Doubly Fed Drives for Variable Speed Wind Turbines

This thesis deals with the use of variable speed wind turbines. Different wind turbine generator topologies are described. In particular, the reduced variable speed turbine, which uses a doubly fed induction generator, is covered. An overview of the power electronic inverters of interest to the field of wind energy is given and a topology for a laboratory model is selected. The discipline of Pulse Width Modulation is presented. The vector control principles of induction machines and grid connected inverters are derived. Having the stator connected direct to the grid gives that the flux level in the machine is nearly constant. This means that changes of either the flux or torque producing current in the rotor circuit is limited by the transient time constant of the machine. A 40 kW laboratory model with a doubly fed induction generator and a 3-level neutral point clamped back to back power converter is constructed. Adaptive active flters are used to reduce harmonics and slip harmonics in the stator current. The flters are implemented in both inverters. The active flters reduce the stator harmonics by 20-30 dB. The flters can reduce the slip harmonics at variable speed.