Wind Turbines: Tiny Sensors Play Big Role
By Barry Manz for Mouser Electronics
Wind turbines may be as high as a skyscraper, but they owe their ability to operate efficiently and safely to the dozens of tiny, inexpensive sensors that monitor their health.
At a fundamental level, a modern wind turbine is a twenty first-century windmill with large rotor blades that turn a breeze into electrical energy rather milling grain or pumping water. Dig a little deeper, and wind turbines reveal themselves as masterpieces of design, combining innovations in mechanical and electrical engineering. Even though their rotors can be more than 79m long and their towers more 183m high, some of their most critical components—sensors—are only centimeters long but play an outsized role in keeping these leviathans functioning in the face of extraordinary stresses, vibration, and various other hazards.
The Role Of Sensors In Wind Farms
Without sensors, wind turbines would arguably be less safe, more costly to operate, unable to accurately predict and solve impending failures, or potentially have lifetimes less than the twenty five years they’re expected to operate. Most important, wind farms need accurate data about every turbine and its most important components that can be provided only sensors linked together and connected to a command center.
What’s more, wind turbines are a classic (if offbeat) example of industrial IoT in action: They have all the necessary ingredients from sensors to the networks that connect them, though typically via Ethernet not wireless connectivity. A wind farm IIoT network can exploit the benefits of historical operating data such as wind speed, power, yaw angles, gearbox temperature, and other metrics to analyze trends, from the entire wind farm down to the lowliest component. From this, operators can create a model that can predict what components to inspect and when. All the information, as well as status alerts and other results from monitoring, can be viewed and actions taken from a smartphone, tablet, or computer.
To see why these sensors are so important, consider a wind turbine and all the places within it where they can be used to monitor all system components including the structure itself (Figure 1). Wind turbines are complex, and typically have more than 8,000 components. Their huge blades and tower structures are anchored to platforms made of thousands of tons of steel and rebar measuring 30 to 15m across and 6 to 10m deep.
Figure 1: The basic components of a wind turbine within the nacelle and some of the types of sensors and where they’re placed. (Source: TE Connectivity brochure)
The gearbox that transforms the slow turning rate of the blades to a faster rotor speed (along with the generator) is housed in a container atop the tower called a nacelle that is the size of a bus and weighs about 45T (tonnes). Some nacelles are large enough to include a helicopter landing pad on the top, and an entire wind turbine platform can weigh more than 272T.
A good example is one of the latest deployments in the UK, which is the world’s largest generator of wind power with a nationwide capacity of 5.3GW that can power more than five million homes. This past May, the Danish company DONG Energy flipped the switch to power up 32 additions to the Burbo Bank Offshore Wind Farm (Figure 2) on Liverpool Bay in the Irish Sea. It was a significant event for renewable energy, as it marked the first commercial use of 8MW turbines, which double the output of the wind farm’s original turbines. The new turbine structures are about 195m high, their rotors are 80m long, and a single revolution of a single rotor can power one home for 29 hours.
Figure 2: Burbo Bank Offshore Windfarm with North Wales in the distance. (Source: Wikipedia)
For the record, the largest-capacity conventional-drive wind turbine is currently the Vestas 164 from Vestas Wind Systems that has an output of up to 9MW, which it generates with its 178m rotor that has a swept area of 20,566m² and weighs 32T. The top of the structure is 219m high (about the height of a 72-story building) and its overall weight more than 1800T. Figure 3 captures how enormous wind turbine rotors really are.
Figure 3: A turbine blade convoy meandering through Edenfield to the Scout Moor Wind Farm, the second largest onshore wind farm in England. (Source: Geograph)
The Crucial Role Of Sensors
There are many different types of electrical and optical sensors used in wind turbines. In general, they
- Detect, monitor, and communicate information about parameters such as changes in the distance between two components near each other
- Monitor levels of vibration that, if excessive, can cause major damage
- Monitor changes in temperature, pressure, and mechanical stresses
Eddy Current Sensors
One of the most common types of sensors in wind turbines are eddy current sensors—also called Foucault currents—which detect changes in the electrical current created when a conductive material enters a moving magnetic field. When this occurs, the strength of the field can be translated into changes in distance.
In wind turbines, eddy current sensors measure the lubricating gap of the shaft to ensure it is always covered by a thin film of oil that is usually applied under pressure. Because these sensors can resist oils and pressure as well as temperature, they can reliably monitor the oil gap under these hostile conditions. If the gap becomes too large and exceeds its specifications, an alert can be sent so that preventive maintenance can be performed before the shaft binds or seizes.
These sensors also measure how the turbine shaft rotates both axially and radially inside its housing, a specification called run-out. Radially, this condition causes the shaft to rotate off-center rather than “true,” and axially results in the shaft rotating at a slight angle. While there is always a tiny amount of run-out, worn bearings can cause it to exceed acceptable limits, and when it is too high, usually the result of high wind loads, the turbine must be shut down for maintenance. Obviously, the ability to monitor run-out over time allows this maintenance to be performed before extreme damage or even catastrophic failure occurs.
Finally, eddy current sensors are also employed to measure the turning effects (moments, or torque) applied to the nacelle—caused by vibration, wind loads, or other factors—that can lead over time to degradation of structural integrity. They can be applied to measure axial, radial, or tangential deflection of the clutch disks that ensure rotor safety by braking during high winds.
