The results of an investigation
A recent (2021) field report investigated the data from 6214 cable failures over a 14-year period (reference below). The report classified the failures into four different types.
57 % - Cable insulation failures
The main cause of failure for an underground cable network is the cable itself. For older cables with oil impregnated paper insulation failure are caused by paper degradation due to moisture, despite their lead-alloy sheath which is waterproof. For XLPE insulated cables the main cause of failure is dielectric breakdown due to the water tree phenomenon.
Cable insulation failures were positively correlated with monthly average temperature, their current loading but not rainfall levels.
23 % - Cable joint failures
Joints are necessary and installing them is time-consuming and must be carefully carried out in strict compliance with manufacturer procedures. The report indicated that “hot” joints are a major reason for the relatively high number of faults from joints.
11 % - Excavation-caused failures
Whenever human behaviour is involved, things get interesting! Failures from external mechanical damage such as during excavations are a serious issue. If an excavator hits a cable, then the insulation will break down due to the mechanical damage. Another less likely cause of failure due to mechanical stress is installing cables and exceeding the minimum bending radius of the cables. What’s not surprising is excavation failures are much less likely to occur when it’s not a workday (weekend) or when it’s lunchtime (12pm).
Very interestingly the report found that excavation-caused failures are 8% higher than expected on non-rainy days and 21% lower than expected on rainy days. It seems that workers are much more careful during times when the weather conditions are poor.
9 % - Secondary substation failures
Faults on connected switchgear resulted in a good proportion of cable faults. Low temperatures and the ambient temperature difference with the dew point as well as high rainfall causes a higher likelihood of switchgear busbar faults which causes failure of connected cables.
Louro, M., Ferreira, A.F.M., “MV underground distribution network failures and correlation to ambient variables.”, IEEE Transactions on Power Delivery, 2021.