In the last few years of the 20th century, variable speed wind turbines were added to the mainstream of large wind turbines. Compared with constant-speed wind turbines, the advantages of variable-speed wind turbines are: at low wind speeds, it can change according to the wind speed, and maintain the optimal tip-speed ratio during operation to obtain the maximum wind energy; when the wind speed is used, the impeller speed is changed. Store or release part of the energy to increase the flexibility of the drive system and make the power output smoother. Therefore, in large-capacity wind turbines, variable-speed wind turbines are replacing constant-speed wind turbines and become the main models of wind power generation.

The control of variable speed wind turbines is mainly achieved in two stages. When the rated wind speed is below, the wind turbine speed is changed to follow the wind speed to adjust the wind speed to obtain the best tip speed ratio. When the wind speed is higher than the rated speed, the pitch of the blade is changed mainly by the variable pitch system. Limit wind turbines to harvest energy and keep wind turbines at rated power. Wind turbines are usually considered as a continuous stochastic nonlinear multivariable system. Linear quadratic optimal control techniques with output feedback can be used. According to the effective model of known systems, the operating requirements of variable speed wind turbines are designed. Controller.

The variable speed operation of the wind turbine is based on the AC excitation variable frequency constant frequency power generation technology. AC excitation variable frequency constant frequency power generation is to apply three-phase low-frequency alternating current in the rotor of the asynchronous generator to realize excitation, adjust the amplitude, frequency and phase sequence of the excitation current to ensure the constant output constant voltage of the generator output power. At the same time, vector transformation control technology is adopted to realize independent adjustment of generator active power and reactive power. Adjusting the active power can adjust the speed of the wind turbine to realize the tracking control of the maximum wind energy capture; adjusting the reactive power can adjust the power factor of the grid, and improve the dynamic and static operation stability of the wind turbine and the connected grid system.

Because modern power electronics technology can construct a PWM rectification-PWM inverter type inverter with good input and output characteristics and power bidirectional flow, using it as an AC excitation power source can not only ensure the synchronous speed up and down. The characteristics of frequency power generation, while ensuring the quality of power generation, truly realize the green transformation of green energy, and make the variable-speed constant-frequency power generation technology have important characteristics of sustainable development.

The wind power generation system using variable speed constant frequency technology is a complex energy conversion system, which includes cross-technical achievements in many disciplines such as aerodynamics, mechanical mechanics, electrical engineering, electronic technology, and control theory. Therefore, it must be systematically Conduct a comprehensive analytical study. This paper will first discuss the wind turbine system speed control principle to achieve maximum wind energy capture from the analysis of wind turbine characteristics. Aiming at the characteristics of AC excitation asynchronous generator, the stator flux linkage oriented sagittal transformation control strategy is adopted to obtain the decoupling and independent adjustment capability of the generator active and reactive power, so as to achieve the maximum wind energy capture and high efficiency power generation operation. Finally, the system simulation research results are provided to verify the correctness and effectiveness of the proposed control strategy.

2 Wind turbine maximum wind energy capture operating mechanism The characteristics of wind turbines are usually expressed by a family of dimensionless performance curves containing the power factor cP, which is a function of the wind turbine tip speed ratio x, as shown.

The cPu) curve is a function of the blade pitch angle. From the figure, we can see the variation of the CP(A) curve on the blade pitch angle: when the blade pitch angle is gradually increased, the CpU) curve will be significantly reduced.

Maximum power. Is a typical CP (A) curve.

The tip speed ratio can be expressed as: a wind turbine rotor angular velocity (rad/s) - a blade radius (m); a dominant wind speed (m/s).

A blade tip speed is based on the mechanical power captured by the wind turbine from the wind: given the wind speed, the power obtained by the impeller will depend on the power factor. If the wind turbine can run at Chi point at any wind speed, it can increase its output power. According to any wind speed, as long as the tip speed ratio of the wind wheel is çºŸ =>, the wind turbine can be kept running at Ck. Therefore, when the wind speed changes, the optimum power factor can be obtained by adjusting the speed of the wind wheel so that the ratio of the tip speed to the wind speed remains unchanged. This is the basic goal of variable speed wind turbines for speed control.

In order to achieve CpCp during wind turbine operation, the wind turbine is controlled by a given power-speed curve. The given value of , varies with the speed, and is calculated from the speed feedback. Based on the calculated value, the generator output power is continuously controlled to track the change in the Popt curve. The system is balanced by the deviation of the target power from the measured power of the generator.

The shape of the optimum power and rotor power-speed characteristic curve is determined by and A.

