Wind turbine structure:
Nacelle: The nacelle contains key equipment for the wind turbine, including the gearbox and generator. Maintenance personnel can access the nacelle through the wind turbine tower. The left end of the nacelle contains the wind turbine rotor, consisting of the rotor blades and shaft.
Rotor blades: They capture the wind and transmit the wind power to the rotor shaft. On a modern 600-kilowatt wind turbine, each rotor blade measures approximately 20 meters in length and is designed much like an airplane wing.
Shaft: The rotor shaft is attached to the low-speed shaft of the wind turbine.
Low-speed shaft: The low-speed shaft of a wind turbine connects the rotor shaft to the gearbox. In modern 600 kW wind turbines, the rotor speed is quite slow, approximately 19 to 30 revolutions per minute. The shaft contains conduits for the hydraulic system, which actuates the pneumatic brakes.
Gearbox: The low-speed shaft is located on the left side of the gearbox, which can increase the speed of the high-speed shaft to 50 times that of the low-speed shaft.
High-speed shaft and its mechanical brake: The high-speed shaft operates at 1500 revolutions per minute and drives the generator. It is equipped with an emergency mechanical brake for use in case the pneumatic brake fails or the wind turbine is under maintenance.
Generator: Commonly referred to as an induction motor or asynchronous generator. In modern wind turbines, the maximum power output is typically 500 to 1500 kilowatts.
Yaw mechanism: An electric motor rotates the nacelle to align the rotor with the wind. The yaw mechanism is operated by an electronic controller, which senses wind direction via a wind vane. The diagram shows the wind turbine yaw. Typically, the wind turbine yaws only a few degrees at a time as the wind changes direction.
Electronic controller: Contains a computer that continuously monitors the wind turbine's status and controls the yaw mechanism. To prevent any malfunction (i.e., overheating of the gearbox or generator), the controller can automatically stop the wind turbine's rotation and call the wind turbine operator via telephone modem.
Hydraulic system: pneumatic brake used to reset the wind turbine.
Cooling components: This includes a fan for cooling the generator. Additionally, it includes an oil cooling element for cooling the oil inside the gearbox. Some wind turbines have water-cooled generators.
Wind turbine tower: The wind turbine tower carries the nacelle and rotor. Taller towers are generally advantageous because the higher above the ground, the greater the wind speed. Modern 600 kW wind turbine towers are 40 to 60 meters high. They can be tubular or lattice-shaped. Tubular towers are safer for maintenance personnel because they can reach the top via internal ladders. Lattice towers are cheaper.
Anemometer and wind vane: Used to measure wind speed and direction.
Wind turbine generators: Wind turbine generators convert mechanical energy into electrical energy. The generators on wind turbines are somewhat different from the power generation equipment you usually see on the power grid. This is because generators need to operate under fluctuating mechanical energy conditions.
Output voltage
Large wind turbines (100-150 kW) typically generate 690 volts of three-phase alternating current. The current then passes through a transformer next to the wind turbine (or inside the tower), where the voltage is increased to between 10,000 and 30,000 volts, depending on the local power grid standards.
Large manufacturers can provide 50 Hz wind turbines (for most of the world’s power grid) or 60 Hz turbines (for the U.S. power grid).
Cooling system
Generators require cooling during operation. In most wind turbines, the generator is housed in a tube and air-cooled by a large fan; some manufacturers use water cooling. Water-cooled generators are more compact and energy efficient, but this method requires a radiator inside the nacelle to dissipate the heat generated by the liquid cooling system.
Starting and stopping the generator
If you connect or disconnect a large wind turbine generator from the power grid by flipping a regular switch, you could very likely damage the generator, gearbox, and nearby power grid.
Design of generator power grid
Wind turbines can use synchronous or asynchronous generators and can be connected to the power grid directly or indirectly. Direct grid connection refers to connecting the generator directly to the AC power grid. Indirect grid connection means that the wind turbine's current is regulated and matched to the grid through a series of electrical devices. With an asynchronous generator, this regulation process is completed automatically.
