The stator of DFIG (double-fed induction generator) is connected directly to the power grid, and the rotor is connnected to the power grid through a back-to-back power electronics converter bridge. the power electronics bridge controls the frequency and amplitude of rotor voltage, so that the stator can generate the electric power with a constant-freuquency even the generator speed is not synchronous. A good feature of the doubly-fed system is that the power exchanging between the rotor side and the power grid is only a fraction of the power generated by the whole wind turbine system, the cost of power eletronics components is reduced. Therefore, many high-power wind generators adopts the double-fed topology.

Control of the DFIG system

The entire experimental system is shown as below. The right part is the DC motor (used to simulate the wind turbine in the laboratory) and the induction generator. The middle electrical cabinet contains a prime mover controller, the IGBT with its driving board, and relay switches, etc. The white box in the left part is PXI, it is used as the converter controller.

The control of double-fed wind power system includes the no-load excitation control and the maximal wind power tracking control.

no-load excitation control：The controller controls the frequency and amplitude of the rotor excitation current and enables DFIG to meet the grid connection condition (the amplitude, phase and frequency of stator voltage are as same as those of power grid).

maximal wind power tracking control：The controller controls the frequency and amplitude of the rotor excitation current and enables DFIG to realize the maximal wind power tracking with the variable speed constant frequency operation. The control algorithm is running on PXI after verified by StarSim Offline.

The picture above shows the generator rotor current and stator voltage waveforms of the grid-connected mode, when the motor speed is 20Hz (electrical speed) and the rotor current frequency is the slip frequency (50Hz-20Hz=30Hz). (Note: the control algorithm is running at 8kHz)