Our stringing crews had a captive audience recently, when thousands of passing motorists were treated to helicopters and linesmen pulling conductor over State Highway One and Lake Karapiro.
The line stringing is the final phase of the project to build a new 400 kV capable transmission line between Whakamaru and South Auckland – known as the North Island Grid Upgrade project. Stringing on the project is now around 60% complete.
For this project, the initial stringing of pilot wires and earthwires is being undertaken by helicopter. Using helicopters also significantly reduces the construction impacts to farms and allows easier access to the lines than ground-based crews.
Once the pilot wires have been flown out, the heavy ground-based machinery rolls in to pull the conductor (wire). This basically involves connecting three conductors to the pilot wires and then using a winch pulling out the pilot wire and replacing it with the conductors. A machine called a “tensioner” is used to ensure enough tension is maintained in the conductor to keep it off the ground.
After the conductor is pulled and “floated” (joining two different sections of conductor), the line is then ready for sagging. Sagging involves making sure that the conductors are installed at the right tension to ensure correct ground clearance and tower loading. When the line is operating, the conductor will expand and contract with varying temperatures.
Once the conductors are sitting in the right place, crews of climbers come through and clamp in and terminate the conductor and earthwires on the towers.
Stringing is expected to be complete well before the intended commissioning of the line. The new overhead line, terminating at a new switching station at Brownhill in South Auckland, will connect with two underground cable circuits (now laid and undergoing testing) to Pakuranga substation.