Most wind turbines are constant speed, the speed at the end of the rotor blade is 64m/s, and the speed at the axis is zero. The rotating speed at a quarter of the blade length from the shaft is16m/s, and the yellow belt in the figure is more inclined to the rear of the wind turbine than the red belt. This is obvious because the rotating speed of the blade tip is eight times that of the wind speed, which impacts the front of the wind engine.
Why are rotor blades spiral?
The rotor blades of large wind turbines are usually spiral. Look at the rotor blades and move to the root of the blades until you reach the center of the rotor. You will find that the wind enters from a very steep angle (much steeper than the usual wind direction on the ground). If the blades are hit from a particularly steep angle, the rotor blades will stop running. Therefore, the rotor blade needs to be designed in a spiral shape to ensure that the blade edge behind the blade is pushed away along the wind direction on the ground.
Wind engine structure
Engine room: Engine room is the key equipment of fan, including gear box and generator. Maintenance personnel can enter the engine room through the wind turbine tower. The left end of the engine room is the rotor of the fan, that is, the rotor blades and shafts.
Rotor blades: capture the wind and transmit it to the rotor shaft. In the modern 600 kW wind turbine, the measuring length of each rotor blade is about 20 meters, and its design is very similar to the wing of an airplane.
Shaft: The shaft of the rotor is attached to the low-speed shaft of the wind turbine.
Low speed shaft: the low speed shaft of the wind turbine connects the rotor shaft center and the gear box. In modern 600 kW wind turbines, the rotor speed is quite slow, about 0/9 to 30 revolutions per minute. There is a conduit for the hydraulic system in the shaft to start the operation of the pneumatic brake.
Gear box: the left side of the gear box is a low-speed shaft, 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 braking: the high-speed shaft runs at the speed of 1500 rpm to drive the generator. It is equipped with an emergency mechanical brake for use when the pneumatic brake fails or the wind engine is repaired.
Generator: usually called induction motor or asynchronous generator. On modern wind turbines, the maximum power output is usually 500 to 1500 kW.
Yaw device: rotate the engine room with the help of motor to make the rotor windward. The yaw device is operated by an electronic controller and can sense the wind direction through the wind vane. The figure shows the yaw of a wind turbine. Usually, when the wind changes direction, the wind engine will only deflect a few degrees at a time.
Electronic controller: includes a computer that continuously monitors the state of the wind turbine and controls the yaw device. In order to prevent any fault (i.e. overheating of the gearbox or generator), the controller can automatically stop the rotation of the wind engine and call the wind engine operator through the telephone modem.
Hydraulic system: used to reset the pneumatic brake of the wind motor.
Cooling element: includes a fan for cooling the generator. In addition, it also includes an oil cooling element for cooling the oil in the gearbox. Some wind turbines have water-cooled generators.
Tower: The tower of the wind turbine carries the engine room and rotor. Usually, a tall tower has an advantage, because the higher it is from the ground, the greater the wind speed. The tower height of modern 600 kW wind turbine is 40 to 60 meters. It can be a tubular tower or a lattice tower. Tubular towers are safer for maintenance personnel because they can reach the top of the tower through internal ladders. The advantage of lattice tower is that it is cheaper.
Anemometer and vane: used to measure wind speed and direction.
wind driven generator
Wind turbines convert mechanical energy into electrical energy. The generator on the wind turbine is a little different from the power generation equipment you usually see on the power grid. The reason is that the generator needs to run under the condition of mechanical energy fluctuation.
Output voltage
Large wind turbines (100- 150 kW) usually generate 690 volts of three-phase alternating current. Then the current passes through the transformer next to the fan (or in the tower), and the voltage rises to 1 10,000 to 30,000 volts, depending on the local power grid standard.
Large manufacturers can provide 50 Hz wind turbines (used by most power grids in the world) or 60 Hz wind turbines (used by American power grids).
cooling system
The generator needs cooling when it is running. In most wind turbines, generators are placed in pipes and large fans are used for air cooling. Some manufacturers use water cooling. Water-cooled generators are smaller and more efficient, but this method needs to install a radiator in the engine room to eliminate the heat generated by the liquid cooling system.
Starting and stopping the generator
If you pull an ordinary switch to connect or disconnect a large wind turbine generator from the power grid, you are likely to damage the generator, gearbox and adjacent power grid.
