North-South power link gets $672m upgrade
BY SIMON EDWARDS
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Hutt News
For the thousands of motorists who whiz past Haywards substation on SH58 it's probably just another electricity facility that barely gets a second thought.
But as reporters at a media briefing learned last week, it's a vital link in the nation's network, and one undergoing an upgrade estimated to cost around $670 million.
Haywards is usually only home to a skeleton systems maintenance crew of five, though Transfield also uses it as a base for its Wellington field workforce.
But in the last few months around 80 civil engineering staff have been swarming over the site creating retaining walls and foundations for new buildings.
Worker numbers will peak at around 160 when electrical engineering specialists get busy later this year.
Since 1965 electricity has flowed between the South and North Islands over the high voltage direct current (HVDC) link. Running 571km of transmission line, plus 40km of submarine cables along the top of the Cook Strait seabed, the HVDC terminates at Benmore substation in the south, and Haywards in the north.
For the vast majority of the time, electricity is flowing northwards as hungry heaters and lights in the bigger population centres in the North Island suck up a share of the output of the big southern hydro power stations.
But during periods of low rainfall in the South Island, or when generators want to preserve water storage, electricity flows along the HVDC from the North to South Islands.
As Transpower's project director for the Hayward's upgrade, Peter Griffiths, explained to journalists last week, the HVDC is a vital cog in the nation's ability to move around electricity to where and when it's most needed.
While upgrading the HVDC is a huge cost, the extra generation capacity we'd have to build without it, using borrowed money, would be much more expensive.
What's more, the network flexibility afforded by the HVDC improves the economics of the new generation plants including wind farms on our horizon.
DC (direct current) is most efficient for transporting power over long distances, so at Benmore and Haywards systems are needed to convert generated AC (alternating current) electricity to DC and back again.
Haywards substation doubles as the Wellington region's major distribution point/switchyard, and lots of extra equipment is needed to "chop" smooth DC current back into lower AC voltages that the local grid can handle.
The big converter systems at Haywards are called Pole 1 and Pole 2. Pole 1 is now more than 45 years old, and is the last substation in the world still using mercury arc valves a safety and reliability risk, and one no longer supported by manufacturers.
Pole 2 uses more modern thyristor technology and that will be the system used in the new Pole 3, now being built to replace Pole 1.
Mr Griffiths likens the advance in mercury arc vs thyristor technology as being like the difference between vacuum valves in old televisions to modern, solid state electricity gear in our flat-screens.
Without Pole 1, the capacity of the HVDC with Pole 2 operating is only 700 Megawatts (MW), insufficient to meet the nation's electricity demands.
But with the new Pole 3, to be completed in two stages, capacity will be boosted to 1200MW by 2014 that's about the same amount of electricity used on a cold winter's evening in the Nelson, Marlborough, West Coast and Canterbury regions combined.
Upgrading work is also needed at Benmore, but at least the site there is flat and roomy.
As Mr Griffiths and Pole 3 site improvement project manager Markus Schoo described, the sloping and cramped Haywards site presents challenges that have required an innovative approach especially given it is a live substation where there can be no disruption to supply.
Foundations for the new Pole 3 building and control room are nearly finished.
Some 84 piles have been sunk, to a depth of 10 to 26 metres.
Now work is underway on the plinths for base isolation lead-rubber bearings, so that the effect of ground shaking in an earthquake on the building housing sensitive equipment is dampened.
Mr Schoo said the older Pole 1 and 2 buildings were built at a time when technology/cost allowed a 1:1000-year earthquake resistance standard.
The new Pole 3 building is at a 1:2500-year standard.
As another earthquake protection, the three state-of-the-art thyristor valves of Pole 3, each weighing 17 tonnes, will be suspended from the ceiling of the 19-metre tall building they'll be housed in, so they can sway in an earthquake without being damaged.
For time-efficiency, the building's roof is being built at the same time and in one continuous piece.
Electrical and fire detection systems are being pre-installed, so workers can do this while it's on the ground, rather than three storeys up.
It's been a boon for Petone steel fabricator MJH Engineering.
The 50m swimming pool-sized roof, weighing 145 tonnes, will be lifted into place by two cranes in a month or two.
"We'll need a calm day," Mr Griffiths says drily.
Another challenge will be getting into place massive transformers weighing 230 tonnes (330 tonnes when filled with oil).
They'll be landed at Wellington Port, but unlike in the 1990s, when two-truck trailer units, each with eight sets of wheels, brought the transformers along the motorway and under the Petone overbridge (the tyres had to be let down), subsequent development means that's no longer possible.
To bypass this pinch point, some time after July this year the massive transformers will be brought from the port by barge, and landed at Petone beach near the Korokoro gateway for loading on truck-trailer units.
The Pole 3 project is due for completion by the end of April next year.
- Hutt News
