Modelling of a controlled doubly fed induction machine for the use in offshore wind power plants

After a brief overview of the actual state of wind energy and planned offshore projects in Germany, state of the art wind power generators are described. It is explained, that a favourite generator system for such offshore applications is the doubly-fed induction machine (DFIM), due to its advantages in weight and operation. Furthermore, a state-space-equation-model of the DFIM in field-oriented coordinates is derived and discussed. This coordinate system offers the possibility of decoupled control of active and reactive power. The structures of the control loops are presented. To enable an effective simulation of large wind farms, two reduction steps can be performed: 1) The differential equation of the rotor is reduced by compensating the transient rotor time constant with the rotor current control. 2) Furthermore, the differential equation of the stator is reduced by means of the singular perturbation method. The derived first order model can appropriately describe the dynamics of controlled DFIM in multi machine installations.