Experimental investigation of yaw damping on a downwind turbine

Abstract

Free yaw downwind wind turbines follow the wind as the wind direction changes. This yaw motion can induce gyroscopic loads, dynamic blade responses and teeter motion in two-bladed teetered rotors. At least three types of problems where yaw damping might be beneficial have been observed. First, during periods of below cut-in wind speeds in complex terrain, variable winds may cause the turbine to yaw preferentially in one direction, possibly damaging droop cables. Second, during operation when the wind speed varies about cut-in, the turbine can yaw up wind, requiring a shut down. Third, during normal operation in higher winds, a sudden change in wind direction or excessive turbulence or wind shear can result in excessive teeter motion. In this study a hydraulic yaw damper has been installed on a two-bladed, teetered, down-wind turbine on a ridge top in Massachusetts. Its operation has been monitored and analyzed to determine if it can reduce or eliminate these problems. Test results focus on the performance of yaw damper itself and changes in the performance and dynamic response of the wind turbine. Output from both YawDyn and a specifically designed damper modeling code were compared with experimental results. The experimental and modeling results do not provide evidenced to indicate that the present yaw damper can be expected to reduce cable wrapping when the rotor is parked nor yawing up wind during low wind operation of a teetered rotor turbine. It may yet prove effective for rapid direction changes on a teetered rotor or for a rigid rotor turbine. Tests reported on in this paper concerned normal operation when winds originate in the