Research On Control Of The DFIG Under Unbalanced Grid Voltage Conditions

Due to the VSCF operation of doubly fed induction generator, it has been widelyapplied in wind power. However, many wind farms connected to electrical main gridsare relatively weak as they are in remote areas, thus the grid voltage is fluctuant andimbalanced, which make the doubly fed induction generator running unsteadily andcause serious harm to doubly fed induction generator. The detection of grid voltagesignal and the control strategy of doubly fed induction generator are analyzed in thispaper when the grid voltage is unbalanced.The space vector model of doubly fed induction generator is established, whichuses the control strategy of the stator voltage oriented vector. When the traditionalcontrol strategy is applied to the doubly fed induction generator under the condition ofunbalanced grid voltage, the output current of the rotor and stator, and the active andreactive power will appear the phenomenon of secondary impulse by the simulationsoftware of MATLAB/Simulink. This will affect the normal operation of the doublyfed induction generator seriously, and also lead to the increased loss, the torquerippled, the fever increased, and the pulsation of reactive power of doubly fedinduction generator.The voltage and current of stator and rotor of the doubly fed induction generatorwould contain negative sequence component as power grid voltage is unbalanced, sothe negative sequence component should be considered according the control of rotorcurrent. The mathematical model of the doubly fed induction generator is establishedin this paper, which is based on the positive and negative rotating coordinating systemcontaining positive and negative sequence. The control of output active power,reactive power, and electromagnetic torque of the stator of doubly fed inductiongenerator can be realized through controlling the positive and negative sequencecurrent. The control system is designed and optimized, when ignoring zero sequencecomponents, and the control volume are DC component in rotating coordinate system.The problem that the detection of the phase angles with traditional phase-lockedloop under imbalanced grid voltage, which can not accurately track the grid voltagephase angle, and also can not separate the positive and negative sequence componentof the grid voltage, a new and improved method is proposed in this paper. With thecondition of the unbalanced grid voltage, the changing frequency of grid voltage, andthe harmonic distortion of grid voltage, the accurate tracking and detection of phaseangle, separating the positive and negative sequence component of grid voltage isachieved. The simulation results show that the improved method is valid.