"Compacting" electricity lines

Pylons carrying extra-high voltage lines stand in open countryside and on the edges of cities, and many people see them as eyesores. A common question is whether modern technology could put these overhead lines underground, or at least carry them on slimmer, less intrusive towers. The question went to Matthias Müller-Mienack, head of the Research and Studies department at GridLab in Germany.

“Compared to the classic design of extra-high voltage overhead lines, there are now much more compact overhead lines available with smaller shapes and requiring less width of route,” he said. “A major component is the application of insulated cross-arms, which allow the wires to be directly fitted to the tower. This reduces both the tower height and the oscillations of the wires in the wind.”

Insulated cross-arms on the Stevin line

The insulated cross-arm technology was applied and tested within the European project BEST PATHS. Because no standards or test methods existed yet, the project prepared new test methods to check its long-term electrical performance.

“The erection and operation procedures for insulated cross-arms have been tested for the new Belgian STEVIN line project,” Müller-Mienack continued. The Stevin line, a 47 km, 380 kV link between Zeebrugge and Zomergem in Flanders, carries power from new offshore wind farms to the mainland and supports green energy projects in the region. Its compact design, he said, had three aims: minimal visual effect on the countryside, low electromagnetic fields and noise, and higher transport capacity.

A photograph published with the interview put the two designs side by side: a tower for the compact 380 kV line with composite insulated cross-arms, next to a traditional 150 kV line.

Going underground

For many years researchers have also looked at burying parts of power lines, alongside overhead sections, where public opposition or environmental limits restrict new overhead routes.

According to Müller-Mienack, cables insulated with cross-linked polyethylene (XLPE) suit underground use very well: their physical and electrical properties are excellent and they last a long time. “These kinds of cables can be directly buried in the ground or installed in tunnels. In urban areas especially, this solution might be the only possibility to set up transmission lines,” he explained.

“There is a restriction on the use of land over buried cables: no deeply rooted trees may be planted within the corridor. Apart from that, there are no limitations to farming.” Because the cables are shielded, he added, no electric field can be detected above ground.

In his account an XLPE cable system needs almost no maintenance, and its failure rate, 0.0307 failures per year according to a 2009 Cigré report, is much lower than that of extra-high voltage overhead lines.

The drawbacks of cable

Underground cables of any kind need trenches, and digging them can be complicated by water pipes, oil and gas pipelines, sewers and other services already in the ground. Rock, loose soil and water along the route add further problems.

“There are disadvantages with XLPE cables compared to overhead lines,” Müller-Mienack said. “In onshore applications, for logistical reasons XLPE cables can only be delivered in lengths of up to 1,150 metres per phase conductor, thus special joint bays need to be erected typically every 700 to 1000 metres. Also, civil works required to erect extra-high voltage cable routes may have a considerable impact on the environment. Access tracks and haulage roads to the site are required for the heavy trenching machinery, cable drums, backfill material and equivalent soil. However, to a large extent they are removed after the works have been completed.”

Cost is the other obstacle. For the same transmission capacity, he said, extra-high voltage XLPE cable typically costs 5 to 10 times as much to build as an overhead line, and repairs on cable routes take much longer.

“According to the Cigré report, in the period 2000 to 2005 more than one third of the cable faults in extra-high voltage cable systems were repaired within one week and more than 75% within one month. Although this statistic doesn’t consider outage times under 1 day or over 6 months, it implies that up to 25 % of recorded extra-high voltage cable faults required repair times longer than one month.”

Each option has its price. Overhead lines are cheaper and easier to build, repair and maintain, but they are visible, exposed to vandalism, strong winds and lightning, and produce electromagnetic fields. Underground lines cost more and intrude far less on the view. The researchers’ view was to combine the two, choosing by area.

The cross-arms, composite towers and other repowering work led by Müller-Mienack are described on the Demonstration 4 page; the Budapest workshop of April 2018 presented the test results.