Importance of lidar measurement timing accuracy for wind turbine control

A turbine-mounted lidar can measure wind speed ahead of a wind turbine, and this preview measurement can be used to improve turbine control performance by reducing structural loads and/or increasing power capture. Effective lidar-based control requires not only an accurate wind speed measurement, but also knowledge of the expected arrival time of the measured wind. Arrival time is the time it takes for the wind to travel from the measurement focus location to the turbine rotor. Typically, arrival time is assumed to be equal to the distance traveled divided by the average wind speed. Field test data show that this assumption can be improved on average through an induction zone correction. In addition, arrival time can temporarily deviate significantly above or below this average value. If we can anticipate how arrival time will change, we can improve control performance. In this study, we post-process turbine and lidar data to show how arrival time varies and to determine an upper limit on possible improvement as a result of accurately predicting arrival time. Results show that this upper limit is a 26% average increase in coherence bandwidth between the measured wind and the wind that arrives at the rotor. In above-rated wind speeds, for example, this corresponds to a 21% improvement in the performance cost reduction due to incorporating lidar into a blade pitch controller, where the performance cost is a combined measure of generator speed error and blade pitch actuation.