Powerful ground

June 9, 2015

Sweden’s SydVästlänken will contain the world’s longest underground cable for high-voltage power transmission.

SydVästlänken’s power cables are being placed underground for a stretch of 270 kilometers.

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.”

Optimized performance

Equipping the Sydvästlänken project was an enormous task due to the magnitude of the project, which runs 430 kilometers through southern Sweden. “Projects that require so many generators and pumps at once are uncommon,” says Mirza Palislamovic of Skanska’s purchasing department, adding that delivering everything to Svenska Kraftnät on short notice was a challenge.

“Atlas Copco did a good job of delivering quickly,” she says. “They have production in Europe and had much of the equipment in stock. Selecting Atlas Copco equipment is always a safe card as it is extremely durable. Their generators and pumps are top-of-the-line and better than most others.”

Atlas Copco’s QAS 14 and QAS 20 on-site generators and WEDA 30 and WEDA 40 submersible dewatering pumps have been running intensively seven days a week. Gunnar Benselfelt, Business Development Manager, Atlas Copco Portable Energy, says there were many special considerations to take into account. “In this project, water is moved rather than lifted, so we offered a three-phase pump with the right curve for pumping lots of water without a high lift.”

Atlas Copco helped Skanska calculate a lower start torque and current to achieve optimum efficiency, and enable more pumps to run on a single generator. “Skanska is able to run three of our pumps on one of our generators,” says Benselfelt. “Normally, they might be able to use only two pumps per generator.”

To add to the efficiency, Atlas Copco offers a level control on generators so that they only run when a pump is pumping.

Growing the Link

Project Engineer Adam Ericsson says the team has become better at placing the pumps well for optimum effectiveness.

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