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How the Northwest Transmission Line was built (and why)

Helicopters place towers on the Northwest Transmission Line

An inside view of the construction of a 344-kilometre transmission line

Posted by Rob Klovance
bchydro.com

Back in November, motorists along a portion of Highway 16 west of Terrace witnessed a spectacular sight — a helicopter stringing power lines across the Skeena River to connect high towers installed earlier in the year.

Those towers were just two of the close to 1,100 that were installed on BC Hydro's Northwest Transmission Line — add up all the steel used in those structures and it's equivalent to 1.5 Eiffel Towers. But towers — which can take three days to assemble and as little as 11 minutes to be picked up by helicopter, lowered into place and then installed — are just part of the Northwest Transmission Line story.

Lost on many British Columbians is the enormity of the task of building a 287-kilovolt line that stretches about 344 kilometres north from near Terrace to a new substation built near Bob Quinn Lake.

"The construction of the Northwest Transmission Line has been an absolutely phenomenal engineering exercise, in extremely rugged terrain," says Lesley Wood, BC Hydro's stakeholder relations and communications lead on a project that will cost between $736 and $746 million. "There's a lot of new right of way that had to be cleared through very difficult-to-access areas. And we had to do an awful lot of work by helicopter in order to install the structures and the lines"

Below, we'll use three videos to show how transmission towers were placed and power lines were strung on NTL, which is just about to go into service.

But first, a quick look at why this transmission line was built.

It's about powering economic development in B.C.'s northwest

Wood has worked on scores of capital projects for BC Hydro, but the Northwest Transmission Line is the one she's most proud and passionate about.

"When I first started to go to Terrace in 2006, 2007, you could shoot a cannon through the main street," says Wood. "It was so dead. Now you go up to Terrace and it's bustling. There are trucks on the road, there are people on the street, new stores opening and new hotels being built."

The Mining Association of B.C. expects that with the transmission line in place, 10 new mines will be developed and seven independent power projects will be able to deliver clean energy back to BC Hydro's electrical grid. The association estimates these projects could generate up to 5,500 direct jobs and over 18,000 indirect jobs over the next several decades.

"Between the NTL, the Forrest Kerr power project, Red Chris Mine and the Kitimat Modernization Project, there's $5 billion worth of work going on in this area," says NTL construction manager Tony Mullin. "I feel like I've helped pioneer the northwest corner of British Columbia!"

How the Northwest Transmission Line was built

After several years of planning, consultation and an environmental assessment, the project got rolling in 2012 with right-of-way clearing and construction access road work. In early June, the last of the transmission towers was put in place, and the last of the power line — known as conductor — strung into place.

In a series of three short videos, we'll show how some of that work was done, starting with a look at how a tower is airlifted by helicopter and installed.

The NTL's towers, most of them Y-shaped lattice towers about 27 metres high and weighing 9.5 tonnes, were assembled at what are known as "fly yards" along the power line route. Assembling the towers is like working with a giant Meccano set — involving as many as 800 parts and the insertion of thousands of bolts. That's why it takes two or three days for a crew to assemble a tower.

The video below shows a helicopter carrying a tower from a fly yard to the site, where it's lowered and held upright by the helicopter while crews — the wind from the helicopter blowing dust and debris around them — work quickly to guide the tower onto its pre-constructed foundation and connect the four guy lines to anchors around the tower that hold it in place.

Conductor (cable) stringing — another role for helicopters

Stringing of cable — running power lines from one tower to the next — is done in segment "pulls" that can range from a single tower-to-tower section to cable strung through a series of towers over a six-kilometre pull.

The helicopter's role is to pull a heavy rope through pulleys, known as travellers, from one tower to pulleys at the next. The rope at the trailing end is then attached to the actual steel-and-aluminum cable, known as "conductor", that spools out from a huge cable reel (or "drum") that can hold up to three kilometres of conductor. The leading end of the rope is attached to a winch that pulls the cable out of the reel and through the pulleys on the tower.

Cable isn't pulled tight. It's pulled to a carefully calculated "sag" — the distance between the ground and the lowest point between two towers. The sag calculation takes into account the air temperature on the day of the stringing, and factors — the amount of energy that will go through the line, high summer temperatures or ice buildup on the lines — that will cause a greater sag in extreme conditions over the years.

The video below shows a helicopter pulling the heavy rope through travellers on one tower, to the next tower.

Bang! Connecting conductors with an implosion

Perhaps the most surprising step in the stringing process is shown in the video below.

Implosive jointing is a method — used throughout the Northwest Transmission Line — to join two ends of a conductor. To splice the two ends together, an explosive sleeve is placed over the ends of two separate lengths of conductor, and the charge is detonated. The implosion compresses the sleeve onto each end of the conductor and forms a permanent connection between them.

On NTL, the majority of these implosion joints spliced one conductor end to another mid-span between towers. But dead-end splices, which use implosions to splice conductor at a tower, were used extensively where towers were placed on an angle to allow for a significant turn in the transmission line.

The video below shows six simultaneous detonations to complete a dead-end splice.