Displacement Sensors
A variety of displacement sensors are used to monitor structural integrity as well. The foundations or platforms required to keep wind turbines in place consist of massive amount of concrete. However, because the towers are very high and the rotors and nacelles housing the generator are huge, the entire structure is effectively “top-loaded,” so monitoring the system's integrity at its base is essential.
Laser displacement sensors can be used to perform this function because they can detect very small movements of the foundation in relation to the tower caused by repeated thrashing by the wind or waves, or caused by resulting structural defects. Laser displacement sensors work by transmitting a beam of light to an optical receiver some distance away. Deviations and movement between the two are transformed into units of distance. Laser triangulation sensors are also used for a similar purpose and are configured with the sensor, transmitter, and receiver in a triangle. As these devices are extremely accurate, they can detect very small changes, so trend data can be created to show whether a problem is developing and how rapidly it is progressing.
Another precision displacement sensor—the capacitive type—measures the distance between the stator and rotor in the turbine, called the generator air gap. Their operation is based on the principle that electrical capacitance exists between conductive surfaces near each other and that the capacitance will change in direct proportion to the distance between the surfaces. These sensors measure those changes and can operate in high-temperature environments, and highly electromagnetic fields.
Draw-wire displacement sensors combine a spring-loaded wire wound onto a spool-type transducer. Because the wire can be quite long, draw-wire sensors have the benefit of being able to measure changes in distance when the sensor is located far away from the object that is moving. As the wire is extended or retracted from the spool, the spool rotation is measured and then converted into a measure of change to an electrical signal. In wind turbines, they measure air flow by detecting the position of the air flaps. A typical draw-wire displacement sensor is shown in Figure 4.
Figure 4: This draw-wire displacement sensor from Bourns shows the spring-loaded spool on which the cable is wound and a rotational sensor mounted to the enclosure. Several types of sensors can be used depending on the requirements of the application. (Source: Bourns)
Draw-wire sensors can also be used with a variety of rotary transducers depending on the application, such as potentiometers, Hall-effect sensors, and analog or digital non-contacting sensors. The Bourns AMS22B5A1BHASL334N non-contacting analog rotary sensor, for example, uses magnetic technology and is resistant to shock, vibration, fluids, and dust, and can operate over a temperature range of -40oC to 125oC. It has 12-bit output resolution and linearity of ±0.3 percent.
Accelerometers
Accelerometers, which measure changes in velocity or speed, are used in wind turbines to detect and monitor vibration within main, yaw, and slew bearings, as well as other rotating components such as the main generator output shafts. The collected vibration data can be used to monitor changes over time and predict impending failures.
The Analog Devices ADXL1001 and ADXL1002 MEMS accelerometers are good examples because they measure vibration with high resolution and low noise density over time. Their sensitivity characteristics are very stable, and they’re immune from shocks up to 10,000mps2. The devices also have integrated self-diagnostic functions and an over-range indicator, and they operate over a frequency range of -40°C to +125°C.
Wind Sensors
Wind sensors are mounted on the top of the nacelle and are either mechanical or ultrasonic. Because ultrasonic types do not need recalibration, they are increasingly used in areas where maintenance is difficult to perform. Ultrasonic sensors measure the distance to an object by using sound waves, sending out a very-low-frequency sound wave and detecting the wave after it has been reflected by the target object. By recording the elapsed time between generation of the sound and its return, it is possible to calculate the distance between the sensor and the object.
The Texas Instruments PGA460/PGA460-Q1 ultrasonic processor and driver SoC has a signal conditioner and Digital Signal Processor (DSP) core that conditions the reflected signal using an analog front-end consisting of a low-noise amplifier and programmable gain stage that sends output to an analog-to-digital converter. The digitized signal is then processed for near-field and far-field object detection using time-varying thresholds.
Temperature Sensors
Temperature sensors are also used in locations where increases in temperature are indicative of the overheating of some type of component of subsystem. TE Connectivity's PTF Platinum temperature sensors measure from -200°C to +600°C and have temperature detectors that use thin-film resistors as the sensing element. They are very small and lightweight, drift little over time, and have a low time constant for rapid feedback.
Conclusion
All this being said, a reasonable question might be: If sensors are so crucial to wind turbine performance and safety, what happens if the sensors themselves fail? The answer is that multiple sensors are used in some locations, the second as a backup that can be switched in, potentially autonomously, upon failure of the first. In addition to this backup approach, sensors used in wind farms (as well as other energy systems) must be specified to meet requirements such as broad operating temperature ranges, certification to IP67 or IP68 for protection from dust and water, and sometimes ruggedized enclosures.
Like any type of evolving technology, creating energy from the wind has had its good and bad days, and some of the latter have resulted from a single electrical component failure rather than a massive generator or turbine blade failure. Sensors are playing a major role in reducing the likelihood of these occurrences, just as they are in all industrial applications. Eddy current and displacement sensors, accelerometers, and wind and temperatures sensors are the key to monitoring turbines and communicating potential and needed maintenance. For this reason, they are likely to be employed in an even greater number of locations on these mammoth machines in the future, as it’s not difficult to make a case for using a $10 part to protect an expensive turbine blade from catastrophic failure.
Barry Manz is president of Manz Communications, Inc. He has worked with over 100 companies in RF, microwave, defense, test and measurement, semiconductor, embedded systems, lightwave, and other markets. He edits for the Journal of Electronic Defense, Military Microwave Digest, and was chief editor of Microwaves & RF magazine
