Fiber-Optical Sensors for Condition Monitoring of Wind Turbines

The Potential of Fiber-Optical Sensors for CMS of Wind Power Plants

Condition Monitoring Systems (CMS) are designed to ensure the long-term efficiency of wind power plants. In the future they will also monitor important components such as rotor blades, towers and foundations. The focus moves here to optical sensor technology as an alternative to electrical technology. Structural and condition monitoring on edifices such as bridges and buildings has been the subject of research and development for a long time already. HBM has been a specialist in Experimental Stress Analysis for more than 6 decades, providing precisely matched solutions for this task along the entire measuring chain. Numerous results of examinations have already been published in this area. [1], [2], [3]

CMS to enable fail-safe maintenance

The purpose of Condition Monitoring Systems (CMS) in Wind Power Plants (WPPs) is to ensure the long-term efficiency of the plant. They also make it possible for operators to practice "fail-safe maintenance," which is based primarily on the loading history that actually occurred. This type of CMS has been mandatory for offshore wind energy systems since 2005 according to the GL Offshore Guideline [4], but is initially required only for the drive train. The various certifying entities (for example DNV GL Renewables Certification) and other relevant expert groups look forward to expanding CMS to monitor other important components in wind energy systems in the future. That includes rotor blades, towers and foundations, for example. [5]; [6] The focus moves here to optical sensor technology as an alternative to electrical technology. Optical sensor technology features many advantages which have already served to establish this approach to a great extent:

The high stability with alternating loads of fiber-optical sensors results in a long service life (relevant issue: high strain). [7]
There is no problem with lightning strikes and thus no danger of the measurement technology and electronics being destroyed.
EMC interference does not occur (electromagnetic signal interference and ground loops, etc.).
The advantage of fiber optic cable compared to copper connecting cables leads to lower costs and material overhead as well as reduced weight.

Significant increase in information content

Because of these advantages, use of fiber optics makes it possible to install sensors distributed over the blade, which significantly increases the information content. This is possible mainly because of the multiplex capability of signal acquisition and routing in fiber optic lines. Hybrid solutions can also be created with a suitable combination of fiber-optical and electrical sensors. These systems are able to record changes very rapidly. [8] Strain gauges are used in most cases to monitor crucial areas such as the tower and foundation (structural dynamic parameters and monitoring of the rigidity matrix). In offshore wind energy systems, however, the sensors must withstand a wide variety of interference effects due to the maritime environment, which requires special care in designing the application and corresponding technical complexity. [9]

Resistant against moisture

For optical technology, in contrast to resistance-based technology, the total complex of interference effects due to moisture is not relevant. OptiMet by HBM, the fiber grid technology offered by HBM, is available in the form of coated fiber (OptiMet PKF with multiple Bragg grids in a chain). [10] The costs per measurement channel in the measuring system for fiber-optical sensors are reduced as the number of connected sensors increases. It is possible to arrange a number of Bragg grids one after the other in a fiber-optical chain (typically 8 to 13). At the same time this reduces the wiring outlay compared to electrical strain gauges by the corresponding factor and the analysis device (interrogator) is used even more effectively. The potential of fiber-optical sensors has its maximum effect from the perspective of a holistic approach. That makes optical solutions an appealing approach with a promising future and possibilities for future expansion.

References:

[1] Henke, V. "Monitoring the Reichenbach and Albrechtsgraben viaducts"; RAM; Reports in applied measurement, No. 1/2007; pages 10-20, Darmstadt, 2007

[2] Liebig, J. P.; Menze, O.: "Keeping an eye on the effects of heavy goods traffic: long-term monitoring of a prestressed concrete bridge," HBM application report 10/2009, Darmstadt, 2009

[3] Gommola, G.; "Are our bridges safe? bridge monitoring with measurement technology from HBM," pp. 22-23; HBM customer magazine "hotline" issue 1/2012

[4] Germanischer Lloyd: Guideline for the Certification of Offshore Wind Turbines, 2005

[5] Steingröver, K.; et al. "Condition Monitoring Systems for Wind Turbines: Current status and outlook on future developments from the perspective of certification"; VDI report on "Vibrations in wind turbines," Bremen, 2010

[6] Steingröver, K.; et al. "CMS für Windenergieanlagen aus Sicht der Zertifizierer" [CMS for wind energy systems from the point of view of the certifier] "ECONOMIC ENGINEERING" Journal, issue 5/2012, Göller Publishing house, Baden-Baden

[7] Frieling, G.; Walther, F.: Tensile and fatigue properties of Fiber-Bragg-Grating (FBG) Sensors. In: Sensors & Transducers Journal 154 (2013), No. 7, p. 143-148

[8] Zerbst, S.; Knops, M.; Haase, K.-H.; Rolfes, R.: "Schadensfrüherkennung an Rotorblättern von Windkraftanlagen" [Early detection of damage on rotor blades of wind power plants], Lightweight Design issue 2010-04, Vieweg +Teubner, Wiesbaden

[9] Haase, K.-H.: Underwater application of strain gauges, UK Environmental, 2004.

[10] HBM/HBK Website, 2014. OptiMet by HBM

(jv)

* Dr. Karl-Heinz Haase, Product and Application Manager Optical Technology & Asset Monitoring; Dr. André Schäfer, Product and Application Manager Calibration/Wind Energy; Hottinger Baldwin Messtechnik GmbH

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