By Paul van Lieshout, PB Power, Auckland, New Zealand
The largest wind farm in the southern hemisphere opened last year, culminating seven years of planning, development and implementation. The Tararua Wind Power project is located on the Tararua ranges south of the Manawatu Gorge, approximately 12 km east of Palmerston North, New Zealand a country with some of the highest recorded wind speeds on the planet.
Figure 1. The turbine units were shipped to the site from Denmark
In the early 1990s, New Zealand forecasters predicted major power shortages in future decades, and plans were developed for an expanding generation system. Power companies viewed wind power as an opportunity to generate electrical energy in their own backyard. Wind power can be utilized near many coastal load centres, increasing the efficiency of the overall power system and reducing losses, thereby giving local power companies control of generation projects. The generation projects could grow in stages along with load growth, avoiding the building of large one-off power stations.
Central Power contracted with DesignPower (now PB Power) in 1992 to evaluate the wind resource in their distribution area and recommend potential wind farm sites, which led to the development of the Tararua Wind Power site.
Initially more than ten sites were chosen based on wind speed characteristics (extrapolated from publicly available meteorological sites), topography, proximity to power lines etc. The top three sites were shortlisted for further evaluation. Three anemometer towers were installed on the Hall Block site to collect wind measurements.
Cross-correlation calculations were conducted with several different long-term meteorological stations (Ohakea, Palmerston North and Wellington), confirming an expected high wind resource. The data indicated the site was ideally suited for the development of a large-scale wind farm. Wind measurements continued to be taken throughout the development of the site.
Building a project team
Central Power was invited by Merrill International, based in the USA and closely associated with the wind turbine manufacturer, Kenetech, to discuss the development of the chosen site. Central Power and Merrill established a 50/50 joint venture, the Tararua Wind Power company, to develop the site.
Merrill International and Central Power continued to develop the wind project. However, after Kenetech became insolvent, the joint venture partners produced tender documents suitable for open tender. Merrill’s share in the company was later bought by Central Power, which then owned 100 per cent of the company. Restructuring initiated by the government led to Central Power’s sale of the wind farm. Trust Power took on ownership of Tararua Wind Power after the full commissioning of the site.
During the five-year development phase of the project, PB Power provided many of the initial services which included: site identification and selection, scientific monitoring and recording of each site, economic reviews, environmental analysis and preparation of evidence in support of the application for resource consent.
Figure 2. High wind speeds hindered construction work
A landowner agreement was reached with the owners of the Hall Block land. Wind measurements were made at the site to verify initial wind resource assumptions. The first data verified many of the assumptions, and a decision was made to work towards obtaining a Resource Consent from the Tararua District Council for development of a wind farm on the land.
Aircraft flight and paths, telecommunication and acoustic noise issues were among the many issues addressed in the consent application and hearings. The Tararua Wind Farm site accommodates several large telecommunication towers that incorporate point to point (microwave antennas) and point to multi point (mobile phone, radio) communication equipment. In order to gain the support of telecommunication companies, the proposed wind farm had to be demonstrated to have no effect on their operation.
The developers’ proposed turbine layout was based on maintaining a distance of 200 m from transmitters and receivers, and keeping the first Fresnel Zone clear for point to point services. This satisfied the telecommunications companies, and the Tararua District Council agreed.
The potential for electromagnetic interference (EMI) from the generators and inverter was also questioned. Detailed measurements conducted within existing wind farms in the USA showed the equipment produced little EMI, which would not cause any interference if reasonable spacing from telecommunication equipment was maintained. PB Power altered the layout to incorporate these requirements, resulting in a layout of 137 wind turbines of about 400 kW in size. This layout also formed the basis for discussions with the Civil Aviation Authority and for the calculations of acoustic sound contours.
Residents were concerned about possible noise emitted by wind turbines as the blades rotated through the wind. A detailed noise contour map based on the layout proved sound produced by the wind farm would be inaudible at most residences around the site.
The investigation determined that the Tararua Wind Farm site would require turbines with a maximum design wind speed in excess of 55 m/s. The turbines chosen for this project are manufactured by Vestas in Denmark. The V47 660 kW wind turbine (47 m rotor diameter) and larger generator (660 kW) incorporates a number of design optimizations. A variable slip induction generator technology controls the electrical resistance of the induction generator, which controls the amount of slip and thus the speed of the generator and turbine.
During gust conditions this technology enables energy storage in the blade rotor system, smoothing the turbines’ power output. An electronic control system provides extra compliance in the drive train. This enables Vestas to use various rotor sizes ranging from the V39 to the V44, optimizing the turbines for different wind regimes. The use of more flexible blades reduced forces on the turbine structure. This allows use of the same bed frame with a larger rotor system, which reduces the cost of the unit.
Design and detailing
Tararua Wind Power decided on a single package contract with Vestas, excluding site road and electrical reticulation. PB Power provided many services as a subcontractor to Vestas including: designs of tower foundations and controller enclosure buildings, review of designs, and construction.
In turn, PB Power subcontracted fabrication and erection of the towers to Eastbridge Ltd, Napier, and the construction of the tower foundations and turbine controller enclosures to the Mainzeal Construction Company, Palmer-ston North. The design and construction of the remaining parts of the project were arranged by the owner under additional separate contracts.
The turbine towers are steel lattice structures with a square cross-section tapering over the lower part of the tower and cross bracing members between the four corner leg members. The towers are made up of bolted connections using high-strength, grade 8.8 bolts, except for the top ring, which is welded to the turbine. All members are hot dip galvanised except for the top ring, which is metal arc zinc sprayed to avoid distortion to this welded section. All galvanising and metal are sprayed to a minimum thickness of 150 microns.
