Wind turbines are very complex machines. Many sensors are needed to ensure their continuous operation, generating green energy. This frequently asked question looks at some of the sensors used to monitor wind turbine operation, such as: Eddy current sensors monitor the shaft lubrication gap, how the turbine shaft rotates both axially and radially inside its housing, and rotational stresses tested by the gondola; Displacement sensors that monitor the structural integrity of towers and gondolas; Accelerometers that measure tower rocking, gondola rotation and gearbox vibration; Bragg grille sensors with optical fibers that allow individual control of the step of the blades and; mechanical, ultrasonic and LiDAR wind sensors that monitor and predict wind speeds.
Eddy current sensors, also called Foucault currents, are used to measure the shaft lubrication gap. These sensors are environmentally friendly and can operate when immersed in oil at high pressures and temperatures. The gearbox and generator have an “elastic” bearing to handle the heavy mechanical loads they experience. The clutches in the wind turbines must balance the different movements of the gearbox and the generator. The “elastic” nature of the bearings is called leakage and allows the shaft to change its rotation both axially and radially inside its housing. The acceptable release is precisely determined for wind turbines. Eddy current sensors can monitor even small changes in leakage, allowing preventive maintenance before damage or catastrophic damage occurs.
Eddy current sensors also measure rotational stresses caused by vibrations, wind loads and other factors experienced by the gondola that can lead to structural problems. And they are used to monitor the axial, radial and tangential deflection of the clutch discs to ensure that the rotor can be safely stopped during high winds. Some eddy current sensors are temperature compensated to operate reliably in conditions of large fluctuations in ambient temperature. They provide higher bandwidth compared to alternative sensor technologies and are suitable for providing accurate measurement of high-speed movements (Figure 1).
Displacement sensors
Various displacement sensor technologies are used to monitor the integrity of the structure. Laser displacement sensors can measure small movements in the tower relative to the base and monitor the structural integrity of the installation. A laser displacement sensor sends a laser beam to a receiver at some distance and measures any change in alignment between the two. These sensors are very accurate and allow trend data to track whether a problem is occurring and how fast it is developing.
Capacitive displacement sensors can measure the air gap of the generator (distance between rotor and stator) in the turbine. Changes in the air gap of the generator lead to changes in capacity and these sensors can operate in high electromagnetic fields and at high temperatures.
Drawn wire displacement sensors can measure airflow by detecting changes in the position of the air valves. These sensors consist of a spring wire wound on a spool-shaped transducer. When the wire is extended or retracted, the spool rotates and this rotation is converted into distance measurement.
Accelerometers
Accelerometers are another type of sensor used to monitor vibrations in main, rotating and rotating bearings, as well as other rotating components, such as the output shafts of the main generator. Vibration data collected can monitor changes in weather and predict impending damage. MEMS accelerometers, which combine a high degree of strength with high sensitivity and stability of measurements, are often used in wind turbines.
Bearing wear can lead to changes in the accelerometer measurement between 0.1 and 1.0 g. Maintenance at 1.0 g is usually required. monitoring the trend of 0.1 and up allows planning preventive maintenance. High frequency monitoring is needed to identify potential bearing malfunctions. The measured g force is proportional to the square of the frequency and a small displacement of the fault at high frequencies leads to a higher g force compared to the same displacement of the fault at lower frequencies. Accelerometers with a measuring range of up to 200 g and 10 kHz are commonly used in wind turbine applications. In addition, they must withstand high levels of impact as a result of abrupt stops or structural impacts.
Gearbox damage prediction
Gearboxes, rotor blades and generator have the highest failure rate of all wind turbine components, and gearboxes are the worst of all. The gearbox is subject to high mechanical loads from changes in wind speed and braking, as well as stresses involved in the transfer of mechanical energy from the rotor hub, which rotates at low speed (20 rpm or less) to the generator, rotating at high speed (3200 rpm) (Figure 2).
Gearbox vibration signals must be captured up to 0.1 Hz to comply with wind turbine design guidelines. The floor noise level must be low in order to detect precisely faults in the bearings at an early stage with low vibration amplitudes. In addition, vibration sensors rated at 10 kHz or higher on both high-speed and low-speed shafts are required to provide the necessary bandwidth to measure harmonics of impending damage.
Fiber optic Bragg grating sensors
With a larger diameter of the turbine blades, the difference in wind speed at the top and bottom of the rotors (wind shear) can be significant. Large amounts of wind shear can lead to imbalance of the rotor, which loads the drive components such as the gearbox and reduces the efficiency of power conversion. Individual blade pitch control (IPC) can solve the challenges posed by wind shear, improving efficiency, extending gearbox life and reducing maintenance costs.
When a broadband light beam is transmitted to an optical fiber, Bragg fiber grids reflect only a certain wavelength (Bragg wavelength) while transmitting all other wavelengths. This reflected wavelength varies with variations in parameters such as voltage, vibration, acceleration and optical fiber temperature. (Figure 3).
Figure 3: Bragg array optical sensors experience changes in optical properties when subjected to voltage and are used to apply IPC to turbine blades. (Image: Eon Photonics)
The load data for each blade must be monitored in real time at high speed in order to implement an efficient IPC algorithm. Electronic sensors are not suitable for the task due to the frequency of lightning strikes received from the turbine blades. Bragg’s fiber optic grating sensors are insensitive to lightning strikes and have become the industry standard for blade monitoring systems.
LiDAR expects changing winds
Conventional wind turbines use mechanical or ultrasonic feedback methods to respond to wind disturbances and to minimize the effects of changing winds after the fact. LiDAR sensors offer a means to predict wind changes, provide additional inputs to the wind turbine controller and allow new control algorithms.
Two types of LiDAR use different methods to calculate wind speed. Continuous wave LiDAR uses a laser beam focused at a certain distance from the tower (Figure 5). Pulsed LiDAR uses a time-based method that searches for reflected laser light at different time delays, allowing it to measure wind speeds at different distances from the tower.
There are additional variables that need to be considered when using LiDAR wind measurement systems. For example, a wider cone angle is better when measuring wind direction, but suffers from reduced accuracy in estimating wind speed. Or LiDAR can be mounted in the turbine hub, resulting in a measurement that rotates at the same speed as the rotor and has a higher correlation with the wind as felt by the blades.
When used during construction, LiDAR measurements can help improve the design of the turbine in such a way as to mitigate extreme events and improve the efficiency of energy production during normal operation, improving operational reliability. In addition, LiDAR measurements are sometimes made as part of a wind farm commissioning to compare actual wind conditions against a specific turbine power curve to identify the possibility of lower performance. In some cases, LiDAR measurements may result in turbines being equipped with larger rotors to improve energy generation, or lighter towers may be used to reduce installation costs.
Summary
A wide range of sensor technologies and hundreds of sensors are needed to monitor the operation of wind turbines and ensure reliable and efficient green energy production. This FAQ reviews some of the key sensor modalities required to operate a wind turbine, including many existing sensor technologies and emerging LiDAR sensors to predict changes in wind speed.
References
Choosing the best vibration sensor to monitor the condition of the wind turbineanalog devices
Bragg grating fiber sensorsEon Photonics
Lidar improved wind turbine management: past, present and futureNational Renewable Energy Laboratory
Merger of sensors and assessment of the state of the wind energy conversion system with activated IoTMDPI sensors
Sensor solutions for predictive maintenance of wind turbines and generatorsMicro-Epsilon
