The crews that work the high towers carrying the country’s high voltage transmission lines are a special breed; well-trained, very skilled and hugely respected. Lawrence Schaffler talks to one of those charged with the task.
THE COUNTRY’S 534 KILOMETRE high voltage direct current (HVDC) transmission line between Benmore and Fighting Bay in the Marlborough Sounds has kept both islands connected in power since 1965.
As part of Transpower’s new HVDC converter stations at Benmore and Hayward’s (Pole3), about 100 of the 1500 steel lattice towers along the high voltage line had to be replaced, or modified.
And this had to be achieved while the lines (most of them running at 350kV) were still alive, as the powerful current on this part of the national grid cannot be disrupted for long periods.
This task is put in the hands of a handful of specialist live-line crews that include those from Electronet Transmission combined with technical support from Transfield Services.
John Claridge, technical manager at Transfield, says the job is not ‘inherently’ dangerous but it does require a strict, regimented methodology – and precise coordination.
“The crews erect temporary structures some eight metres on either side of each tower. Using cranes insulated from the live conductors, the conductors are transferred from the original tower to the temporary structures.
“With the conductors now much further apart, the old tower is disassembled in the safer environment, and its new, higher replacement erected.
“Reversing the transfer process, the conductors are reattached to the new towers.”
Depending on the terrain, each tower swap takes about five days.
All in a day’s work
While the HVDC project is unusually demanding, Claridge says it’s simply an extension of a much more common practice; the regular maintenance of live, high-voltage lines (66, 110 and 220kV).
“All are vital links in the country’s electrical network – they have to be maintained while they’re alive – there isn’t any other option.”
And in many cases that means the crews climb on to the lines, wearing high-tech Faraday suits that are raised to the same potential as the lines on which they’re sitting – as high as 350,000 volts.
Routine live-line maintenance, he adds, includes pole structure replacement, cleaning and/or replacing insulators, fitting line spacers, replacing vibration damping equipment or applying shunts across poor joints in the conductors.
“Poor joints increase resistance and heat and weaken the joint. A shunt across the joint is a temporary fix, giving the line operator a breather until the affected circuit can be removed from service and the joint repaired.”
Most of the damage is normal wear and tear, but lightning strikes and a build-up of salt on insulators (particularly where towers are close to the sea) are common problems.
“All are vital links in the country’s electrical network – they have to be maintained while they’re alive – there isn’t any other option.”
Salt build-up causes corona discharges and can lead to a short, and a possible tripping of a circuit.
Depending on the nature of the maintenance, as well as the line’s voltage, one of two live-line techniques are used: “hot-stick” and “bare-hand”.
Live-line techniques
With hot stick, direct contact by the line mechanic with the conductor is avoided. The crews stay at “earth potential” (i.e. they work from the tower, grounded to the earth) and using insulated sticks with specialised fittings work on the conductors from a distance.
Prior to each live-line job, the sticks are marked at the minimum approach distance (MAD) to ensure that crews don’t get within ‘flashover’ distance of the conductor. Hot stick line mechanics are only required to wear their standard Personal Protective Equipment (PPE).
To replace a string of insulators, the crews use a complicated configuration of strain poles and support yokes to lift and take the weight of the conductor. They then disconnect the insulator string from the conductor (still using sticks), before lowering the old string to the ground then raising and installing a new set.
With the bare-hand technique, the line mechanic (hot end worker) wears a Faraday conductive suit and is placed directly on to the conductors. This approach is used when the area requiring maintenance is either too far from the tower for using the sticks or there is a greater MAD. Claridge says sitting on a line isn’t a problem in itself (birds do it all the time).
“The tricky part is getting the line mechanic on to the conductor. He typically scoots out over long, insulated ladders suspended between the tower and the conductor. The ladder’s length ensures that he can maintain the required MAD at all time while accessing and returning from the conductor.
“Alternatively, the mechanic can be raised from the ground using an insulated (live-line) rope and a winch or he is suspended below a chopper on a long, insulated rope and placed on to the conductor.”
Because there is an “energy potential” difference between the energised conductor and the mechanic, a small electrical arc occurs as they reach out to grab the conductor. To facilitate this initial contact, they reach out with a “wand”. The arc occurs and disappears with contact, leaving them to climb on to the line.
Claridge says the electricity flows around the Faraday suit (complete with hood, gloves and conductive socks) rather than through the body. These conductive suits are manufactured from a blend of Nomex flame resistant fibre that includes a 25 percent stainless steel weave. They are rated for voltages up to 765kV.
Which begs the question; who volunteers for this kind of work?
“They are a special breed,” Claridge concedes.
“They are well-trained, very skilled and hugely respected in the industry.”
Claridge himself is a 20 year-plus veteran of live-line work and still gets a “buzz” from conducting live-line training both within New Zealand and offshore, and especially enjoys working with the crews in the field.
All live-line training is provided by Transpower Grid Skills at its Blenheim training facility.
