The vital link

At the time of its commissioning, the Cook Strait transmission link was the most ambitious high voltage direct current scheme in the western world. An essential part of that link, Pole 1, was designed to last 30 years and is only finally going to be replaced.  By Alan Titchall.

The original HVDC Pole 1 was completed within budget and on schedule in May 1965, but the project concept goes back to 1950 when the chief engineer of the State Hydro-electric Department, Bill Latta, suggested that the transfer of electricity from the South Island was a promising way of meeting future North Island demand. The idea was debated for over a decade before the project got off the ground.

Forty three years later, the inter-island transmission link was back on the front page after Pole 1 finally showed signs of giving up the ghost and had to be stood down by Transpower which then pushed for its replacement.

But first, any new Pole project would have to get past the Electricity Commission and its infamous Grid Investment Test, and then there was the matter of who pays for it – another decision in the hands of the commission.

The HVDC inter-island link is a key part of our national grid, providing the South Island with access to the north’s gas and coal generation, and the North Island with access to the south’s large hydro generation capacity.

Running from Benmore substation in the South Island to Haywards substation just north of Wellington, the HVDC link is made up of two separate circuits, each with its own major converter system at each substation (Pole 1 and Pole 2). The HVDC link also includes three undersea cables across the Cook Strait, two connected to Pole 2 and one to Pole 1, that are in good condition.

Pole 2 was commissioned in 1992 and has a design life of 30 years. The overload capacity of the Pole 2 plant is 700MW, but its present transmission capacity is restricted to 500 MW because of the submarine cable configuration.

Pole 1 was stood down in late September 2007 after it became so degraded it could not be insured against “catastrophic failure” and potential environment, life and property loss. There was even talk of a possible “massive hydrogen explosion with mercury vapour fallout over populated areas”.

Since then, half of the pole was returned to service for use in critical periods. While Pole 2 has been able to handle additional electricity load since Pole 1 was stood down, anyone with an interest in generation agrees that the link needs a new Pole urgently.

Transpower submitted an investment proposal to the Electricity Commission earlier in 2008 to replace Pole 1 with a new one that would feature a state-of-the-art thyristor valve unit and increase the capacity of the overall HVDC link to 1000MW from 2012, and 1200MW from 2014.

“The HVDC link is an important link for New Zealand, as it balances the energy use between islands, which is more sustainable for the country. It also helps to facilitate renewable generation.” says Transpower chief, Patrick Strange

Transpower says analysis shows the benefits of the project significantly outweigh the costs and its watchdog – the Electricity Commission – agreed, granting preliminary approval for the replacement in September 2008.

Strange says that since submitting the plan in May, Transpower has been busy with the planning and procurement for the project, on the assumption that it would get final approval. “We are on track to commission the new pole in 2012,” he says.

Bun fight over costs

South Island generators were angered by an earlier decision by the Electricity Commission to place the cost of the HVDC upgrade on their shoulders, because it allows them to profit from selling electricity to the North Island.

Meridian and other South Island generators such as Contact Energy and TrustPower said the commission’s  reasoning was nonsense, and the cost of the link upgrade should also be shared by the North Island generators.

Meridian spokesman Alan Seay said that in recent winters electricity flow had reversed – regularly flowing from north to south across the link, benefiting northern generators. Meridian formally requested a public conference on the link upgrade and this was held by the Electricity Commission in Wellington during October 2008.

The commission then confirmed its decision to approve the revamp Pole 1 of the HVDC link immediately after.

It appears all parties left the conference happy campers and were in agreement over the urgency of the Pole replacement project.

Typically, Meridian chief Tim Lusk, who took over from Keith Turner in March 2008, stood up at the Pole 1 Conference and acknowledged the country needs a new Pole 1 and needs it now. Lusk, who was involved in the building of the Poles earlier in his career, admitted that the original projects had proved their worth within a very short time. Meridian’s opposition had always been only been about who pays for the upgrade, he said.

Transpower had initially asked for $728 million for the project but that was reduced to $672 million following further analysis and discussion with the commission.

Commission chief David Caygill says the existing transmission pricing methodology stands until it is reviewed and amended while any review process would likely take some time.

“As matters stand the South Island generators will be liable for the increased costs of the HVDC link once it is upgraded (presumably from 2012).” Meantime, Transpower has warned not to expect the Pole upgrade to solve all our grid probems.

“The upgrade will be underway in 2012 but the grid is tight in other places, and once pole 1 is replaced we will still have constraints to the north on the AC route, so it won’t move all the constraints,” says Transpower chief Patrick Strange.

HVDC technology

HVDC technology (High Voltage Direct Current) is used to transmit electricity over long distances by overhead transmission lines or submarine cables, and are used for transmission of power over long or very long distances (such as the inter-island link between Benmore substation in the South Island to Haywoods just north of Wellington).

Of the 70,000MW of HVDC transmission capacity currently installed over the world, more than half has been supplied by ABB, a company that pioneered the first HVDC line technology on the Swedish island of Gotland in 1954. Since then, the company’s HVDC achievements include installing the most impressive HVDC transmission in the world at Itaipu in Brazil, with a total rated power of 6300MW (and a world record voltage of 600kV DC).

Having operated in New Zealand for over 70 years, ABB was involved in the original 250kV, 600MW HVDC Inter-island Link between the South and North Island commissioned in 1965. ABB was also involved in the upgrade of the inter-island HVDC link in 1992 (Pole 2), and is one of three short-listed suppliers involved in the new Pole 1 Replacement Project, due to be completed in 2012.