Inter-island link can manage jolt

19:24, Jul 22 2013

In an earthquake, it is probably the safest building in the country, able to handle a magnitude-9 shake, an event expected just once every 2500 years.

The valve hall for Transpower's recently completed $600 million Pole 3 inter-island project, at the Haywards substation, was revealed to about 100 industry leaders, customers, staff and key contractors at a ceremony just north of Wellington yesterday.

The Pole 3 inter-island power link is a critical part of the electricity system which allows New Zealand to be one of the highest producers of renewable power in the world, at about 80 per cent.

Transpower chief executive Patrick Strange joked that, if there was another big quake during the ceremony, they were in the best place possible.

"Don't worry about where the Haywards faultline is . . . it is directly beneath us, but we designed for a 2500-year [quake]," he said, alluding to piles drilled 30 metres down into bedrock.

The massive valve hall is built on base-isolators and can move up to 1m in a big quake. The valve equipment inside, used to convert DC electricity back to AC, is suspended from the ceiling so it can sway in a quake without being damaged.


It could handle a quake about as big as the devastating 2011 Japanese quake.

Karlheinz Springer, power transmission chief executive of German-based Siemens, a key contractor for Pole 3, felt Sunday's 6.5 quake while in his ninth-floor hotel room in Wellington. It was his first big quake and brought home why seismic strengthening was so important for the Pole 3 project.

Springer confirmed that Siemens lost money on the contract to build Pole 3 by underestimating how much seismic strengthening would cost. But even so, Pole 3 was "a showcase for all areas around the world which have seismic requirements", he said.

Strange said Pole 3 and Pole 2, built 20 years ago, were like "twin outboards".

"The boat will go like a rocket and be very manoeuvrable with them both running, but it can run on just one," he said.

Pole 3 would make the supply of power to Wellington and elsewhere in the North Island much more stable and secure - for example, when overhead lines clashed during recent big storms. Once a new control system was completed later this year, there was potential for "massive" savings worth tens of millions a year.

Pole 3 enabled Transpower to balance up the power supply in the two islands. For example, if the hydro lakes in the South Island were low, then power could be shipped south.

The inter-island link was critical in putting New Zealand near the top of the world in renewable power generation, with South Island hydropower like "a big storage battery" reducing the need for more gas or coal-fired stations in the North Island.

KEY POINTS Pole 3 had an initial budget of $672 million, but came in about $600m. Work has been under way since 2010 and involved about 3000 staff and contractors over the past 3 1/2 years, in 2.3 million man-hours of work. Pole 3 is the latest version of a high-voltage direct current (HVDC) converter system installed at Benmore in the South Island and Haywards just north of Wellington. Pole 3 replaced Pole 1, aged 47, which was turned off last year after running in a limited way since 2007. HVDC link first built 50 years ago to ship power north from the big hydro schemes in the South Island. Benmore and Haywards stations are joined by 600 kilometres of overhead lines and three big undersea cables across Cook Strait, carrying power at 350,000 volts on a DC line.


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