Analysis, Modeling and Control of Doubly-Fed Induction Generators for Wind Turbines

This thesis deals with the analysis, modeling, and control of the doubly-fed induction generator (DFIG) for wind turbines. Different rotor current control methods are investigated with the objective of eliminating the influence of the back electromotive force (EMF), which is that of, in control terminology, a load disturbance, on the rotor current. It is found that the method that utilizes both feed forward of the back EMF and so-called “active resistance” manages best to suppress the influence of the back EMF on the rotor current, particularly when voltage sags occur, of the investigated methods. This method also has the best stability properties. In addition it is found that this method also has the best robustness to parameter deviations. The response of the DFIG wind turbine system to grid disturbances is simulated and verified experimentally. A voltage sag to 80% (80% remaining voltage) is handled very well. Moreover, a second-order model for prediction of the response of small voltage sags of the DFIG wind turbines is derived, and its simulated performance is successfully verified experimentally. The energy production of the DFIG wind turbine is investigated and compared to that of other wind turbine systems. The result found is that the energy capture of the DFIG wind turbine is almost the same as for an active stall-controlled fixed-speed (using two fixed speeds) wind turbine. Compared to a full-power-converter wind turbine the DFIG wind turbine can deliver a couple of percentage units more energy to the grid. Voltage sag ride-through capabilities of some different variable-speed wind turbines has been investigated. It has been found that the energy production cost of the investigated wind turbines with voltage sag ride-through capabilities is between 1–3 percentage units higher than that of the ordinary DFIG wind turbine without the ride-through capability. Finally, a flicker reduction control law for stall-controlled wind turbines with induction generators, using variable rotor resistance, is derived. The finding is that it is possible to reduce the flicker contribution by utilizing the derived rotor resistance control law with 40– 80% depending on the operating condition.