Power control of wind turbine

Figure 1 shows the relationship curve between the power of the wind turbine and the wind speed. With the help of this graph, the comprehensive performance of the wind turbine can be evaluated. Obviously, when the wind speed is less than the cut-in wind speed, the wind turbine will stop; when the wind speed is greater than the cut-in wind speed, the wind turbine will start, and the output power will increase approximately to the third power of the wind speed until it reaches the rated wind speed; when the wind speed is greater than or equal to the rated wind speed, take appropriate measures to limit the increase in the output power of the wind turbine and keep it at the rated power to avoid overload and damage to the wind power generation system; when the wind speed is too high and exceeds the cut-out wind speed, the wind turbine must be shut down. Judging from the current actual application situation, the cut-out wind speed of a large wind turbine is generally around 25m/s. Normally, people control the rated wind speed v_N and cut-out wind speed through stall control, active stall control, and pitch control.

Stall control

For a fixed-speed and fixed-pitch wind turbine, the pitch angle of the blades is fixed. It uses the aerodynamic characteristics of the blade to cause it to stall at high wind speeds to limit the power of the wind turbine. As shown in Figure 2(a), when the wind speed increases from v₀ to v₁, since the linear velocity u of the wind turbine blades is constant, the angle Φ between the synthetic wind speed vector and the plane of rotation increases; and because the angle β between the leaf element chord line and the rotation plane of the wind wheel is constant (for the convenience of research, a micro element with radius r and length δ_r is selected on the blade, which is called leaf element), the angle of attack increases from α₀ to α₁. Once the angle of attack α is greater than the critical value, the airflow on the upper side of the blade separates, forming drag, and the corresponding drag coefficient increases, while the lift coefficient decreases, as shown in Figure 2(b). The change in lift F_L and drag F_D causes the axial thrust F_T acting on the blade to increase, and the tangential rotational force F_R decreases slightly, which in turn will cause the aerodynamic torque and power to decrease at the same time. This phenomenon is called stall.

As shown in Figure 3(a), the power curve of stall control is given. It can be seen from Figure 3(a) that there is a significant difference between the actual power curve and the ideal power curve. This is due to the constant speed of the wind turbine. In the interval below the rated wind speed, the tip speed ratio can only be optimized at a certain wind speed [see point E in Figure 3(a)], the output power is equal to the ideal power, and the wind energy utilization efficiency is the largest, and at other points, the tip speed ratio λ deviates from the optimal value, so the power absorbed from the wind is less than the ideal power; in the interval above the rated wind speed, due to poor stall regulation performance, the power absorbed by the wind turbine can only be equal to the rated power at a certain wind speed [see point F in Figure 3(a)]. When the wind speed is less than or greater than the wind speed, the output power is lower than the rated power.

In summary, the fixed-pitch stall wind turbine has a simple structure and low cost, but it has poor power regulation, large output power fluctuations, low wind energy utilization, and large aerodynamic loads. Therefore, people have gradually developed variable-pitch wind turbines to replace fixed-pitch wind turbines.

Active stall control

Generally, when the wind speed reaches the rated wind speed and above, the control method of artificially controlling the pitch angle to deepen the stall of the wind turbine is called active stall control. As shown in Figure 2, when the wind speed increases from v₀ to v₁, since the linear velocity u of the wind turbine blades is constant, the angle Φ between the synthetic wind speed vector and the plane of rotation increases. At this time, the pitch angle β is reduced by the actuator, and the angle of attack α=Φ-β increases rapidly, which strengthens the stall of the wind turbine and achieves the purpose of quickly adjusting the power of the wind turbine. Similar to the stall control, at this time, the lift is reduced and the resistance is increased, resulting in an increase in the axial thrust FT acting on the plane of the wind turbine rotor, and the tangential rotation force remains unchanged, so the aerodynamic torque and power remain at the rated value. As shown in Figure 3(b), the power curve during active stall control is given. It can be seen from Figure 3(b) that the power curve is in good agreement with the ideal power curve, especially when the wind speed is above the rated wind speed, the power of the wind turbine is stably controlled at the rated value. Active stall control has good power adjustment performance and simple control, but the axial thrust acting on the rotor plane increases and the aerodynamic load of the wind turbine increases.

Pitch control

During the operation of the variable-pitch wind turbine, if the wind speed exceeds the rated wind speed, the blade can be rotated around its axis through the variable-pitch mechanism to increase the angle between the blade element chord line and the rotating plane. That is, the pitch angle β reduces the angle of attack α, so that the power of the wind turbine remains unchanged. As shown in Figure 4(a), assuming that the wind speed increases from v₀ to v₁, since the linear velocity u of the wind turbine blades is constant, the angle Φ between the relative wind speed and the plane of rotation increases. At this time, the pitch angle β is increased by the pitch mechanism, and the angle of attack is reduced from α₀ to α₁. Further through the curve of the drag coefficient with the angle of attack shown in Figure 4(b), it can be seen that the lift coefficient is reduced, while the drag coefficient is still maintained at a small value. Therefore, it can be considered that the controller keeps the tangential rotation force F_R acting on the rotor plane unchanged by adjusting the lift F_L, thereby ensuring that the power of the wind turbine remains unchanged. At the same time, it can be further seen from Figure 4 that the axial thrust F_T acting on the rotor plane is also reduced. Therefore, the pitch control can not only control the power of the wind turbine to be constant, but also reduce the aerodynamic load of the wind turbine. The power curve of pitch control is basically the same as that of active stall control, as shown in Figure 4(b).

In summary, the variable pitch control has good power adjustment performance and small aerodynamic load, but it requires a complex control mechanism, which increases the complexity of the wind power generation system.

Finally, it is necessary to emphasize that although both active stall control and pitch control adjust the power of the wind turbine by adjusting the pitch angle, their adjustment directions are different, the adjustment frequency is different, and the change law of the axial thrust is also different.

Pitch system and yaw system

Pitch system

Pitching is to make the blades rotate around its installation axis, change the pitch angle, and thus change the aerodynamic characteristics of the wind turbine. As shown in Figure 5, a schematic diagram of the basic principle of pitch control is given. By detecting the output electric power of the generator as the input of the controller, the pitch angle is adjusted according to the set control strategy. In modern large-scale wind turbines, variable pitch wind turbines are generally used.

The pitch system is a kind of pitch angle adjustment device. As shown in Fig. 6, a block diagram of a typical three-blade wind turbine pitch system is given. Generally speaking, there are two types of pitch system: hydraulic pitch system and electric pitch system.

Yaw system

Practice has shown that the magnitude of the wind speed and the direction of the wind always change continuously over time. In order to ensure the stable operation of the wind turbine, there must be a device to make the wind turbine change with the wind direction and keep the wind turbine perpendicular to the wind direction. This device is called a yaw system, also called a windward device. Generally, there are two types of yaw systems, passive yaw systems and active yaw systems, and their structure is shown in Figure 7(A). Among them, the active yaw system is essentially an automatic control system, and its composition block diagram is shown in Figure 7(B). It is important to note that the yaw system can sometimes be used to adjust the power of the wind turbine to deviate the wind turbine heading from the wind direction by a certain angle, so that the power captured by the wind turbine is reduced.