Generator power grid design
Wind turbines can use synchronous or asynchronous generators and directly or indirectly connect the generators to the power grid. Direct grid connection means that the generator is directly connected to the AC power grid. Indirect grid connection means that the current of wind turbines passes through a series of power equipment and is adjusted to match the power grid. Using asynchronous generators, this adjustment process is completed automatically.
rotor blade
Rotor blade profile (cross section)
The rotor blades of a wind turbine look like the wings of an airplane. In fact, rotor blade designers usually design the cross section of the farthest part of the blade to be similar to the wing of an orthodox aircraft. However, the thick profile of the inner end of the blade is usually specially designed for wind turbines. Choosing a profile for a rotor blade involves many trade-offs, such as reliable operation and delay characteristics. The contour design of the blade enables the blade to work normally even when there is dust on the surface.
Rotor blade material
The rotor blades of most large wind turbines are made of glass fiber reinforced plastic (GRP). Using carbon fiber or aramid fiber as reinforcement is another option, but such blades are not economical for large wind turbines. Wood, epoxy wood or epoxy wood fiber composites have not yet appeared in the rotor blade market, although they have been developed in this field. Steel and aluminum alloy have some problems such as weight and metal fatigue, so they are only used in small fans at present.
Wind turbine gearbox
Why use the gearbox?
The energy generated by the rotor rotation of the wind turbine is transferred to the generator through the main shaft, gear box and high-speed shaft.
Why use the gearbox? Why can't the generator be driven directly through the spindle?
If we use an ordinary generator and use two, four or six electrodes to directly connect to a 50 Hz AC three-phase network, we will have to use a wind engine with a speed of 1000 to 3000 rpm. For a wind turbine with a rotor diameter of 43 meters, this means that the speed at the end of the rotor is more than twice the speed of sound. Another possibility is to make an alternator with many electrodes. But if you want to connect the generator directly to the power grid, you need to use a generator with 200 electrodes to get 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, there is a great opportunity for direct-drive power generation.
Lower torque, higher speed
Using the gearbox, you can convert the lower speed and higher torque on the rotor of the wind turbine into the higher speed and lower torque used on the generator. The gearbox of a wind turbine usually has a single transmission ratio between the rotor and the generator speed. For 600 kW or 750 kW machines, the transmission ratio is about 1 50.
The following figure shows the 1.5 MW gearbox of the wind turbine. This gearbox is a bit unusual because the flanges are installed on two high-speed generators. The orange fitting installed on the right side below the generator is an emergency disc brake driven by hydraulic pressure. In the background, you can see the lower half of the nacelle of the 1.5MW wind turbine.
Wind turbine yaw device
The yaw device of the wind turbine is used to rotate the rotor of the wind turbine to the windward direction.
yaw error
When the rotor is not perpendicular to the wind direction, the wind turbine has yaw error. Yaw error means that only a small part of energy in the wind can flow in the rotor area. If this happens only, yaw control will be an excellent way to control the power input of the wind turbine rotor. However, the part of the rotor near the wind source is subjected to greater force than other parts. On the one hand, this means that the rotor tends to automatically deflect against the wind, which is the case for both upwind and downwind turbines. On the other hand, this means that every time the rotor rotates, the blades will bend back and forth along the stress direction. Wind turbines with yaw errors will bear more fatigue loads than those with yaw perpendicular to the wind direction.
Yaw mechanism
Almost all horizontal axis wind turbines will be forced to yaw. That is, a mechanism with a motor and a gearbox is used to keep the wind motor deflected against the wind. This picture shows the yaw mechanism of 750 kW wind turbine. We can see the outer edge around the yaw bearing, as well as the wheels of the internal yaw motor and yaw brake. Almost all manufacturers of headwind equipment like to stop the yaw mechanism when it is unnecessary. The yaw mechanism is excited by an electronic controller.
Cable twist counter
Cables are used to carry current from the wind turbine to the bottom of the tower. However, when the wind turbine deflects in one direction for too long, the cable will become more and more twisted. Therefore, the wind turbine is equipped with a cable twist counter to remind the operator that the cable should be untied. Similar to the safety mechanism of all wind turbines, the system is redundant. The wind engine will also be equipped with a cable switch, which will be activated when the cable is excessively twisted.
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