Underground cable research

Recent years have witnessed considerable discussion about the increased use of underground cables. Although Landsnet’s oldest underground cables date from the 1950s, in Iceland and overseas the use of underground cables was in previous years largely limited to low-voltage distribution in urban areas. Thanks to great technological strides and increasing cost-effectiveness, the use of underground cables has been on the rise. Most of Landsnet’s infrastructure projects in the last few years have been underground cable projects. This trend is expected to continue in the coming years.

To obtain a clearer picture of the costs involved, the environmental impacts and the use of underground cables at higher voltages, we organised an extensive research project in co-operation with the Danish TSO, Energinet.dk, the Danish consultancy firm StellaCable, Reykjavik University and Icelandic engineering firms.

The project report was presented in February 2015. Among the most salient findings was that the maximum length of an underground cable across the Sprengisandur plateau was 50 km. This is owing to technical limits due to the grid’s variable strength in different regions of the country. The factors affecting the possible length of underground cables in the system include a low short-circuit power, or the strength of the system. The research also confirms that the price of underground cables with a high transmission capacity has come down considerably, or by almost half for 220 kV cables, and that he purchase price of an underground cable accounts for 30-50% of the total capital cost of a 220 kV underground cable installation. It should be borne in mind, however, that providing a general reference price is challenging. Each case needs to be considered individually on the basis of available facts.

The report also confirms that soil thermal conductivity in Iceland is generally lower than in our neighbouring countries, conventional cable laying in an open trench is the least costly option in the vast majority of cases and minimising environmental impact through effective underground cable routeing is important. Restoring the ground surface is easy in sandy terrain, well vegetated heathland or cultivated land, whereas in forests and scrubland a treeless corridor must be left above the cable. Furthermore, there are good reasons to avoid undergrounding in lava fields as far as possible, as the ground surface cannot be restored to its original condition, in addition to which Holocene lava fields are protected under the Nature Conservation Act.