POWER ELECTRONIC SYSTEMS DESIGN

Wind turbine modelling using doubly-fed induction generators is a well-known subject.  However, studies have tended to focus on optimising the components of the system  rather than considering the interaction between the components. This research examines  the interaction of the control methods for a doubly-fed induction generator (DFIG) in a  wind turbine application integrating them with the crowbar protection control and DC
link brake control to make the best use of the converter.

The controls of the rotor-side and the grid-side converters of the DFIG model are both  well established and have been shown to work. Typically the crowbar protection is  designed in order to protect the rotor-side converter and the power electronic  components of the DFIG system from high currents occurring in the rotor due to the  faults. The DC-link brake-overvoltage protection is also designed to prevent the  overcharging of the DC-link capacitor placed between the rotor-side converter and the  grid-side converter. In order to show that these protection schemes work and with  thought can co-ordinate with each other, tests consisting of a number of balanced three-,  two- and one-phase voltage sags are applied to the network voltage.

The main contributions of this thesis are establishing operational tuning and design  limits for the controllers and system subassemblies. This is to minimise the electrical  subsystem interaction while maintaining adequate performance, and have an improved  DC-link control. This work also includes a full electrical system study of the wind  turbine and an essential literature review on significant references in the field of the  DFIG wind turbine system modelling, control and protection.

Specifically this research project makes a number of novel contributions to the  literature: enhanced DC voltage control including operating point sensitivity analysis  and dynamic stiffness assessment, sensitivity and robustness analyses of the power loop  control and control loop segmentation by appropriately tuning the controller loops.

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