeguard reliable power delivery. Since the start of phasing out oil-paper insulated cables, polymeric insulation is most widely applied. As phenomena associated with high electric field stresses are not expected, little is known about the degradation mechanism of LV cables with polymeric insulation. External damage is known to be a major cause of defects, which directly causes a failure or initiates degradation resulting in a failure later on. To study the degradation mechanisms, short cable sections with artificial damage have been subjected to periodic water exposure in the laboratory. This shows that dry-band arcing is a major mode of degradation, with accumulation of pollutants due to chemical decomposition. Several factors such as water conductivity, presence of oxygen, soil type and insulation material have been investigated. Strong dependencies upon these parameters are observed. However, it may be hard to quantify the degradation process in practical situations. The presence of a water film that can sustain dry-band arcing will occur only in certain situations and for limited amount of time. However, the accumulation of these events, mainly caused by periods of precipitation, will eventually lead to breakdown at the damaged spot.