June 9, 2015
Sweden’s SydVästlänken will contain the world’s longest underground cable for high-voltage power transmission.
Demand for electricity and renewable energy is growing in many parts of the world, and electrical grids are expanding to keep up with the need for robust systems. One example is SydVästlänken (South West Link), designed to expand the Swedish power grid. When completed, the high-voltage power cable will stretch a world-record-breaking 270 kilometers underground, surpassing the current record holder, the Murraylink cable in Australia.
Svenska Kraftnät (Swedish National Grid), a state-owned public utility company, decided to bury most of the transmission cables in the ground to eliminate the risk of electrical outages caused by falling power lines. There is also much less maintenance when cables are tucked safely underground, and local residents generally favor this more aesthetic approach.
However, it is a costlier alternative to traditional overhead power lines. Much of SydVästlänken runs along the existing infrastructure of the E4 highway. Alongside this main transportation artery, workmen are digging trenches that are 1.5–2 meters deep. Almost one million cubic meters of rubble will be dug out by 2015, the completion date of this three-year project. One of the main companies on the project is Skanska, a global project development and construction group. At the project’s peak, about 90 Skanska employees and subcontractors worked 10-hour days to meet the tight deadlines. “We completed three kilometers every week,” says Magnus Johansson, Skanska’s SydVästlanken Project Manager.
Johansson says that time pressures and the enormous physical size of Sydvästlänken were big challenges. “Moving all of the personnel, equipment and transport from one stretch of the project to the next is a major operation.” Other challenges included getting clearances to dig on the properties of 250 different landowners, working in remote areas with no electricity and keeping the trenches as dry as possible. Atlas Copco generators and submersible dewatering pumps have helped solve the last two issues.
“ When you are above groundwater level, it’s just rainwater that you have to take care of,” says Johansson. “But below groundwater level you need more pumps. Sometimes you need a lot of pumps every 30–40 meters, while in other places it can be drier and you don’t need any pumps at all. The soil conditions can vary every second meter. ”
When it rains, overflow from the E4 highway runs into the trenches. “Then we’re dealing with enormous quantities of water,” says Johansson.
Project Engineer Adam Ericsson says the team has become better at placing the pumps well for optimum effectiveness.
Project Engineer Adam Ericsson says that the project team has gotten better at managing the water. “We have learned and taken the time to really place the pumps well for optimum effectiveness, and this has resulted in operational improvements.”
The base of each cable trench must be kept completely dry to protect the 14 different transmission cables that will eventually rest here. A special gravel backfill is meticulously laid, following exact specifications for the best compaction, drainage and cohesiveness. Only then can the transmission cables, tucked into sturdy tubes, be laid in the trenches, which are then refilled.
During the period of 4–8 weeks when the trenches are open, some 60 Atlas Copco generators power about 250 pumps. “We are putting extremely high demands on the pumps and generators, which are running around the clock,” says Ericsson. “It’s a tough environment outside, and the pumps and generators are running much more intensively than normal. The equipment has been under high pressure daily, but it is keeping up with the demands.”