Transpower’s live line tower replacement project on the Benmore to Haywards transmission corridor was a massive undertaking that required innovative techniques and procedures never before used in New Zealand.
The project was in association with the HVDC Pole 3 project – the new 700 MW thyristor valve unit that enables 1,000 MW to flow north and south between Benmore in Otago, and the Haywards substation north of Wellington.
Transpower was tasked with reviewing the clearances on the 350 kV transmission line for potential clearance violations, designing options to eliminate these violations and safely undertaking the required remedial works.
The company concluded that 89 towers would need replacing, nine towers would need to be raised and around 50 hardware changes would need to be made, some in the most rugged and remote parts of the South Island. And because the line is critical for the country’s security of supply, the replacement of the towers would need to be done live.
The process began with a series of workshops involving Transpower’s projects team, Electrix, Transfield Services, and Electronet Transmission services. A working group was set up 18 months before any construction to develop a plan that would impose the least risk to the crews and the assets involved. This was necessary given the project involved a large number of machinery movements, heavy lifting, manual handling, potential exposure to extremely high-voltage, working from heights and multiple work sites.
The final method developed saw two guyed steel poles with insulation used to hold the conductor in place while the towers were replaced within the developed envelope. An 80-tonne crane was used to transfer the conductors while still live and to assemble the new tower. The old tower was removed, the new tower erected and the conductors transferred back using the crane. This was carried out without any disruption to the line which remained in service.
The techniques were then practised and developed on a de-energised circuit in Balmoral Forest, which proved the plan was viable.
Practice and further proof of concept followed and a special training area was established at Islington for crews to practise and safely prove their procedures.
In all, three options were developed to allow for the limitations of the different sites, including the presence of distribution lines, irrigation channels and trees.
The one crane and two temporary support poles system was the preferred method. Other methods involved the use of one pole and two cranes, or just three cranes.
All 89 towers were replaced on time, with no live-line incidents and only a couple of minor medical treatment accidents. And the companies involved now have a new set of skills and procedures which Transpower says will be invaluable in future structure replacements.
The Innovation in Electricity award category is sponsored by Gentrack.