Operational Modal Analysis of the Idling CART3 Research Wind Turbine Using BladeVision Data

Wind turbine dynamics and especially rotor vibrations are of rising interest, since longer and slender rotor blades are more prone to vibrations. To understand the underlying phenomena, it is necessary to analyze vibration data from the rotor. Due to lightning protection sensor-based systems with classical accelerometers are rarely available. During the research project SmartBlades2 a rotor with a diameter of 40m was tested at the National Renewable Laboratory (NREL) in Boulder (Colorado) with a number of research instrumentation. Among these the camera-based BladeVision system which uses reflector marks installed inside the blades at four different spanwise positions with the last one installed at 75%. All three blades are measured synchronously which enables the usability for operational modal analysis. This paper focuses on the identification of the turbine in idle mode under different wind speeds and temperatures. In detail the application of different identification methods (SSI and LSCF) and their advantages and disadvantages will be discussed for dedicated modes. Altogether a number of 12 rotor modes could be identified within the frequency range up to 20 Hz. Results are shown as eigenfrequencies, damping ratios and mode shapes.