The Research On Control Strategy For Low Voltage Ride Through Of DFIG Under Unbalanced Grid Conditions

As the increased capacity of wind power generations, the effect on the stability of wind power generation to the grid system gradually increases. So the wind turbine must have good anti-jamming characteristic. It also called low voltage ride through. Unbalanced gird which often happens in daily life is caused by asymmetric voltage fault. So today’s research has gradually turned to the unbalanced power grid under continuous operation.The traditional vector control strategy is capable of performing very well under the balanced grid conditions. But this control strategy could not ensure the complete decoupling of control system under unbalanced grid conditions which could lead to the weakening control ability due to the existence of negative sequence components. Moreover the negative sequence components could also cause the oscillation of stator/rotor current and power output with the twice frequency of the fundamental. The negative sequence components and DC components presented after voltage dip are the crucial factors which cause the over current and voltage of the rotor winding.Doubly-fed induction generator(DFIG) is widely applied to wind power system. Because of its stator connect power grid directly, the generator is extremely sensitive to the grid voltage fault. In order to improve the low voltage ride through capability of DFIG, the mathematical model of a DFIG is firstly built. And the dynamic behaviors of DFIG during asymmetrical grid fault are analyzed. After the theoretical analysis, the rotor side and the grid side converters are modeled based on double d-q positive and negative sequence decomposition of 1.5 MW DFIG in Simulink. A new control strategy which is combined vector control based on double d-q positive and negative sequence decomposition with Crowbar circuit is proposed. The simulation results are compared with the traditional one. The results show that the oscillation of power output could be effectively restrained under with this improved control strategy. The endurance ability and operating stability are also enhanced.