Instability character study on the torsional vibration of wind turbine drive system caused by control system

Based on the experience obtained from diagnosing and dealing with a violent torsional vibration in two 1500 kW-wind turbines in a wind park, the models of torsional vibration for doubly fed wind turbines and directly driven wind turbines are set up, the torsional vibration and its stability of drive train of the models are investigated. In the investigation, the influences of two control modes on the stability of torsional vibration of the drive train are revealed. One mode is that the load torque of generator is controlled in terms of rotational speed of the generator, and another one is based on the rotational speed of turbines. The results shown that the torque control of a generator using the rotational speed of the generator will enhance the stability of the drive train, while the control based on the rotational speed of turbines will provide with destabilizing torque which will lead to an unstable torsional vibration of the turbines as the turbines run near to rated speed. Therefore it is suggested that the input signal for the torque control of generators should be always the rotational speed of the generators for any type of wind turbines (no matter doubly fed wind turbines or directly driven wind turbines).