The world’s largest commercially-built wind turbine
by G Lund, the CADDET Danish National Team

A 2 MW wind turbine is now in operation at Lammefjorden in Denmark, on a site with good wind conditions. The turbine is designed to produce maximum power in high wind speeds and is intended for future offshore application. Five further 2 MW turbines were built in May 2000 on the same site. The six turbines are expected to produce a total of 24 GWh/year – equivalent to the electricity consumption of about 6,000 Danish households.

Background

Denmark’s first wind power programme was initiated in the 1970s. By 1998, total power production was 477 GWh from utility-owned wind turbines and 2,302 GWh from privately owned machines. To date, wind turbines have been built mainly on land. In future, though, erection offshore is expected to increase due to the lack of appropriate land-based sites. In June 1999, the Danish Government approved five specific sites for large-scale offshore wind farms with a total capacity of 750 MW. These wind farms will be built and owned by power utilities. The first two farms, each of about 150 MW, are expected to be operational in 2002.

The Lammefjorden project is the result of a commitment by the Danish electricity utilities, in agreement with the Government, to install wind turbines. When completed, it will consist of six

2 MW onshore machines – three owned by the local power utility, North-West Sealand Energy Supply Company, and three owned privately. The price of each turbine, including installation, is about DKK 15 million (where DKK is the Danish krone).

The Lammefjorden turbines

The new prototype 2 MW turbines at Lammefjorden are currently the largest being built commercially anywhere in the world. Developed and manufactured by the Danish company NEG Micon, using its experience gained over the last five years in manufacturing wind turbines in the 600-750 kW, 1.0 MW and 1.5 MW classes, particular emphasis is placed on suitability for offshore installation. The aim has been to produce a high-tech product, simple in concept, ruggedly built, efficient and reliable in operation, and designed in harmony with the landscape. The manufacturer co-operated with design company Jacob Jensen Design to produce a streamlined cabin with a harmonic rotor.

The turbine is 104 m high with a rotor diameter of 72 m and hub height of 68 m. It is the first wind turbine in Denmark to exceed a height of 100 m. Special permission was required from the Civil Aviation Administration prior to erection. In order to warn low-flying aircraft, a 24-hour red warning light is mounted on the cabin.

Features

The machine bed and its components are completely enclosed in a fibreglass cabin. This cabin is divided into a large upper section containing the transformer, gear mechanism, generator, control system and main shaft, and a lower section containing the yawing system around the tower. The upper section has a wide mast at the rear that contains cooling radiators, an anemometer, wind vanes, warning lights and a lightning conductor. Access to these parts is via a hatch and safety rail system. The lower section has service hatches allowing most of the components to be replaced without using external cranes.

The rotor consists of a steel hub with three fibreglass blades mounted on rotating bearings. The hydraulically controlled blade-tip angle is variable, from 0° to -95°. Power control is based on a fixed rotating speed and power regulation through active stalling. The tower is a trumpet-shaped double cone made of steel plates.

The control system cuts the generator in and out of the electricity grid and monitors the turbine’s vital functions in order to avoid hazardous operating situations. On the basis of received data, the computer automatically operates the turbine in the optimum way. The turbine control cabinet is installed in the rear of the nacelle and there is an operating desk in the base of the tower.

The gear mechanism consists of a three-step gear in which the first step is a planetary gear and the subsequent steps are bevel-cut gearwheels with parallel shafts.

The generator, installed at the rear of the nacelle, is an asynchronous two-speed liquid-cooled engine with an operating voltage of 960 V. It is designed and built specially for wind turbines and is particularly efficient at relatively low loads. The generator is connected via the control cabinet to a 2,000 kVA transformer fixed on a bracket bolted to the machine bed. By placing the transformer at the top of the tower, large losses of heat and energy in the generator cables are avoided.

Operation

During operation, blade pitch is usually at 0° to give optimum lift on the aerofoil and thus optimum power production, as well as minimum blade noise. Once maximum power has developed, it is maintained at wind speeds of about 14 m/s and above by gradually turning the blades and making fine adjustments to the pitch angle of between 0° and -4°.

The rotor normally runs in idling mode at low wind speeds. When the wind increases to around 4 m/s, blade pitch changes to a negative angle so that the rotor can measure wind speed. At a rotation speed over a pre-set threshold value, the blades turn into the operating blade pitch range. During cut-in to the grid, the pitch angle is corrected to make rotor acceleration suitably low and uniform, both at low and high wind speeds. Thus, under all conditions, cut-in should be smooth and controlled, without peak loads.

The braking system is designed to be fail-safe. Active hydraulic pressure is used to maintain the wind turbine in operating mode. When the control system emits a stop signal or if the power supply fails, the integral braking forces are released, slowing down the turbine. The braking system has two elements, both of which are hydraulically controlled: blade feathering and a mechanical disk brake located on the gearbox.

Performance

Based on normal wind conditions in Denmark, expected total electricity production from the six turbines at Lammefjorden is estimated to be about 24 GWh/year. The first turbine is still in its running-in period and has, therefore, been stopped from time to time in order to adjust and improve operational performance. From 12 August 1999 to 31 January 2000 it generated about 565 MWh of power; almost 40% of this was produced in January.

The turbine has lived up to expectations and performed robustly in wind speeds up to 25 m/s, which occurred during the hurricane that hit Denmark in December 1999. (Above that speed the turbine was shut down to avoid damage.) Overall, the Lammefjorden project is expected to underline the potential of the new generation of large-scale wind turbines currently under development.

For more information contact the CADDET Danish National Team at Tølløse.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by  the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors.  They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44  1235 432968, Fax: +44 1235 433595.