Driving the Turbine innovations

Over the past decade, windfarms turbines have continued to grow in size and sophistication to meet increasing market demands for clean energy generation. Today, the 300MW Stateline project located in the USA's Pacific Northwest is the world's largest windfarm, and standard-size turbines range between 1.5-2MW.

In the coming years, however, a series of even larger offshore wind-farms are being proposed. To meet the rigors of offshore development and attain cost efficiency, technology also is growing in size and capability.

For example, the proposed Arklow Banks development in the Irish Sea near Dublin, Ireland has been granted a Foreshore Lease that can provide for more than 500MW of offshore windpower. In America, the proposed Horseshoe Shoal windfarm in Nantucket Sound, near Cape Cod, Massachusetts calls for 130 wind turbines, with a 468MW capacity.

Construction of these giant projects will begin with a 25 MW demonstration considered to be Phase I of the Arklow Banks power plant. This demonstration, among other proposed offshore projects, will utilize GE's new 3.6MW offshore wind technology.

Spanish prototype

Last September, GE Wind Energy unveiled the first commercial prototype 3.6MW wind turbine. Positioned on land for testing, the prototype currently operates at a site about 200km south-east of Madrid in Castilla la Mancha, supplying power to Spanish energy supplier Iberdrola.

The technology behind this machine is based on the successful GE Wind Energy 1.5MW wind turbine series, with over 1,500 of' these, units operating at onshore and offshore sites around the world. To, create the 3.6MW model, however; the development team worked to extend wind technology to a larger form factor; one that could withstand the rigors of offshore life and provide cost-competitive power generation over the life of the project.

Innovation centred on a few key areas: • Nacelle components - the term 'nacelle' encompasses the generator, gearbox, drive train, the power electronics, and the material that houses them. The 3.6MW machine features a doubly-fed asynchronous generator, a pulse-width modulated IGBT frequency converter and an improved three-step planetary spur gearbox, as well as several other additions aimed at reducing maintenance costs. A wind direction sensor ensures greater energy capture and automatic cable unwind reduces downtime.

The nacelle comes with its own optional two-ton internal gantry crane to enable ease in conducting repairs and maintenance. An optional 40-ton hoisting system makes it easy to deal with major components making external cranes unnecessary. A container protects the converter, low and medium voltage switchgear, transformer and control system, and allows easy access during maintenance.

• Rotors - the power rating of wind turbines increases substantially as the rotor diameter is increased. The three-bladed, upwind 3.6MW machine utilizes a 104metre diameter rotor, providing a swept area of 8,495m2. A new blading concept includes fail-safe electromechanical pitch control for each blade as well as a hydraulic parking brake. The turbine has a cut-in speed of 3.5m/s and cuts out at 25m/s. The rotor speed is maintained in the range of 8.5-15.3rpm, Annual energy yield, per turbine is approximately 15 million kWh

• Tower - the hub height at the 3.6MW prototype in Spain is 100m, This height was chosen to approximate the wind conditions that prevail at an offshore hub height of 75m - offshore winds are stronger closer to the ground than onshore so the towers installed at sea will not require a height taller than 75m.

The 3.6MW machine's offshore towers will be made of tubular steel specially treated to prevent corrosion. Power electronics - the new turbine has a variable speed, constant frequency design that automatically adjusts blade rpm level in order to achieve optimum lift under varying wind speed conditions. A frequency converter controls generator torque to prevent gusts from adding torque to the drive train.

Grid connection - the 3.6MW model also incorporates closed-loop voltage control advances known as Wind Volt-Amperes Reactive (WindVAR), a unique feature of most GE Wind 1.5MW turbines. This prevents voltage variations from causing flicker, dip, and power loss and enables the wind turbine to supply reactive power to the grid at the time it's needed.

Additional GE technology advancements announced earlier this year prevent wind turbines from tripping offline during major grid disturbances. Whenever voltage levels drop below 70 per cent, turbines trip offline as a form of self protection. GE's new, revolutionary Low Voltage Ride-Through (LVRT) technology delivers ride-through capability of below 15 per cent grid voltage for up to 500 milliseconds. In addition, it remains engaged until after the fault is cleared.

The first test results on the 3.6MW turbine have been favorable. Conducted by German testing company Windtest, initial results indicate that the prototype is performing within its design parameters.

The power curve, for example, meets or exceeds the theoretical machine power curve. Similarly, the decibel level from the turbine tested within the required range. Despite the much greater size of the turbine compared to its predecessor, it is only 2dB louder. This was achieved by adding impact noise insulation to the gearbox and generator a noise reduced gearbox and nacelle, and a blade design that minimized noise level Further tests have demonstrated that grid compatibility, naturally occurring frequencies within the structure, vibration level, mechanical loads and internal component performance are within parameters.

Offshore opportunities

With many future windpower projects expected to be offshore installations, the new 3.6MW wind turbine has been designed to address the specific requirements of that industry segment. Since its introduction, the 3.6MW wind turbine technology has attracted the interest of several offshore project developers. In addition to the Arklow Banks project, the new technology has been selected for Gunfleet Sands, a 108MW facility proposed to be built off the east coast of England; plus two proposed projects in the U.S. oft the coasts of Cape Cod, Massachusetts and Long Island.

N.Y. IPG