Wind Turbine Overspeed Control and Power Generation

Wind Turbine Overspeed Control and Power Generation
  1. Overspeed control

If the wind turbine rotates faster and faster, it will generate a huge force and cause the equipment to self-destruct. In order to protect the equipment, corresponding measures will be introduced when designing the wind turbine, and it will start when the wind speed is high. Such protection mechanism designs can be divided into three basic categories.

1.1 Mechanical brake
When the rotational speed of the wind turbine reaches the cut-out speed, the centrifugal force will be used to force the wind turbine to stop running (similar to the braking mechanism of a car). Wind turbines operating at cut-out speeds generate a lot of heat and cause wear and tear to the wind turbine, which is unacceptable. Large horizontal axis wind turbines with 100m long blades must be completely shut down in the event of high wind speeds.

1.2 Feathering
Some generators will feather when the rpm reaches the cut-out speed. The generator set can feather a single blade by rotating the blades to reduce the windward angle; or use the tail rudder to control the yaw, the principle is similar to that of a pumping windmill, so that all the blades are not facing the wind to reduce the rotation speed.

1.3 Variable axis
Another way of feathering is to use a variable shaft. At low wind speeds, the blades are similar to those of the downstream horizontal axis generator, and the blades face the wind and have a large swept area. This allows efficient operation of power generation when the wind speed is not high. These blades are airfoil-shaped in cross-section, and when they begin to rotate, they apply a lifting force to the body of the generator, as shown in Figure 1. As the wing is lifted, the effective swept area of ​​the blades exposed to the wind decreases, and the rotational speed of the blades is almost constant over a wide wind speed range. At maximum rpm, the machine will be level, making it a vertical axis. The tail acts as a buffer to lift the machine to prevent damage to the blades and to counteract the rotational torque applied to the machine if the wind suddenly drops. Even in a cyclone, wind turbines can still generate electricity.

Figure 1 - The inclination of the wind turbine for speed limiting
Figure 1 – The inclination of the wind turbine for speed limiting

1.4 Downwind wind turbine with pitch system
Downwind wind turbines now use a pitch system, which enables the wind turbines to operate in light winds or strong winds to generate electricity. The pitch system allows the blades to bend and flex. When the wind gets stronger, the blades rotate on the windward side to reduce aerodynamic force. This allows the wind turbine to maintain high output even in the harshest conditions, so that the wind turbine does not lose power to protect itself at high wind speeds. The blades also regulate their speed, preventing damage, especially if a power outage or power failure disconnects the wind turbine from the load. This design helps reduce noise and wind turbine maintenance.

  1. Power generation

The blades of the wind turbine are connected to the shaft of the generator, which drives the motor to rotate and generate electricity. If the generator is directly connected to the shaft (direct drive), the system maintenance costs will be low, but the generator will become bulky. Adding a gearbox can increase the speed of the generator and also reduce the weight of the generator.

The output frequency of the generator will depend on the angular speed of the blades and the wind speed, and when the alternator is used for direct power supply, the change of wind speed can cause many problems. These problems can be overcome by installing speed control devices on wind turbines. But these increase the cost of the system, and also increase the likelihood of the system crashing.

In order to solve these problems, DC generators began to be used. The current produced by the DC generator is fed into a static inverter, where it is converted into a fixed frequency AC current. Wind turbines are often far away from the load and the power losses on the cables are quite significant. The usual practice is to convert the output voltage of the wind turbine to a higher voltage to reduce power loss. At the load end, the voltage is transformed into the voltage level required by the load.

Likewise, the electricity produced by wind turbines can also be used to charge batteries. This means that changes in wind speed will only change the charging rate of the battery. Similar to in photovoltaic systems, a charge regulator is required to limit the charge rate within the range required by the battery to protect it from damage. This is different from the control system on the motor, which needs to keep the load running at the motor load at all times, otherwise the motor will overspeed and cause self-destruction. The load is usually a resistive load and can be a hot water element, an auxiliary battery pack, or something like a water pump.

Read more: Safety Equipment and Site Safety in Independently Powered Systems