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 machine used as a wind turbine generator. The energy efficiency of wind turbine systems equipped with doubly-fed induction generators are compared to other wind turbine generator systems. Moreover, the current control of the doubly-fed induction generator is analyzed and finally the sensitivity of different current controllers with respect to grid disturbances are investigated.
The energy efficiency of a variable-speed wind turbine system using a doubly-fed induction generator is approximately as for a fixed-speed wind turbine equipped with an induction generator. In comparison to a direct-driven permanent-magnet synchronous generator there might be a small gain in the energy efficiency, depending on the average wind-speed at the site. For a variable-speed wind turbine with an induction generator equipped with a full-power inverter, the energy efficiency can be a few percentage units smaller than for a system with a doubly-fed induction generator.
The flux dynamics of the doubly-fed induction machine consist of two poorly damped poles which influence the current controller. These will cause oscillations, with a frequency close to the line frequency, in the flux and in the rotor currents. It has been found that by utilizing a suggested method combining feed-forward compensation and "active resistance," the low-frequency disturbances as well as the oscillations are suppressed better than the other methods evaluated.
The maximum value of the rotor voltage will increase with the size of a voltage dip. This means that it is necessary to design the inverter so it can handle a desired value of a voltage dip. For the investigated systems the maximum rotor voltage and current, due to a voltage dip, can be reduced if the doubly-fed induction machine is magnetized from the stator circuit instead of the rotor circuit. Further, it has been found that the choice of current control method is of greater importance if the bandwidth of the current control loop is low.