The towers were designed based on the loading from extreme winds of 70 m/s. The design was evaluated for the specific location in terms of the New Zealand Loadings Code with some strengthening at the waistline level of the towers necessary to meet code seismic requirements.
Comprehensive quality assurance and control procedures were documented with full tractability on all the welded top ring assemblies. A trial assembly prior to galvanising of a tower lying horizontally was undertaken to confirm fit-up and tolerances.
The tower foundations comprise square reinforced concrete slabs or rafts set approximately 1.5 m below ground level. The gravity load of the concrete rafts and the ground above them act to resist the overturning forces from the towers. The lower sections of the four legs of the towers are embedded and anchored into four reinforced concrete upstands extending from the foundation raft to just above ground level.
Foundations were kept simple as relatively level sites were selected or established during the micrositing of the turbine locations. Pre-design geotechnical data was gathered with a single shallow bore hole with penetrometer tests at regular intervals at each turbine location. Design assumptions were verified and at a number of sites ground improvement was undertaken as a result of further penetrometer probes on the bases of the foundation excavations.
Insulated panel (Portacom) buildings were selected to enclose the turbines because of their ability to withstand the harsh site climate at the site. However, significant modification to the ventilation of the buildings was necessary in order to keep temperatures inside the buildings at an acceptable level on warm windy days. The prime source of heat creating the temperature problem inside the buildings was the lengths of generator cables within the buildings themselves.
Construction commenced on the central group of 16 turbines, then extended out to the northern end of the site, and finished with the last group of 16 at the southern end of the site. This construction sequence allowed the central group of turbines closest to the substation to be commissioned first.
At each turbine site the only interface with other peripheral activities was the work associated with the installation of the turbine transformer and the electrical connections from this to the turbine controller unit. A re-installed cable conduit in the controller building floor allowed this interface to be effected smoothly.
Wherever possible, components of the complete turbine facility were pre-assembled or were delivered to site in packaged form to facilitate on-site erection. The towers were erected on site in two sections. The upper 23 m long 1.6 m2 section of each tower was fully assembled off site, transported to the site in one piece and erected. All the bolts on this section of the tower were tightened to full torque at the off site assembly location. The lower 17 m long tapered sections of the towers were assembled lying horizontally on site. Bolts were initially only taken to snug tight condition before the section was lifted into place. Once verticality and top level tolerances on the fully erected tower were checked, the bolts in the lower section were then tightened to full torque.
The nacelles (turbine units) comprising the drive unit, gear box, generator and ancillary equipment were shipped from Denmark as fully assembled, ready to install units. The 23.5 m long blades were also shipped from Denmark. The rotor and blades were assembled lying horizontally on the ground adjacent to their respective turbine location. The full assembly was then erected in a single lift.
Full time monitoring of the progress of each facet of the work ensured that the progress of the work on site was well coordinated. Very little time was lost to the project schedule through inter-related activities causing delay.
The work associated with the construction and erection of the wind turbines followed a logical order at each turbine location:
- Excavation for foundation
- Construction of foundation raft
- Installation and levelling of base section of tower
- Construction of foundation upstands
- Backfill around foundation
- Construction of turbine controller building
- Erection of tower
- Erection of nacelle
- Installation of cabling and turbine controller
- Assembly and erection of rotor and blades
Figure 4. The towers had to be erected on site in two sections
Transportation to the site of the blades, nacelle and tower presented a technical challenge as it required an access road with specific curve and slope characteristics. The existing public access road is a very narrow shingle road, with several tight hairpin bends, steep gradients, culverts and bridges. This hurdle was overcome by upgrading another private access track into an access road. Nevertheless, the new 4 km long gravel access road to the site had grades of up to 14 per cent, which provided a challenge to heavy vehicles accessing the site. Only minor corrections were necessary to the road profile, but regular maintenance was necessary to ensure that the vehicles carrying tower sections and turbine blades maintained traction on the road surface.
Much of the site is at an altitude in excess of 400 m and extends along the crest of the Tararua Ranges for approximately 4 km. To meet the project schedule, work on the tower foundations commenced at the start of winter and erection of the lattice towers commenced at the start of spring. Rainfall, wind and wind chill all contributed to construction difficulties during this period.
Wind had the most significant effect, particularly in relation to tower erection. Selection of appropriate plant and equipment for the conditions was essential for the success of the project. Wind speed was continuously monitored at the site throughout the duration of the construction period. Much of the erection work was either hindered or stopped altogether when wind speeds exceeded 36 km/h. Approximately 84 days were “lost” during the construction period when winds exceeded this speed. During October alone, the winds were in excess of this speed on 27 days.
Crawler cranes were used exclusively for erection of all tower sections, nacelles and rotor assemblies. Although not as mobile over this extensive site, they were able to operate safely in conditions which would have been marginal, if not impossible, for a mobile crane. Mobile hydraulic platforms were also utilized for access to the towers for bolt torquing operations. This equipment allowed the riggers to work safely in windier conditions.
Approximately 3500 m3 of concrete was used in constructing the tower foundations. In view of the quantity involved and the remoteness of the site, a concrete batching plant was set up on site. This plant produced approximately 95 per cent of the concrete used in the foundations. The balance supplied over the latter stages of the work was trucked from a central batching plant in Palmerston North.
The Tararua Wind Farm project is an accomplishment for everyone who has been involved in the project, particularly the people of Central Power who had the foresight to investigate wind generation technology back in 1992. Producing non-subsidized energy at competitive market prices is a testament to their perseverance and to the maturity of this generation technology.
With the successful commissioning of this project, New Zealand has been clearly put on the map of international wind power: today it has what is considered one of the world’s best producing large-scale wind farms.