The Power of the Wind: Cube of Wind Speed
Download scientific diagram | Relationship between wind speed, wind direction and the power output at Sotavento wind farm for three months, January to March . Download scientific diagram | Relationship between wind speed, wind direction and the power output at Sotavento wind farm for three months, January to March . Most U.S. manufacturers rate their turbines by the amount of power they can safely produce at a particular wind speed, usually chosen between.
Control Methods You can use different control methods to either optimize or limit power output. You can control a turbine by controlling the generator speed, blade angle adjustment, and rotation of the entire wind turbine.
Blade angle adjustment and turbine rotation are also known as pitch and yaw control, respectively. A visual representation of pitch and yaw adjustment is shown in Figures 5 and 6. Yaw Adjustment The purpose of pitch control is to maintain the optimum blade angle to achieve certain rotor speeds or power output. You can use pitch adjustment to stall and furl, two methods of pitch control.
Wind profile power law - Wikipedia
By stalling a wind turbine, you increase the angle of attack, which causes the flat side of the blade to face further into the wind. Furling decreases the angle of attack, causing the edge of the blade to face the oncoming wind.
Pitch angle adjustment is the most effective way to limit output power by changing aerodynamic force on the blade at high wind speeds. Yaw refers to the rotation of the entire wind turbine in the horizontal axis. Yaw control ensures that the turbine is constantly facing into the wind to maximize the effective rotor area and, as a result, power. Because wind direction can vary quickly, the turbine may misalign with the oncoming wind and cause power output losses.
You can approximate these losses with the following equation: You can achieve this dynamic control with power electronics, or, more specifically, electronic converters that are coupled to the generator.
The two types of generator control are stator and rotor. The stator and rotor are the stationary and nonstationary parts of a generator, respectively. In each case, you disconnect the stator or rotor from the grid to change the synchronous speed of the generator independently of the voltage or frequency of the grid. Controlling the synchronous generator speed is the most effective way to optimize maximum power output at low wind speeds.
Figure 7 shows a system-level layout of a wind energy conversion system and the signals used. Notice that control is most effective by adjusting pitch angle and controlling the synchronous speed of the generator.
Control Strategies Recall that controlling the pitch of the blade and speed of the generator are the most effective methods to adjust output power. The following control strategies use pitch and generator speed control to manage turbine functionality throughout the power curve: Figure 8 shows the power curves for different control strategies explained below, with variable-speed variable-pitch, VS-VP, being the ideal curve.
Fixed-speed fixed-pitch FS-FP is the one configuration where it is impossible to improve performance with active control.
14. Wind turbine power ouput variation with steady wind speed.
These turbines are regulated using passive stall methods at high wind speeds. The gearbox ratio selection becomes important for this passive control because it ensures that the rated power is not exceeded. Figure 8 shows the power curve for FS-FP operation. From the figure, it is apparent that the actual power does not match the ideal power, implying that there is lower energy capture.
Notice that the turbine operates at maximum efficiency only at one wind speed in the low-speed region. The rated power of the turbine is achieved only at one wind speed as well.
Wind Turbine Control Methods - National Instruments
This implies poor power regulation as a result of constrained operations. Fixed-speed variable-pitch FS-VP configuration operates at a fixed pitch angle below the rated wind speed and continuously adjusts the angle above the rated wind speed. To clarify, fixed-speed operation implies a maximum output power at one wind speed. You can use both feather and stall pitch control methods in this configuration to limit power. Keep in mind that feathering takes a significant amount of control design and stalling increases unwanted thrust force as stall increases.
Figure 8 shows the power curve for FS-VP using either feather or stall control. Exceeding the rated wind speed, the pitch angles are continuously changed, providing little to no loss in power.
A stall-regulated variable speed wind turbine has no pitching mechanism. However, the rotor speed is variable. The rotor speed can either be increased or decreased by an appropriately designed controller. In reference to the figure illustrated in the blade forces section, it is evident that the angle between the apparent wind speed and the plane of rotation is dependent upon the rotor speed.
This angle is termed the angle of attack. The lift and drag co-efficients for an airfoil are related to the angle of attack. Specifically, for high angles of attack, an airfoil stalls. That is, the drag substantially increases. The lift and drag forces influence the power production of a wind turbine.
This can be seen from an analysis of the forces acting on a blade as air interacts with the blade see the following link. Thus, forcing the airfoil to stall can result in power limiting.
So it can be established that if the angle of attack needs to be increased to limit the power production of the wind turbine, the rotor speed must be reduced.