Abstract: In wind power generation systems, pitch control technology is crucial for the safe and reliable operation of wind turbine generators and affects their service life. By controlling the pitch angle, output power is stabilized, torque oscillations are reduced, and nacelle oscillations are minimized. This not only optimizes output power but also effectively reduces noise, stabilizes generator output power, and improves the stress conditions of the blades and the entire turbine. Variable pitch wind turbines have better wind energy capture characteristics than fixed pitch wind turbines, and most modern large wind turbines adopt variable pitch control. This paper focuses on a hydraulic variable pitch wind turbine from a well-known international wind power company, using a programmable logic controller (PLC) as the pitch controller. This pitch controller features flexible control methods, simple programming, and strong anti-interference capabilities. This paper introduces the working principle of the hydraulic variable pitch system and designs the software system of the pitch controller. Finally, experiments were conducted on a wind turbine generator set from a well-known international wind power company, verifying that the pitch controller can operate safely and stably on variable pitch wind turbines.
Keywords: Variable pitch; Wind turbine; Programmable logic controller
1 Introduction
With the continuous maturation and development of wind power technology, the advantages of variable pitch wind turbines have become increasingly prominent: they can improve the reliability of wind turbine operation, ensure a high wind energy utilization coefficient, and continuously optimized output power curves. Wind turbines employing variable pitch mechanisms can reduce rotor weight, greatly improve the overall stress distribution, and enable the wind turbine to maintain optimal conversion efficiency at different wind speeds, maximizing output power and thus improving system performance. As the power rating of wind turbines increases, the adoption of variable pitch technology has become an inevitable trend. Currently, there are two main types of pitch actuators: hydraulic and electric, which can be further divided into unified pitch and independent pitch based on their control methods. Independent pitch technology, developed from unified pitch, allows each blade to independently change its pitch angle according to its own control rules. This effectively solves the problem of uneven load on components such as blades and towers, and offers advantages such as compact and simple structure, ease of application of various controls, and high reliability, making it increasingly popular in the international wind power market.
Megawatt-class variable-speed constant-frequency variable-pitch wind turbines are currently among the most technologically advanced wind turbine models internationally. Looking at future development trends, they will inevitably replace fixed-pitch wind turbines and become the mainstream wind turbine model. Variable pitch technology plays a crucial role in the research of variable-speed constant-frequency wind turbines and is a prerequisite for their realization. Researching this technology to improve the flexibility of wind turbines and extend their lifespan is currently a hot research topic abroad, but domestic research in this area is scarce. Funding this forward-looking research project and mastering independent pitch control technology with independent intellectual property rights is of great significance for breaking the monopoly of developed countries on advanced wind power generation technology and promoting the further development of my country's wind power industry.
To obtain a highly reliable controller for the pitch control system, this paper adopts the OMRON CJ1M series programmable controller as the controller for the pitch control system, and designs the PLC software program. Experiments were conducted on the wind turbine generator set of a well-known foreign wind power company.
2. Variable pitch wind turbines and their control methods
Variable pitch speed control is one of the main speed control methods for modern wind turbines. Figure 1 shows a simplified diagram of a variable pitch wind turbine. The speed control device reduces the tendency for the rotor speed to increase due to increased wind speed by increasing the pitch angle. When the wind speed increases, the variable pitch hydraulic cylinder is activated, pushing the blades to rotate in the direction of increasing pitch angle, thereby reducing the wind energy absorbed by the blades and maintaining the wind turbine operation within the rated speed range. When the wind speed decreases, the opposite operation is performed to keep the power absorbed by the wind turbine basically constant. The hydraulic control system has advantages such as large transmission torque, light weight, high rigidity, accurate positioning, and fast dynamic response speed of the hydraulic actuator, which can ensure that the blades are adjusted to the predetermined pitch more quickly and accurately [4][5]. At present, most of the large wind turbines produced and operated in China use hydraulic systems as the power system for their variable pitch devices.
Figure 1. Schematic diagram of variable pitch wind turbine.
Figure 2 shows the principle block diagram of the variable pitch controller. Before the engine is connected to the grid, the speed controller performs variable pitch control based on the engine speed feedback signal, and determines whether the blade is in standby or feathering position based on the speed and wind speed signals. After the engine is connected to the grid, the power controller takes effect. The power regulator usually adopts PI (or PID) control. The power error signal is processed by PI to obtain the pitch angle position.
Figure 2 Control block diagram of variable pitch wind turbine
When the wind turbine is in a stopped state, the pitch angle is at 90°, at which point the airflow does not generate torque on the blades. When the wind turbine transitions from a stopped state to an operating state, the pitch angle decreases from 90° to the standby angle (15° in this system) at a certain speed (approximately 1°/s). If the wind speed reaches the grid connection speed, the pitch angle continues to decrease to 3° (the optimal wind energy absorption coefficient is found at around 3°). After the generator is connected to the grid, when the wind speed is lower than the rated wind speed, the pitch angle is maintained at 3°. When the wind speed is higher than the rated wind speed, based on the power feedback signal, the controller outputs a -10V to +10V voltage to the proportional valve, controlling the direction and magnitude of the output flow from the proportional valve. The variable pitch hydraulic cylinder manipulates the blade pitch angle according to the flow and direction output by the proportional valve, maintaining the output power near the rated power. In case of a fault or a shutdown command, the controller will output a rapid feathering command, allowing the wind turbine to stop quickly; the feathering speed can reach 20°/s.
