Investigation of voltage sag impact on wind turbine blade vibrations

Recently the size and the capacity of wind turbines increase rapidly, the interactions between the wind turbines and the electric grid becomes more obvious. The disturbances from the grid and the faults of the grid will have impact on mechanical components of wind turbines. In order to obtain the dynamic responses of the blade under voltage sag, the blade was modeled as a cantilever and discretized by applying 3D Timoshenko beam element, considering the centrifugal stiffening of the rotating blade. The structural dynamic equation of the blade was established and the time-varying loads acting on the blade were calculated based on the data from a 1.5 MW wind turbine LVRT (low voltage ride through) test. The dynamic responses of the blade under the three-phase symmetrical voltage dip and three-phase asymmetry dip were calculated by applying Newmark method. The results show that, the voltage dip produces the transient loads and excites the vibrations of the blade. The amplitude of vibration in the flap wise is bigger than that in the edgewise. The vibration increases with the increase of the load of wind turbine and the magnitude of voltage drop. The dynamic responses of the blade under the three-phase symmetrical voltage dip are larger than those under three-phase asymmetry dip.