>>AEOLIC SYSTEMS

THE WIND TURBINE GENERATORS

The wind turbine generator converts mechanical energy to electrical energy. Wind turbine generators are a bit unusual, compared to other generating units you ordinarily find attached to the electrical grid. One reason is that the generator has to work with a power source (the wind turbine rotor) which supplies very fluctuating mechanical power (torque).

All wind systems consist of a wind turbine, a tower, wiring and the balance of system components: controllers, inverters and/or batteries. Wind turbines consist of blades on a rotor, a generator mounted on a frame and a tail. The spinning blades turn the rotor capturing the kinetic energy of the wind. This is converted to rotary motion to drive the generator. The best indication of how much energy a turbine will produce is the diameter of the rotor. This determines the amount of wind that will intercept the turbine. Wind speeds increase with height in flat terrain, thus towers are used to mount the wind turbine. Generally speaking, the higher the tower the more power the wind system can produce. A general rule of thumb is to mount the wind turbine 30 feet above any obstacle that is within 300 feet of the tower.

Wind turbines are typically installed in small groups of 2 to 5 units directly connected to an existing utility grid, or in larger groups of 10 to 30 units with an indirect grid connection, where the current from the turbine passes through a series of electric devices that adjust the current to match that of the grid. When the wind speed increases above a certain speed, known as the cut in speed - typically about 3 to 4m/s (meters per second), the turbine begins to generate electricity and will continue to do so until the wind speed reaches the cut out speed, (about 25m/s). At this point the turbine will shut down, rotate out of the wind and wait for the wind speed to drop to a suitable speed to allow the turbine to start again. The turbine's optimum operating wind speed at which maximum output will be achieved is typically about 13 to 16m/s. During operation the generator ensures that the blades maintain a constant speed of about 20 revolutions per minute, which the gearbox transforms into 1500 revolutions per minute. Higher wind loads acting on the blades result in increased power production but not a higher number of revolutions per minute.

On large wind turbines (above 100-150 kW) the voltage generated by the turbine is usually 690V three-phase alternating current (AC). The current is subsequently sent through a transformer next to the wind turbine (or inside the tower) to raise the voltage to somewhere between 10,000 and 30,000 volts, depending on the standard in the local electrical grid. Large manufacturers will supply both 50 Hz wind turbine models (for the electrical grids in most of the world) and 60 Hz models (for the electrical grid in America).

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