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Öğe A new analysis methodology for estimating the eigenfrequencies of systems with high modal damping(Academic Press Ltd- Elsevier Science Ltd, 2016) Ozbek, Muammer; Rixen, Daniel J.Contemporary system identification algorithms are well proven to provide accurate eigenfrequency estimates in analyzing the systems with low modal damping. Since most engineering structures usually have low damping ratios the corresponding response characteristics can easily be obtained by conventional methods. Indeed, these modes can be extracted by using relatively short measurement durations (150-200 cycles of the lowest frequency included in the data block). However, some specific applications such as analyzing the in-operation vibration behavior of MW scale large wind turbines also require an accurate estimation of the modes with high damping. For a rotating wind turbine, some important turbine modes (e.g. flapwise rotor modes) have very high aeroelastic damping, which make them very difficult (if not impossible) to be detected. Extracting these high damping modes is a challenging task for almost all system identification techniques that are currently in use. In this work, a new method, which is based on Natural Excitation Technique (NExT), is proposed as an alternative approach for extracting the eigenfrequencies of high damping modes in an efficient way. NExT is a well established experimental dynamic analysis tool which was specifically developed to extract the dynamic characteristics of wind turbines in the early 90s. However, during the analyses it was observed that conventional NExT algorithm requires analyzing very long measurement durations (4500-5000 cycles) to be able to estimate the high damping modes accurately. A new method proposed in this work enables the eigenfrequencies of high damping modes to be estimated by using data series which are approximately 30 times shorter (around 150 cycles) than those required for a standard NExT algorithm. (C) 2015 Elsevier Ltd. All rights reserved.Öğe Stability Control of Wind Turbines for Varying Operating Conditions Through Vibration Measurements(Springer Int Publishing Ag, 2015) Ozbek, Muammer; Rixen, Daniel J.Wind turbines have very specific characteristics and challenging operating conditions. Contemporary MW-scale turbines are usually designed to be operational for wind speeds between 4 and 25 m/s. In order to reach this goal, most turbines utilize active pitch control mechanisms where angle of the blade (pitch angle) is changed as a function of wind speed. Similarly, the whole rotor is rotated toward the effective wind direction by using the yaw mechanism. The ability of the turbine to adapt to the changes in operating conditions plays a crucial role in ensuring maximum energy production and the safety of the structure during extreme wind loads. This, on the other hand, makes it more difficult to investigate the system from dynamic analysis point of view. Unexpected resonance problems due to dynamic interactions among aeroelastic modes and/or excitation forces can always be encountered. Therefore, within the design wind speed interval, for each velocity increment, it has to be proven that there are no risks of resonance problems and that the structure is dynamically stable. This work aims at presenting the results of the dynamic stability analyses performed on a 2.5-MW, 80-m-diameter wind turbine. Within the scope of the research, the system parameters were extracted by using the in-operation vibration data recorded for various wind speeds and operating conditions. The data acquired by 8 strain gauges (2 sensors on each blade and 2 sensors on the tower) installed on the turbine were analyzed by using operational modal analysis (OMA) methods, while several turbine parameters (eigenfrequencies and damping ratios) were extracted. The obtained system parameters were then qualitatively compared with the results presented in a study from the literature, which includes both aeroelastic simulations and in-field measurements performed on a similar size and capacity wind turbine.Öğe Structural Health Monitoring of Multi-MW-Scale Wind Turbines by Non-contact Optical Measurement Techniques: An Application on a 2.5-MW Wind Turbine(Springer Int Publishing Ag, 2015) Ozbek, Muammer; Rixen, Daniel J.Optical measurement systems utilizing photogrammetry and/or laser interferometry are introduced as cost-efficient alternatives to the conventional wind turbine/farm health-monitoring systems that are currently in use. The proposed techniques are proven to provide an accurate measurement of the dynamic behavior of a 2.5-MW, 80-m-diameter wind turbine. Several measurements are taken on the test turbine by using four CCD cameras and one laser vibrometer, and the response of the turbine is monitored from a distance of 220 m. The results of the infield tests show that photogrammetry (also can be called as computer vision technique) enables the 3-D deformations of the rotor to be measured at 33 different points simultaneously with an average accuracy of +/- 25 mm while the turbine is rotating. Several important turbine modes can also be extracted from the recorded data. Similarly, laser interferometry (used for the parked turbine) provides very valuable information on the dynamic properties of the turbine structure. Twelve different turbine modes can be identified from the obtained response data. The measurements enable the detection of even very small parameter variations that can be encountered due to the changes in operation conditions. Optical measurement systems are very easily applied on an existing turbine since they do not require any cable installations for power supply and data transfer in the structure. Placement of some reflective stickers on the blades is the only preparation that is necessary and can be completed within a few hours for a large-scale commercial wind turbine. Since all the measurement systems are located on the ground, a possible problem can be detected and solved easily. Optical measurement systems, which consist of several CCD cameras and/or one laser vibrometer, can be used for monitoring several turbines, which enables the monitoring costs of the wind farm to reduce significantly.
