INVESTIGATING RISK FACTORS OF UNDERGROUND POWER CABLES IN ICELAND

In designing, installing and managing an underground electric power cable system in Iceland, efficient thermal dissipation in the soil surrounding the cable is critical in preventing thermal breakdown due to high temperatures at cable jackets. Two underground power cables extend from the Nesjavellir geothermal plant to Reykjavik. Previous work has shown that the thermal conductivity of soil is dependent on soil water content. There are various environmental factors that may affect soil water content, causing the soil to dry up. This study identified the various factors that should be considered when dealing with underground power cables, particularly with regard to Icelandic environmental conditions.

Data on temperature and moisture around cables collected in the field at several sites, as well as a compilation of published meteorological information, and power production at Nesjavellir over a 90 day period, were used to determine relationships among these factors. A mineral analysis was performed on soil samples from sites, and the backfill material. Maps of the cable route, weighing different environmental factors that could possibly affect the soil water content were evaluated. These maps show the risk areas for cable failure to coincide with drier areas. There is a strong correlation between thermal conductivity and soil water content, supporting previous research. The backfill material surrounding the cable needs to have certain physical properties that allow for consistent, unrestricted thermal dissipation from the cable jacket. Due to environmental conditions in Iceland, the risk of cable failure is minimal, with the exception of extended droughts in the summer, due to decreased soil moisture causing decreased thermal conductivity in the surrounding soil. The risk maps indicate that the area between the Nesjavellir power plant and Reykjavik is of relatively low risk. These results support the hypothesis that the soil surrounding an underground cable needs to maintain high enough water-holding properties for the thermal conductivity not to be affected by decreased soil moisture, thereby allowing for unrestricted thermal dissipation from the cable jacket.