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Products & Applications Predictive maintenance: How sensors monitor wind turbines in real time

| Author/ Editor: Michael Kuran* / Erika Granath

Lightning strikes, defective rotor blades, or tower movements. Wind turbines must be permanently monitored in real time. Predictive maintenance based on precise sensor technology plays an important role here. It not only ensures timely shutdown but can also reduce maintenance and repair costs.

Humans have always used the wind’s power in one way or another, for example, through windmills or sailing boots. In the past few years, wind has been given a new, extremely important, task: to generate electricity.
Humans have always used the wind’s power in one way or another, for example, through windmills or sailing boots. In the past few years, wind has been given a new, extremely important, task: to generate electricity.
(Bild: gemeinfrei / CC0)

Humans have always used the wind’s power in one way or another, for example, through windmills or sailing boots. In the past few years, wind has been given a new, extremely important, task: to generate electricity.

Wind turbines are used both on land and in water. The average length for turbine is around 110 meters and have a diameter of up to 2,000 meters. These large-scale constructions can weight several thousand tons. Due to their large size, a failing turbine can be extremely expensive to repair, should one fail unexpectedly. Some wind farms, especially offshore ones, are also difficult to reach with the material and machines necessary to repair a broken turbine.

Living in tough environments, the wind turbines often get damaged caused by their surroundings. However, there are a lot of factors that can cause the turbines to break, for example: shaft problems, gear faults, gear wear, material fatigue, unbalance, temperature differences, lubrication faults, and bearing play. Predictive maintenance plays an important role in preventing damages on the turbines.

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Predictive maintenance: How to prevent damages

The work with predicting potential risks for the wind turbines starts already at the manufacturing stage where the turbines are equipped with sensors that can detect minor damages on them in real-time. By doing so, actions that prevent the turbine to get further damaged can be taken. Detecting and repairing damages while still minor saves both time and money.

Both actual damages to the turbine and trends in its behavior are documented. This helps researchers and constructors plan reparations in a manner that ensures shortest possible down-time for large wind power plants.

Micro-Epsilon offers ultra-sensitive sensors that can use data to measure and evaluate different constructions performance. For wind turbines, a few factors are extra critical for their functionality: misalignment of the coupling ring, gap measurements on the plain bearing, temperature monitoring of the generator, and air gap monitoring in the generator.

Eddy-current sensors for determining misalignment of coupling bores

The gearbox and generator are therefore elastically mounted. The couplings in wind turbines must compensate for the relative movements of the gearbox and generator.. This determines the load profile.

The strong force at the high of 100 meters is making the wind turbines large rotor blades rotate, despite their heavy construction. The gearbox and generator are not fixed to the wind turbine but attached in a manner that allow them to rotate slightly. When measuring the coupling ring offset, the distance is measured with eddy current sensors on the metallic coupling ring. This helps researchers measure the constructions load profile. By monitoring these values, it’s possible to predict wearing of couplings, bearings, or shaft seals and, in extreme cases, serious damage to the wind turbine.

Measurements are taken in various directions: axial, radial and tangential. The eddy-current sensors of the EddyNCDT 3001 and 3005 series from Micro-Epsilon are temperature-compensated and offer high stability even with strongly fluctuating ambient temperatures. They are factory-adjusted to ferromagnetic or non-ferromagnetic materials, eliminating the need for on-site linearization.

Thanks to the factory calibration, the sensors offer high accuracy and temperature stability. The M12 thread, among other things, allows the sensors to be replaced quickly. They have a robust IP67 design and can be integrated into the smallest of installation spaces thanks to their compact design with integrated electronics.

Therefore, the sensors are mainly used for monitoring potential damages on the wind turbines and determining their current condition. Compared to inductive switches and sensors, these models provide a higher bandwidth and are therefore ideal for monitoring and accurately detecting fast movements.

Measuring the bearing gap in wind turbines

Hydrostatic bearings are used in many large plants such as stone mills, telescopic plants, and wind turbines. Their task is to measure the gap between the bearing surface and the shaft in real time. There is an oil film in the lubrication gap which prevents direct contact between the bearing surface and the shaft. In the event of a malfunction in the hydraulics, the oil pressure drops, and the gap can, in extreme cases, close. The result would be damage to the bearing, which in turn could lead to the entire system failing.

The sensor is placed on the side of the bearing shoe. It measures through the oil film and the plain bearing layer directly onto the shaft. Non-contact eddy-current displacement sensors are used to measure the current pressure, humidity, and temperature in the turbines.

Optical and capacitive sensors can detect unbalance in wind turbines

For very large generators or electric motors, it’s important to determine the concentricity of the rotor inside the motor relative to that of the stator. Unbalance in operation, which occurs in wind turbines due to wear caused by extreme wind and weather conditions, among other things, can cause the rotor to touch the stator. This can then result in serious damage. During the operation, the distance between the stator and the rotor, the so-called rotor gap, is monitored with optical, and capacitive sensors are always measured. Capacitive sensors use sensors with a measuring 0–8 millimeters.

Capacitive sensors from Micro-Epsilon are designed for non-contact displacement, distance, and position measurement. They are characterized by their long-term stability, reliability, and temperature stability. The average temperature of the air gap monitoring in the generator is about 120 °C. The sensors used have been specially optimized for measurements in the generator. They are particularly vibration-resistant and protected by a special housing.

In addition, the triaxial design allows flush installation in conductive materials. This is used as the protective ring electrode and the earthing are located at the front edge of the sensor in addition to the measuring electrode. According to the manufacturer, these capacitive sensors have a high electromagnetic compatibility. In addition, the sensors can be exchanged without being recalibrated.

Predictive maintenance can be used for more than wind turbines

By using sensors, repairs can be avoided, breakdowns reduced, maintenance cycles planned, and costs reduced. This is true not only on wind turbines but for almost all applications in most industries.

This article was first published in German by Elektronik Praxis.

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