Correlation between Rotating LIDAR Measurements and Blade Effective Wind Speed

Preview wind speed measurements from a forward looking Light Detection and Ranging (LIDAR) system located in the hub of a wind turbine can be used by a feedforward blade pitch control system to mitigate structural loads. For individual blade pitch control, a separate preview estimate of the effective wind speed encountered by each blade must be available. One way of providing an estimate of the blade effective wind speed is to implement a hub mounted spinning lidar that scans the wind field at the rotational rate of the rotor such that the measured wind will reach the blade after some delay. In this way, both the lidar measurement and the wind turbine blade rotationally sample the wind field. The benefit gained by using preview wind speed measurements strongly depends on the correlation between the measured wind and the wind that interacts with the blades. In this research, the coherence between rotating lidar measurements and blade effective wind speed is calculated and analyzed using a spectral model of the wind field. The simulated wind field uses an isotropic von Karman spectrum and contains a model of wind evolution described by a longitudinal spatial coherence function. The coherence between stationary measurements and stationary blade effective wind speeds decreases to zero near the 1P rotational frequency of the turbine. However, measurement coherence between rotating lidar measurements and blade effective wind speed remains much higher and does not decay until higher frequencies.