Harvesting the wind

The answer to our future energy supply is blowing in the wind, according to the Government’s renewable energy strategy, and there’s no finer example of wind generation in the Southern Hemisphere than Taraura Wind Farm on the eastern boundary of Palmerston North.  By Alan Titchall.

Whatever your prejudice concerning wind turbines, you can’t help but be impressed by the distant sight of hundreds of these graceful monsters peppering the horizon as you drive east from Palmerston North city along SH3 towards Ashhurst and the Manawatu Gorge. The turbines on the northern side of the gorge belong to the Te Apiti farm owned by Meridian Energy, while those on the southern side make up Trustpower’s larger wind farm.

Trustpower commissioned the third and final stage of its Tararua Wind Farm during the spring and organised a site inspection for the magazine soon after. “Bring warm clothes,” they warned, “we predict strong winds and wet weather.”

Prevailing westerly winds sweep in from the Tasman Sea across the rich, green Manawatu plains and whistle their way to the Pacific Ocean through the ridges separating the Taurua and Ruahine Ranges. It is this relentless wind, not too strong and not to weak, that makes the Tararua and Te Apiti wind farms so efficient. They are deemed the most productive wind farms in the world, generating electricity over the past five years of operation for 45 percent of the time – compared to a world average of around 30 percent.

With 134 turbines scattered along a 5.3 kilometre ridge, the Tararua Wind Farm is the largest in the Southern Hemisphere with a potential generation of 200MW, and the hardest working in the world. Neighbouring Te Apiti, by comparison, can pump out 90MW from 55 turbines. Power from the farm is feed into the local grid (split evenly between Linton and Bunnythorpe substations) and the national grid.

Consent was granted in the beginning of 1996 for 103 turbines. The first stage, commissioned in June 1999 (and built by the first owner Central Power), involved 48 V47-660kW wind turbines perched on steel latticework pylons at a cost of $50 million. A second stage, commissioned in May 2004, added another 55 V47 turbines, while the third stage with 31 tubular V90 3MW turbines (the biggest in the country) was commissioned in September 2007. Both turbine designs, and those of the neighbouring Meridian farm, are designed and operated by the Danish company Vestas.

A fortuitous lull in the weather provided Energy NZ with a fine welcome to the Tararua Wind Farm, and the scale and size of the turbines from the maintenance depot on the ridge top is awesome. Standing under a V47, the sound is a gentle and constant undulating swoosh as the 1.5 tonne, 23-metre blades are maintained at 29 revolutions a minute regardless of the wind speed. The whopping seven tonne, 43-metre V90 blades turn at a more leisurely 14 revs per minute. Both blades need a stiff breeze of over five metres per second to start generating electricity, and the annual mean wind speed at hub height on the farm is a productive 20 metres per second.

To my surprise, the turbines are all facing into an easterly breeze. Brian Towler, Vestas’ site supervisor, takes a quick glance out of his office window. “Yes, that is unusual,” he admits. “And you can see a lone V90 on a lower section that has pitched back towards the direction of Palmerston North to catch the gusts.”

Towler or ‘BT’, as his staff call him, points to the nearest V47 from his office and the weather vane and anemometer on the back of the turbine nacelle (the container-size box housing the gearbox and generator casing). There’s another anemometer half way down each tower where the tip of the blades reach. “They keep the turbines faced into the wind and detect wind speeds and wind sheer from those two points. They trigger shut-off sensors if the wind get over 25 metres per second, or 95kph, for more than 10 minutes.”

The direction of the turbines and the pitch of the blades can also be controlled remotely. “Even from home from my laptop,” he says.

The Vista switchgear located in the bottom of each of the V90 turbines were provided by Electropar. The V90 Vestas turbines generate at 1kV and immediately step up to 33kV. The turbines are ‘daisy-chained’ together at 33kV using multiple cable terminations at the switchgear bushings. Cable size for switchgear connection depends on the position of the turbine/switchgear within the daisy chain, so no ‘collector’ switchgear (collecting the output from, say, six to 10 turbines) is required. The wind farm generation is exported at 33kV to a local substation and exported at 220kV to the grid.

There’s no Trustpower personnel permanently based at the depot and the 14 Vistas staff and a number of sub contractors are mostly employed on maintenance, and there’s a lot of it.

Each turbine receives a six-hour clean up and a major overhaul every year. Staff safety is, understandably in these elements, a priority. Two staff work on the turbine nacelle at the same time. Access is via a one-man lift in the towers of the 65-metre high V90, and 127 outside rungs on the 40-metre V47.

“And it’s not much fun on a windy day,” says BT. No work in done in the hub if the wind gets over 15 metres per second, he adds.

Maintenance scheduling is a constant battle with the elements with two dangers peculiar to the job – lightning strike and hyperthermia. The generators produce quite a bit of heat inside the nacelles that is some comfort to the workers during the winter.

Staff keep constant vigil on an internet-based lightning detection service based in Burwood. Even though they are heavily earthed, a lightning strike this year blew out the end of a blade. On top of these threats are the cramped working conditions and dizzy heights. Safety harnesses are checked over every month and sent away for auditing every six months.

Worth over $1 million each, much work goes into keeping the nacelles clean. “Two hours of the six hour maintenance schedule of a V47 is taken up cleaning out after possums,” BT says with disgust. And dried possum poop is toxic. Rentokil gets plenty of work out of the farm.

An advantage of a wind farm is being able to use the land for other productive purposes. Tararua’s turbines sit on farmland leased from a number of owners. The sheep enjoy a benign relationship with the turbines, huddling around their warm transformers against the worst of the hill country weather. Not so the local Magpie population.  

What sort of carnage do the blades cause in terms of avian mortality, or dead birds in plainer language, I ask?  BT is hesitant, but claims bird strike is very low. The wind farm is now obligated to keep a logbook of strikes. I don’t get to see it, but a worker turns up at his office door not long after holding a mangled Magpie by its legs and enquiring about the new tally book. 

“Don’t bring it in here,” BT growls, eyeing my camera bag.

There are other sizeable wind farms around the country, but none so close to a major city or so publicly accessible, which has nurtured the country’s first wind farm tourism with regular bus tours. The views, on a good day, extend from Foxton beach to Mt Ruapehu.

Local opinion towards the wind farm, in general, has become more positive, says BT, and a fascination with local schools. Both Trustpower and Meridian pump plenty of PR money into community projects, and fashionable ‘green power’ messages.

“I wear a very green hat,” says BT, making sure I get the message that the farm practices what it preaches. “We recycle everything.” Including the oil from the gearboxes and filters, nut and bolts, and even the little batteries playing a minor role in the turbine computers.

There are bigger wind farm plans on the horizon, but until they get through their consents, they are only proposals and Tararua holds on to its mantle as the country’s biggest Tararua would have been even larger, says BT, if the third stage built closest to the Manawatu Gorge and near a flight path had been given the full consent of 40 V90 turbines. Opposition whittled that stage down to 31 – close to making it uneconomic – and the rest of the farm can’t accommodate any more turbines.

However, the V47s always have the potential to be re-powered in the future, says BT, looking into what must be the most benevolent wind in the country.