Optical monitoring and operational modal analysis of large wind turbines

Abstract

Identification of the dynamic properties and the corresponding structural response of wind turbines is essential for optimizing the energy produced, ensuring safe and reliable operation and increasing the life-time of the system. As the sizes of modern wind turbines increase, their dynamic behaviors get more complicated and it becomes more important to predict the response characteristics of new designs through simulations. Modern computation and simulation tools provide designers with great opportunities to detect and solve most of the possible problems at very early stages and improve their designs. Indeed, several important system properties such as eigenfrequencies and mode shapes, which govern the dynamic response of the turbine, can be estimated very accurately by using structural analysis programs. However, some important dynamic parameters (e.g. damping) cannot be modeled precisely without supplementary information obtained from in-field tests and measurements. Considering the fact that only the models validated by real response measurements are able to represent the complicated dynamic behavior of the structure, various tests have been applied on both parked and rotating turbines for several decades. However, some further improvements are still needed for testing and analyzing the dynamic characteristics of these specific structures in an accurate and efficient way. This thesis aims at making a contribution to this challenging field of experimental and operational modal analyses through several aspects; - Two non-contact optical measurement systems (laser interferometry and photogrammetry) are proposed as alternative turbine monitoring systems. In Chapter 2 and 3, it is demonstrated that optical measurement systems enable the dynamic response of the turbine to be measured with a high precision and spatial resolution both at parked condition and in operation. The pros and cons of the methods and the acquired accuracies are discussed in detail. - In Chapter 3, the vibration data recorded on a 2.5 MW -80 meter diameter- wind turbine by using 3 different measurement systems (laser interferometry, photogrammetry and conventional strain gauges) are analyzed by using modal analysis algorithms based on NExT (Natural Excitation Technique) and LSCE (Least Square Complex Exponential) techniques. Several important turbine parameters (eigenfrequencies and damping ratios) are extracted and compared with the results presented in literature. - In Chapter 4, the main challenges in testing and analyzing the in-operation vibration characteristics of wind turbines are discussed in detail. The factors affecting the accuracies of the estimated modal parameters and the applicability limits of some state of the art system identification tools are examined. In order to investigate specifically the performance of the identification algorithms, numeric response data generated by an analytical model and an aeroelastic simulation tool were used. - In Chapter 5, an alternative method (based on NExT) is proposed for identification of the systems with high modal damping. The introduced technique aims at improving the capabilities of NExT in extracting the highly damped eigenmodes such as the aeroelastic modes of an operating wind turbine. It is demonstrated that the proposed approach enables the eigenfrequencies of the high damping modes to be estimated by using data series which are approximately 30 times shorter than those required for standard NExT algorithm. Results of the analyses show that eigenfrequencies of highly damped modes can be estimated with an average accuracy of 95%. The stability of the proposed method and the possible effects of measurement noise on the estimated modal parameters are also investigated in Chapter 5.