Unstable torsional vibration of wind turbines caused by nacelle yawing

In order to reveal the cause and mechanism of the vibrations, a torsional vibration model of tower was set up based on the structure and principle of the yawing system. The motion equation of the tower was established and analyzed. The result shows that the observed violent vibration is an unstable torsional vibration of the towers caused by dry friction between nacelle and tower. The mechanism of this instability is that the yawing brakes exert large friction moment on both surfaces of tower flange. Because the difference of static friction and moving friction coefficient, the decrement of friction moment becomes destabilizing moment driving the tower into unstable torsional vibration driven by yawing motors when the wind turbines yaw. A threshold of the instability is defined by the friction destabilizing factor described in this paper. Simulating a real type of wind turbines gives a confirmation of this instability. The results suggest that reduction the friction moment of the yawing brakes by reducing the moment in pressing bolts in the brakes can suppress the unstable vibration. Another solution may use a rolling-element bearing in yawing systems.