Fiber-optic blade monitoring for loads reduction and condition-based maintenance
Ton Veltkamp1, Jan Willem Wagenaar
1, Frank Kaandorp1, Peter Kooren4, Maarten van Balveren2, Gert Kwikkers3, Mar van der Hoek5
1ECN, Petten, The Netherlands, 2Voestalpine SIGNALING Siershahn GmbH, Waddinxveen, The Netherlands, 3Railstress Consultancy B.V, Bergen, The Netherlands, 4Technobis tft-fos, Alkmaar, The Netherlands, 5vanderhoekPhotonics, Vlaardingen, The Netherlands
Loads monitoring can be used for individual pitch control (IPC) and condition based maintenance (CBM), decreasing the overall loads on wind turbine and maintenance requirements. In this way, the costs of wind energy can be reduced significantly. Fiber-optical blade load monitoring (FOBM) is a promising alternative for common electrical strain gauges due to better long-term stability and ease of installation and calibration. Accurate and maintenance-free sensors for monitoring blade root bending moments are a necessity for implementing control algorithms based on blade loads. In the Dutch LoadWatch R&D project, innovative FOBM equipment and software is under development. In this presentation, recent improvements in the fiber-optics configuration, sensor design and interrogator are discussed in terms of applicability, reliability and cost reduction. Results collected from laboratory tests and field campaigns in 3 different, 2-3 MW sized wind turbines demonstrate the large potential of the FOBM system for IPC and CBM.
The sensor consists of 3 basic components: a pre-assembled optical fiber sensor designed for easy installation in the blade root, the interrogator in the hub and software for data acquisition, filtering and analysis. The sensor assembly is based on ECN's patented plug-and-play design. Two most promising interrogator configurations have been tested. The software determines every 10 minutes which operational mode has occurred (power production, start-up, shutdown, emergency shutdown, etc.) and filters erroneous data. The software generates monthly reports with a.o. statistics, capture matrices, and load spectra. This can be used to assess the consumed lifetime of the wind turbine components. FOBM systems have been laboratory tested and installed for long-term field studies in 2-3 MW wind turbines of 3 different turbine manufacturers.
The pre-assembled and pre-calibrated sensor assemblies are connected to the FOBM system as "plug-and-play" in the blade-root. The temperatures were measured with the optical sensors with PT-100 element for comparison. The average difference between the two sensors was 0,46 °C and remained constant over the measurement period. The fiber-optical temperature measurements are therefore considered as reliable and adequate for the purpose for which they are designed. A fair agreement was found between optical and electrical strain measurements, with a maximum difference between the optical and electrical strain gauge glued under the FOBM sensor of less than 4%.
The FOBM system has been extensively tested, both in laboratory tests and in 3 long-term outdoor field campaigns. We conclude that the FOBM system is more versatile and more reliable as compared to electrical strain gauges for long-term monitoring of blade root bending moments. The sensor assembly allows for very easy installation in the rotor blade as a retrofit or into new blades, and does not require frequent maintenance or on-site calibration. It is therefore perfectly fitted to enable for IPC and CBM in off-shore windparks in support of lowering the costs of wind energy.
Easy auto-calibration of sensors when commissioning the turbine or after replacement is a huge improvement, saving engineering effort and shortening time to re-start the turbine. Measuring the mechanical loads will inform operators and OEM's about the degradation of turbine components and help to prioritize the need for maintenance. ECN is developing algorithms that combine blade root bending moments with turbine geometry data and SCADA data (e.g., pitch angle, azimuth angle) to determine the loads for other main turbine components. In a favorable configuration, two or three turbines from an off-shore windpark (the Fleet Leaders) are instrumented with FOBM systems. With ECN's Fleet LeaderTM software  the loads on all other turbines in the windpark can be derived from the FOBM data from the Fleet Leaders.