Research on variable pitch control system of megawatt wind turbine generator

0 Introduction
Wind energy is the fastest growing clean energy among renewable energy sources, and it is also the renewable energy with the most promising prospects for large-scale development and commercial development. With the increasing energy consumption and further deterioration of the environment, countries around the world have made the development of renewable "green" energy the focus of their energy strategies. Wind power generation is the main way to utilize wind energy. In recent years, China has made great progress in wind power technology and wind power industry, but the design technology and manufacturing technology of megawatt-class wind turbines are still in the initial stage, the independent innovation ability is still weak, the practical experience accumulation is insufficient, and the control technology is quite different from foreign advanced technology.
The variable pitch control system, as one of the core parts of the control system of megawatt-class wind turbines, plays a very important role in the safe, stable and efficient operation of the unit. Stable variable pitch control has become one of the hot spots and difficulties in the current research on the control technology of megawatt-class wind turbines. Therefore, it is necessary to conduct a detailed comparative analysis and study on the variable pitch control system of megawatt-class wind turbines. Combined with the development status of foreign megawatt-class wind turbines, this paper analyzes the structure and control principle of the variable pitch system control of wind turbines. The model is briefly simulated using the PID control method to verify the correctness of the model.

1 Analysis of aerodynamic characteristics of wind turbines
Under the action of external wind, the wind rotor rotates to generate mechanical energy, driving the generator to output electrical energy. However, in reality, the wind turbine cannot convert all the wind energy on the surface swept by the wind rotor into rotational mechanical energy. There is a wind energy utilization coefficient Cp:

Where: Pin is the total wind energy within the rotor sweep surface; Pout is the mechanical energy absorbed by the rotor; ρ is the air density; A is the rotor sweep area; v is the wind speed upstream of the rotor.
The wind energy utilization coefficient Cp of the variable pitch wind turbine is nonlinearly related to the tip speed ratio λ and the pitch angle β of the blade. The tip speed ratio is the ratio of the linear velocity of the blade tip to the wind speed:

Where: n is the rotation speed of the wind rotor; ω is the angular velocity of the wind rotor; R is the radius of the wind rotor.
According to relevant records and research, the wind energy utilization coefficient Cp of the wind turbine can be approximately expressed by the following formula:

Based on the above formula, the ratio of the tip speed ratio of the wind rotor variable pitch to the wind energy utilization rate can be obtained by calculation through Mat1ab:

The following conclusions can be drawn from Figure 1:
1) At a certain pitch angle β, no matter how the tip speed ratio changes, there is only one maximum wind energy utilization coefficient Cpmax, and it is only about 0.5;
2) For any tip speed ratio λ, the wind energy utilization coefficient at the blade pitch angle β=0° is relatively the largest, and as the blade angle continues to increase, the wind energy utilization coefficient decreases rapidly.

2 Model of wind turbine generator set
This paper takes the megawatt-class variable pitch and variable speed wind turbine generator set as the research object. It is assumed that the wind turbine generator set used consists of a horizontal axis variable pitch wind rotor connected to the generator through a speed increaser. The system block diagram is as follows:

In order to design a good controller, it is a necessary prerequisite to establish a dynamic model of the wind turbine. From the perspective of the control system, the wind turbine can be divided into three subsystems: wind rotor aerodynamic characteristics, transmission characteristics and generator model.
2.1 Wind rotor aerodynamic characteristics
In the system, we assume that the variable pitch blades are rigid.
According to formula (1), the power (mechanical energy) absorbed by the wind rotor is:

The dynamic model of the wind rotor is represented by the following motion equation:

Where: Jr is the moment of inertia of the wind wheel, kgm2; ωr is the angular velocity of the wind wheel, rad/s; Tr is the aerodynamic torque of the wind wheel, N·m; n is the transmission ratio of the gearbox speed increaser; Tm is the torque transmitted from the rotating shaft to the rigid gear, N·m.
The relationship between the wind wheel torque and power is:

2.2 Dynamic characteristics of the transmission system
The wind wheel converts the kinetic energy of the wind into mechanical energy on the wind wheel shaft, and then this energy is converted into the required electrical energy, which is generated by a high-speed rotating generator. Due to the limitation of the tip speed, the wind wheel rotates slowly, and the generator cannot be too heavy, and the number of pole pairs is small. The generator speed should be as high as possible, so a gearbox speed increaser should be connected between the wind wheel and the generator to increase the speed to the speed of the generator.
According to the aerodynamic characteristics of the wind wheel, the torque Tr generated by the wind wheel acts on the wind wheel with a moment of inertia Jr. The wind wheel is connected to the generator with a moment of inertia Jg through a speed increaser with a speed increase ratio of n, and the generator will generate a counter torque Te. Due to the rigidity continuity between the wind wheel, the input shaft and the speed increaser, the total friction in the transmission system and the relative angular displacement on the output shaft are ignored.
2.3 Generator
The generator involved in this paper is a wound three-phase asynchronous generator, so the speed change is achieved by changing the stator voltage to change the generator counter torque and speed.