Design of 3 Pitch Controller
3.1 System Hardware Configuration
This experiment uses a wind turbine generator set from a well-known foreign wind power company as the experimental object. Its rated power is 550 kW, and it employs a hydraulic pitch control system. The schematic diagram of the hydraulic pitch control system is shown in Figure 3. As can be seen from Figure 3, the feed or retraction speed can be changed by altering the voltage of the hydraulic proportional valve. In the event of a wind turbine malfunction or emergency shutdown, the solenoid valves JB can be closed, and JA and JC can be opened, allowing the hydraulic oil in the pressure tank 1 to quickly enter the pitch cylinder, pushing the blades to the feather position (90°).
Figure 3. Schematic diagram of hydraulic pitch control system
This system uses an OMRON CJ1M series PLC. The generator power signal is input to the PLC in analog form (0-10V corresponds to a power of 0-800KW) by a high-speed power transmitter. The pitch angle feedback signal (0-10V corresponds to a pitch angle of 0-90°) is input to the PLC's analog input unit in analog form. Hydraulic sensors 1 and 2 are also input in analog form. Here, a 4-channel analog input unit CJ1W-AD041 is selected; the analog output unit CJ1W-DA021 is selected, with an output signal of -10V to +10V, which is output to the proportional valve to control the advance or retraction speed; in order to measure the generator speed, a high-speed counting unit CJW-CT021 is selected. The generator speed is detected by a photoelectric encoder connected to the generator, which outputs 10 pulses per revolution, which are input to the counting unit CJW-CT021.
3.2 System Software Design
The main functions of this system are implemented by a PLC. When the wind turbine's starting conditions are met, the PLC issues a command to reduce the blade pitch angle uniformly from 90°. After the generator is connected to the grid, the PLC adjusts the power based on the feedback power, maintaining a high wind energy absorption coefficient below the rated wind speed, and adjusting the pitch angle above the rated wind speed to keep the output power at the rated power. In the event of a fault shutdown or emergency stop signal, the PLC controls solenoid valves JA and JC to open and JB to close, causing the blades to quickly change to a 90° pitch angle position.
The pitch control program flow during wind turbine startup is shown in Figure 4. When the wind speed is higher than the starting wind speed, the PLC outputs a 1.8V voltage to the proportional valve through the analog output unit, causing the blades to change to 15° at a speed of 0.9°/s. At this time, if the generator speed is greater than 800 r/min or the speed remains greater than 700 r/min for one minute, the blades continue to advance to the 3° position. When the PLC detects that the speed signal from the high-speed counting unit is greater than 1000 r/min, it issues a grid connection command. If the blades are not connected to the grid within 2 minutes after reaching the 3° position, the analog output unit outputs a -4.1V voltage to the proportional valve, causing the blades to return to the 15° position.
Figure 4 Flowchart of wind turbine start-up pitch control program
After the generator is connected to the grid, its output power is adjusted by regulating the pitch angle. The power regulation flowchart is shown in Figure 5. When the actual power exceeds the rated power, the PLC's analog output unit CJ1W-DA021 outputs a voltage signal proportional to the power deviation, and uses the LMT instruction to limit the output voltage to within -4.1V (corresponding to a pitch speed of 4.6°/s). When the power deviation is less than zero, the pitch needs to be increased to boost the power. During pitch increase, the maximum voltage output to the proportional valve is 1.8V (corresponding to a pitch speed of 0.9°/s). To prevent frequent reciprocating pitch changes, pitch changes are not performed when the power deviation is within ±10kW.
In a pitch control system, the pitch regulation power in the high-wind-speed range is crucial. If the pitch retraction speed is too slow, overpower or overcurrent phenomena may occur, potentially even burning out the generator. Conversely, if the pitch regulation speed is too fast, over-regulation will occur, causing large fluctuations in output power, and will shorten the service life of the pitch cylinder and pitch bearings. This will also affect the generator's output power, reducing power generation. In this system, different pitch speeds are used for overpower retraction and underpower advance. The retraction speed is greater than the advance speed to prevent excessive generator power during strong gusts.
Figure 6 shows the ladder diagram program for the variable pitch power regulation section. 100.08 is the command to start power regulation. When the power regulation conditions are met, relay 100.08 changes from 0 to 1. D2100 stores the deviation between the engine's rated power and actual power. When the deviation ΔP satisfies -10kW < ΔP < 10kW, 0 is assigned to D2100. When 60.07 is 1, meaning the power deviation is negative, the power deviation in D2100 is scaled proportionally and output to the proportional valve via the LMT command limit switch. The minimum output corresponds to a voltage of -4.1V. If relay 60.07 is 0, meaning the power deviation is positive, the value of D2100 is scaled proportionally via the SCL3 command and output to the proportional valve via the LMT command. The maximum output voltage is 1.8V.
Figure 6 Pitch Power Adjustment Program
4. Conclusion
An OMRON CJ1M series PLC was used as the controller for the pitch control system of a large wind turbine, and experiments were conducted on a pitch control wind turbine belonging to a well-known foreign wind power company on Nan'ao Island, Guangdong Province. Field test records show that this PLC control system ensures safe operation of the wind turbine, enabling rapid feathering shutdown in case of a shutdown failure. During operation, it adheres to the principle of optimal power output, maintaining a constant pitch angle of 3° below rated wind speed. At high wind speeds, it adjusts the pitch angle according to the output power, maintaining an output power of approximately 550kW. During high-speed gusts, power fluctuations do not exceed 10% of the rated power, meeting design requirements. Because the pitch control system uses a PLC as the controller, complex logic control can be achieved with simple software programs, and it exhibits strong anti-interference capabilities and reliable performance.
