Risk Model for Medium Voltage Underground Cables in Sweden

The society of today is largely dependent on electricity and the dependence does not show any signs of declining. Reliability of supply will become increasingly important in the future and it is therefore important to investigate factors affecting the failure rates in electrical components. Sweden was hit by several severe storms during the 00’s. As a result of this many overhead lines in forests were cablified. In urban areas underground cable systems have been the norm for many years. Despite the increase in cablification in forest areas and the large amount of underground cables in urban areas there is relatively little known about the life expectancy of different cables and the external factors affecting the durability of the cables. To increase this knowledge the purpose of this master thesis was to develop a risk model for medium voltage underground cables. The risk model consisted of two parts; a statistical model to determine the probability of failure for different types of cables, and a model to determine the consequences from a power outage on a specific cable section. The result was a risk assessment which can be used when making investment decisions. The result from the statistical model showed that paper insulated-lead covered cables (PILC) and an older type of cross-linked polyethylene cable (XLPE) seemed to increase the probability of failure. The model also indicated that grids in urban areas and grids with many joints and different cable types increased the probability of failure. However, the model needs further improvement to ensure these results. The consequence from a power outage was calculated as a community cost. The cost increases with the length of the outage and the average power on the cable section examined. The highest cost occurs when consumers in the group commerce and services are affected. Based on the results, a strategy when renewing the grid should be focusing on replacing cable types that increase the probability of failure, but also focusing on replacing larger parts of a cable section rather than several small parts. By doing this the number of joints and cable types will be reduced, also decreasing the probability of future failures. To minimize the community cost, focus should be on ensuring the security of supply on cable sections with a high average power and with customers sensitive to power outages (commerce and services, industry and the public sector), but also where there is a large risk for long power outages.