In the formula: p is the number of pole pairs of the generator; m1 is the number of phases of the motor stator; U1 is the grid voltage, V; Cl is the correction coefficient; ωg is the angular velocity of the generator, rad/s; ω1 is the synchronous speed of the generator, rad/s; r1, x1 are the resistance and leakage reactance of the stator winding, Ω; r2, x2 are the reactance and leakage reactance of the rotor winding after calculation, Ω.
Generator rotation equation:

Where: Jg is the moment of inertia of the generator, kg·m2; Te is the generator counter torque, N·m.
The relationship between the angular velocity of the wind wheel shaft and the generator speed is expressed by the following formula:

3 Variable pitch control strategy for MW variable pitch variable speed wind turbine generator set
According to the operation of variable pitch variable speed wind turbine generator set in different areas, the basic control strategy is determined as follows: when the wind speed is lower than the rated wind speed, track the Cpmax curve to obtain the maximum energy; when the wind speed is higher than the rated wind speed, track the Pmax curve and keep the output stable.
Assume that the pitch angle of the blades of the generator set is at a constant angle before starting. When the wind speed reaches the starting wind speed, the rotor speed increases from zero to the speed that the generator can cut in, the Cp value continues to rise, and the wind turbine generator set starts to generate electricity. By controlling the generator speed, the wind turbine generator set gradually enters the Cp constant zone (Cp=Cpmax), at which time the unit operates in the best state. As the wind speed increases, the speed also increases, and finally reaches a maximum allowable value. At this time, as long as the power is lower than the maximum allowable power, the speed remains constant. After reaching the power limit, the generator set enters the constant power zone. At this time, as the wind speed increases, the Cp value must be reduced so that the tip speed ratio decreases faster than in the constant speed zone, thereby allowing the wind turbine to operate at a constant power at a smaller Cp value.

4 PID controller and MATLAB simulation results
PID control is the basic and most common method in industrial control. The PID controller is relatively simple in form, it consists of proportional, integral and derivative (Proportional-Integral-Derivative), and its transfer function is:

Where: Kp, Ki and Kd are proportional, integral and differential gains respectively.
The PID parameter setting is to determine the three parameters (Kp, Ki, Kd) according to the characteristics of the controlled object and the desired control performance requirements.
When the wind speed is lower than the rated wind speed, the control goal is to seek the maximum power coefficient to capture the maximum wind energy. From the experimental data of the wind power plant, it can be seen that the pitch

When the blade tip speed ratio is 9, the wind energy utilization coefficient Cp is the largest (about 0.4623).

The pitch angle is set to 0, and the best wind energy utilization system can be obtained by adjusting the wind rotor speed so that its ratio to wind speed remains unchanged (λ=ωrR/v=9).

The PID controller is used to change the generator stator voltage, thereby adjusting the generator counter torque to change the speed, and Kp=150, Ki=2.

5, Kd = 7.5 (the blade pitch angle is initially set to 00).
When the wind speed is higher than the rated wind speed, the control goal is to keep the output power stable at the maximum allowable value. Therefore, when the wind speed is high, it is usually adjusted

The blade pitch angle is adjusted to adjust the power utilization coefficient Cp value, so as to keep the output power at the maximum allowable value.

The value of Cp is changed by changing the angle, and Kp=0.00007, Ki=0.00001, Kd=0.000001 are selected. Therefore, the PID controller is used in MATLAB.

The constructed system model is shown in the figure:

When the wind speed changes, the simulation results of the output power and generator speed under various wind conditions are shown in the figure below.

When the wind speed v=6m/s, that is, before the wind turbine reaches the rated power, the simulation of the output power and speed of the asynchronous motor is shown in Figures 4 and 5 below:
When the wind speed v=19m/s, that is, after the wind turbine reaches the rated power, the simulation of the output power and speed of the asynchronous motor is shown in Figures 6 and 7 below:

5 Conclusion
Taking a 1.3MW wind turbine as an example, this paper studies the control strategy of maximum wind energy tracking and rated power maintenance based on the analysis of wind energy, wind turbine characteristics and asynchronous motors, and simulates it with the traditional PID control method. The results show the correctness of the model.