Negative sequence control of DFG based wind turbines

The paper deals with the control of negative sequence voltages and currents in wind turbine systems caused by grid fault or unsymmetrical system operation. The ensuing stator and rotor currents lead to additional thermal stress. Moreover, the interaction between the different sequence components of the current and voltage in the stator as well as rotor cause oscillating torque leading to mechanical strain on the drive-train. A control approach for limiting or eliminating the negative sequence current and the resulting alternating torque is discussed. This is followed by the description and derivation of the rotor side converter (RSC) for the positive as well as negative sequence current controllers. The procedure is repeated for the grid side converter (GSC), and the limitations imposed on the controllers by practical operational considerations are explained. On the basis of simulation examples using representative wind turbine system data, the effectiveness of the proposed control methods has been demonstrated.