Torque transient alleviation in fixed speed wind generators by Indirect Torque Control with STATCOM

Gearboxes for wind turbines must ensure high reliability over a period of 20 years withstanding cumulative and transient loads. Grid disturbances affecting wind turbines with induction generators directly connected to the grid generates electromagnetic torque transients that will result in significant stresses and fatigue of the gearbox. This paper presents a technique by which the transient torques during recovery after a grid fault can be smoothed for fixed speed wind turbines with induction generators directly connected to the grid. The technique labeled indirect torque control (ITC) is suggested in order to reduce the drive train mechanical stresses caused by the characteristics of the induction machine when decelerating through the maximum torque region. The basis of the approach consists of controlling the induction generator terminal voltage by the injection/absorption of reactive current using a STATCOM. By controlling the terminal voltage with the STATCOM, the electromagnetic torque of the generator is indirectly smoothed. The control concept is shown by simulations on a wind generation system model built in PSCAD, where the smoothing effect of the proposed technique is seen during the recovery after a three phase to ground fault condition. The influence of shaft stiffness and mutual damping on the proposed control is further investigated on a two mass model of the wind generation system.