Sensorless control for active damping of torsional vibrations in wind turbine drivetrains with doubly-fed induction generator

Until now, the most common drivetrain concept for multi-megawatt wind turbines is still the classic design with multi-stage gearbox and fast running doubly-fed induction generator (DFIG). Some control strategies for damping of drivetrain torque oscillations already exist, but they based on mechanical sensors, e.g. speed and rotary encoders which can fail within the harsh environment of a wind turbine. To reach the highest possible system reliability it is obvious trying to minimize the number of installed sensors. This contribution shows an innovative sensorless control method with great performance for DFIG coupled to highly oscillatory mechanical drive systems, such as in wind turbine drivetrains. The aim of the approach is, to reduce wear and increase service life by means of active damping torsional vibrations in the mechanical drivetrain during normal operation using a high reliable sensorless control system for the converter-fed generator. The outstanding feature of the control method is, that it requires no sensors for rotational angle, speed or shaft torque and considers the inherent highly oscillatory characteristic of geared drivetrains mostly equipped with DFIG. Therefore measuring of electrical quantities, that is the stator phase voltages and currents and rotor phase currents of a DFIG is sufficient, when using a combination of different observer structures for the electrical and mechanical subsystem of the drivetrain. The outcome is a sensorless field oriented vector and a state controller for effective active damping. The presented algorithms are tested on a laboratory test rig, some of the results and measurements are presented here.