New Zealand Engineering 1999 March

Energy - Tararua Wind Farm

The wind turbines
The wind farm
Performance - comparison with other wind farms
The New Zealand electricity industry and the climate for generation investment

In April of this year, the largest wind farm in the Southern Hemisphere will become fully operational in New Zealand. The Tararua Wind Farm with a generation capacity of 32 MW will take advantage of one of the world’s best wind sites with an annual mean wind speed in excess of 10m/s. The site is located on the Tararua Ranges overlooking Palmerston North.

The wind farm is owned and operated by Tararua Wind Power Limited which is presently a wholly owned subsidiary of CentralPower Limited. CentralPower made a decision in 1992 to investigate the feasibility of wind energy and in 1996 a decision was made to form Tararua Wind Power Limited (TWP), the company which at the time was a joint venture of CentralPower and Merrill International. More recently, CentralPower took 100 percent ownership of Tararua Wind Power.

A resource consent was granted by the Tararua District Council in February 1996 as public support for the project has been strong and few objections were raised at the planning hearing. Potential interference to telecommunication facilities close to the site and adjustments to the flight path into Palmerston North airport were the main issues that had to be addressed.

Later in 1996, a Request for Proposals was sent to six selected wind turbine manufacturers, with responses being received from five. From this, two suppliers, Vestas Wind Systems and Zond, were shortlisted for further negotiation. In December 1997, TWP signed a contract with Vestas for the supply of 48 wind turbines.

As a result of the Electricity Industry Reform Act, CentralPower has had to sell the project. In December of last year, the project was sold to TrustPower and the formal handover will occur once commissioning is complete.

The wind turbines
Vestas V47 wind turbines have been on the market since 1997, and are a variant of the V39 design. They are an upwind, three bladed, variable slip, active pitch design. They have a rated output of 660 kW at 13 m/s wind speed and a cut-out speed of 25 m/s1. The turbines have a hub height of 40 m and rotor diameter of 47 m. Rotational speed is fixed nominally at 28.5 rpm.

The V47 generator is an induction generator which has its slip controlled electronically through the Vestas opti-slip drive. This capability allows the generator to slip up to 10 percent, ie. allowable generator speeds of 1500 to 1650 rpm, and gives similar benefits to that of a variable speed turbine.

The tower design for Tararua is a 40 m lattice tower with a lower section which is tapered and a top section which has a constant cross section. The V47 rotor uses 23 m flexible blades which is a variation on previous Vestas designs. The V47 turbines are designed for a 20 year operational life.

The wind farm
The Tararua Wind Farm is located about 10 km NNE of Palmerston North. The terrain is an undulating ridge having relatively steep upwind and downwind slopes. The site has a very high annual mean wind speed, at above 10 m/s, and the predominant wind directions are northwest and southeast.The Tararua Wind Farm consists of 48 wind turbines located along a 4.3 km ridge line. The turbines are placed in two rows following the crest of the ridge with a spacing of at least 2.5 rotor diameters between adjacent machines.

Wind farm construction began in March 1998 with the commencement of the access roads and all construction was completed in March 1999. The first turbine came online early in November and all 48 turbines are now operational.

The wind farm will be maintained by Energy Connections Limited, CentralPower’s contracting subsidiary, supported by Vestas.

Performance - comparison with other wind farms
(Click here to see table)
The site’s high mean wind speeds will provide excellent performance by this wind farm. The table shows performance statistics from eight wind farm sites around the world. The capacity factor and the specific energy yield (energy per square metre of swept rotor area) are a measure of the turbine’s reliability, efficiency and the site’s wind resource and are independent of turbine ratings. These parameters allow performance at different sites using different turbine sizes to be compared. Comparing mean wind speeds between sites can be misleading if the shape of the distribution and extent of the topographical effects (such as wind speed variation over the site) are unknown.

With a predicted wind farm capacity factor of 50 percent and a specific output of 1,650 kWh/m2/year, the performance comparison shows that the Tararua Wind Farm will significantly out perform other wind farms around the world.