With the world's energy and environmental problems becoming increasingly serious, wind energy has become one of the most widely used and fastest-growing renewable energy sources. The diameter of wind turbine rotors has been continuously increasing; for onshore models alone, it has increased from 72m for 1.5MW units to 156m for current 3.XMW units. As the diameter of wind turbine rotors continues to increase, the reliability and accuracy of blade angle calibration schemes are receiving increasing attention.
1. Functions and Classification of Pitch Systems
The pitch system is an important component of variable speed constant frequency wind turbines. Its function is to adjust the blade angle in real time according to the wind speed change near the rated wind speed, control the absorbed mechanical energy, and reduce the impact of wind on the wind turbine while ensuring the maximum energy (corresponding to the rated power). When shutting down, the pitch system adjusts the blades to the feather position to achieve aerodynamic braking and ensure the safe shutdown of the wind turbine.
Currently, there are two main technical approaches to pitch control systems internationally: hydraulic and electric. Hydraulic pitch control systems use hydraulic cylinders in a hydraulic unit to drive a linkage mechanism, which in turn moves the blades to achieve pitch control. Electric pitch control systems, on the other hand, use a servo motor to drive the blades via a reduction gear and transmission system to achieve pitch control.
2. Composition of the electric pitch system
An electric pitch control system typically includes a servo motor, controller, motor driver, uninterruptible power supply, pitch reducer, and sensors. Among these, the sensors are mainly divided into limit switches, proximity switches, and redundant encoders.
When the electric pitch control system is operating, the programmable logic controller (PLC) controls the servo drive to output according to the position or speed commands given by the wind turbine master controller. The servo drive converts the torque and speed signals into current and frequency signals to further control the operation of the servo motor. The servo motor achieves the transition from high speed and low torque to low speed and high torque through a speed reducer. Finally, the pitch reducer gears control the blade angle change by meshing with the blade root bearing.
3. Significance of blade angle calibration
The pitch motor, through a gearbox and blade root bearings, forms a reliable mechanical connection. During the operation of the wind turbine, the blade angle frequently changes to adapt to the real-time changes in wind speed, accompanied by wear and tear on the mechanical connection. Wear on the gearbox or bearings will cause inconsistencies in the angles of the three blades, resulting in aerodynamic imbalance in the wind turbine.
Field applications have revealed that aerodynamic imbalance can affect the power generation efficiency of wind turbines and cause additional problems, such as damage to critical components like the pitch system, drivetrain, and yaw system. Therefore, accurate blade angle calibration technology is crucial for the safe operation of wind turbines.
4. Introduction to Blade Angle Calibration Technology
For blade angle calibration, pitch manufacturers need to solve two problems: ① The calibration scheme needs to be effective in the long term, that is, it can accurately detect the predetermined angle deviation within the design life of 20 years; ② The calibration scheme needs to consider sufficient design margin to ensure that the pitch system has a certain fault tolerance mechanism to avoid wind turbine shutdown due to false alarms caused by angle calibration failure.
Currently, the blade angle calibration technologies used in electric pitch systems mainly include redundant encoder schemes and proximity switch schemes. Both schemes install an absolute encoder at the end of the pitch system motor, and the feedback value of this encoder is called the default angle value.
In addition, to calibrate the actual blade angle, a sensor (redundant encoder or proximity switch) is installed on the blade root bearing. The feedback value from this sensor is called the calibration angle value. When the deviation between the default angle value and the calibration angle value in the pitch system exceeds the design limit, the system will report a fault.