The relationship between wind turbine power and target power at different wind speeds is given. Assuming that the wind speed is %, point A2 is the operating point of the generator at a speed of 600 r/min, and the point is the operating point of the wind turbine, which are not optimal points. Due to the wind power (point is greater than the electric power (point Az), the excess power increases the speed (generating the acceleration power), and the latter is equal to the difference between the power of A and A2. As the speed increases, the target power follows the P curve. Continued to increase. Similarly, the operating point of the wind turbine also changes along the % curve. The working point and A2 will eventually meet at point A3, and the wind turbine and generator balance the power at point A3.

The working point of the generator is B, and the working point of the wind turbine is B. Since the generator load is greater than the mechanical power generated by the wind turbine, the rotor speed is reduced. As the rotor speed decreases, the generator power is continuously corrected and varies along the curve. Wind turbine output power also varies along the % curve. As the rotor speed decreases, the difference between the wind turbine power and the generator power decreases, and eventually the two will meet at point B3.

The key to achieving maximum wind energy capture operation is the speed control of the wind turbine. In this study, the control of the speed of the wind turbine is realized by adjusting the output power of the generator to adjust the electromagnetic resistance torque of the generator.

3 AC excitation variable speed constant frequency power generation principle AC excitation variable speed constant frequency doubly-fed power generation system schematic diagram As shown, the generator is generally a three-phase wound-type asynchronous generator, the stator winding is connected to the grid, and the rotor winding is connected to the three-phase slip frequency. The inverter realizes AC excitation. When the wind speed changes cause the generator speed to change, the frequency of the rotor current should be controlled/2 to make the stator output frequency A constant. According to the relationship, when the speed/1 of the generator is lower than the speed of the rotating magnetic field of the air gap, the generator is running at sub-synchronous speed. At this time, the inverter supplies positive phase excitation to the generator rotor, and the output voltage component is adjusted, and then With the voltage compensation component, the rotor voltage command U2 can be obtained. After the rotary transformation, the three-phase voltage control of the generator rotor is obtained = 1.251/1113, and the reactive power value Cr=350W is increased between the wind turbine and the generator shaft. A gearbox with a speed ratio of N = 7.864 is connected. Therefore, the relationship between the angular velocity of the generator and the angular velocity of the wind turbine is called == 7.8460. The theoretical calculation shows that the optimum angular velocity of the generator at the wind speed is assumed to increase from 4 m/s to 6. 8 m/s at the 10th second. The theoretical optimal speed of the generator at the two wind speeds indicates the process of adjusting the generator speed with the change of wind speed. The first adjustment starts from 167.5 rad/s at the grid-connecting time, and the speed reaches stable after 5 s. The secondary regulation starts at 10s of the step of the wind speed. After 20s, the speed tends to be stable. After adjustment, the two optimal angular velocities are respectively stabilized at 122. 92 rad/S variable speed constant frequency wind turbine simulation s, and theoretical calculation Very consistent. The model of the variable-speed constant-frequency wind power generation system based on the above stator flux linkage vector control is used to realize the running simulation of the maximum wind energy tracking control under wind speed variation using Matlab/Simulink software.

See the appendix for the parameters used in the simulation. Assume air density and rotor current waveform. It can be seen that as the output power of the generator increases, the amplitude of the stator current also increases accordingly, but the frequency is always constant at 60 Hz, achieving a constant frequency. For the change process of the rotor current, the rotor current frequency changes continuously with the change of the generator speed, and the frequency is zero when the speed exceeds the synchronization point. And 0 describe the power of the stator and rotor side of the generator. It can be seen that with the change of wind speed, the generator can output the maximum power after adjustment; at the same time, it can be seen that when the generator output power P changes, the reactive power Q remains unchanged, and the P is realized. Decoupling control with Q. It shows the flow direction of the generator rotor side power åœ¨ during the tracking of the maximum wind energy. When the generator is at the synchronous speed, P2>, indicating that the power flows from the grid to the generator; when super-synchronous operation, the wind turbine characteristics are analyzed. Based on the mathematical model of AC excitation doubly-fed generator, the control strategy of tracking the maximum wind energy is studied. The stator field-oriented loss-of-conversion control is applied to the generator to achieve the decoupling of the generator active power and reactive power. The active power is controlled to control the electromagnetic torque, thereby controlling the speed of the wind turbine to achieve tracking of the optimal power curve. The modeling and simulation using Matlab/Simulink software verifies the success of the control strategy of the variable-speed constant-frequency wind power generation system proposed in this paper, and meets the requirements of variable-speed constant-frequency wind power generation, which provides theoretical basis and design basis for the development of actual system in the future.

Power three-phase wound asynchronous generator, four poles, rated power 2.1kW, rated voltage 220V / rated frequency 60Hz; stator resistance r, and leakage inductance U are respectively 0. 435n, 2mH; rotor resistance r2 and leakage inductance U respectively for.

=2.3m, rated power 2.21, optimal wind energy coefficient (1 and best tip speed ratio; 1.1), respectively 0.43 and 9.

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