Rotor blades
Rotor blade profile (cross section)
Wind turbine rotor blades resemble the wings of an aircraft. In fact, rotor blade designers typically design the cross-section of the outermost part of the blade to resemble the wing of a traditional aircraft. However, the thicker profile at the inner tip of the blade is usually specifically designed for wind turbines. Choosing the profile for rotor blades involves many trade-offs, such as reliable operation and delay characteristics. The blade profile is designed so that the blade can operate well even when its surface is dirty.
Rotor blade material
Most rotor blades on large wind turbines are made of glass fiber reinforced plastic (GRP). Using carbon fiber or aramid as reinforcement is another option, but such blades are uneconomical for large wind turbines. Wood, epoxy wood, or epoxy wood fiber composites are not yet available on the rotor blade market, although there is progress in this area. Steel and aluminum alloys have issues with weight and metal fatigue, and are currently only used in small wind turbines.
Wind turbine gearbox
Why use a gearbox?
The energy generated by the rotation of the wind turbine rotor is transmitted to the generator through the main shaft, gearbox, and high-speed shaft.
Why use a gearbox? Why can't we drive the generator directly through the spindle?
If we use a conventional generator and connect it directly to a 50Hz three-phase AC grid with two, four, or six electrodes, we would have to use a wind turbine with a rotational speed of 1000 to 3000 revolutions per minute. For a wind turbine with a rotor diameter of 43 meters, this means the speed at the rotor tip would be more than twice the speed of sound. Another possibility is to build an AC generator with many electrodes. But if you want to connect the generator directly to the grid, you would need a generator with 200 electrodes to achieve a speed of 30 revolutions per minute. Another problem is that the mass of the generator rotor needs to be proportional to the torque. Therefore, a directly driven generator would be very heavy.
Lower torque, higher speed
Using a gearbox, you can convert the lower speed and higher torque on the wind turbine rotor into a higher speed and lower torque for the generator. The gearbox on a wind turbine typically has a single gear ratio between the rotor and generator speeds. For a 600 kW or 750 kW machine, the gear ratio is approximately 1:50.
The image shows the 1.5 MW gearbox used for the wind turbine. This gearbox is somewhat unusual because flanges are mounted on the two generators at the high-speed points. The orange fitting mounted under the generator on the right is a hydraulically driven emergency disc brake. In the background, you can see the lower half of the nacelle used for the 1.5 MW wind turbine.
Wind turbine yaw device
A wind turbine yaw device is used to rotate the wind turbine rotor to face the wind.
Yaw error
When the rotor is not perpendicular to the wind direction, a wind turbine exhibits yaw error. Yaw error means that only a small portion of the energy in the wind can flow within the rotor region. If this were the only occurrence, yaw control would be an excellent way to control the electrical input to the wind turbine rotor. However, the portion of the rotor closest to the wind source experiences greater forces than other parts. On one hand, this means the rotor tends to automatically yaw towards the wind, a phenomenon present in both headwind and tailwind turbines. On the other hand, it means the blades bend back and forth along the direction of the force with each rotor rotation. A wind turbine with yaw error will experience greater fatigue loads compared to a wind turbine yawed perpendicular to the wind direction.
Yaw mechanism
Almost all horizontal-axis wind turbines employ forced yaw. This involves using a mechanism with a motor and gearbox to keep the turbine yawed against the wind. This diagram shows the yaw mechanism on a 750 kW wind turbine. We can see the yaw bearing surrounding the outer edge, and the wheels housing the internal yaw motor and yaw brake. Almost all manufacturers of upwind equipment prefer to disable the yaw mechanism when not needed. The yaw mechanism is activated by an electronic controller.
Cable Twist Counter
Cables are used to carry current from the wind turbine to the base of the tower. However, if the wind turbine accidentally deflects in one direction for too long, the cable will become increasingly twisted. Therefore, wind turbines are equipped with cable twist counters to alert operators that the cable should be untangled. Similar to safety mechanisms on all wind turbines, the system has redundancy. Wind turbines are also equipped with pull switches that are activated when the cable twists too severely.
