Study On Single - Phase Asymmetrical Low - Voltage Crossing Of Doubly - Fed Wind Power Generator

With the increasing capacity of wind farms connected to grid, the impact between wind farms and grid is increasingly evident, especially when the grid voltage drop. In order to regulate the wind power accessed to the power system, various countries all formulate electricity supply network technical specifications associated with wind turbines based on individual circumstances.Currently, research on the LVRT technology of DFIG mainly focused on symmetrical grid faults, while in actual operation, the probability of asymmetry grid fault is even bigger(the single-phase grounding fault is 70%, the two-phase grounding fault is 15%, the phase to phase grounding fault is 10%, and the three-phase grounding fault is 5%), therefore, this paper analysis single-phase asymmetric fault. Analyze its electromagnetic transition process under single-phase asymmetric voltage drop, and propose appropriate control strategy, to satisfy the LVRT standard.To facilitate the research of the characteristic of DFIG under single-phase asymmetric voltage drop, on the basis of studying the structure and fundamental principle of DFIG, established the dynamic mathematical model under single-phase asymmetrical voltage drop. And through the state space equation which describes the electromagnetic transient process of DFIG with the current as variable, according to the principle of superposition, the transient process of single-phase asymmetric voltage drop is equivalent to the superposition of the stable operation and the operation condition of stator side adding with the asymmetric reverse voltage. Then obtain the analytical results of DFIG stator and rotor current dq component under single-phase asymmetrical voltage drop according to the Laplace transform and its inverse transformation. According to the protection requirements of the rotor side converter of DFIG, researched the analytical expression of DFIG stator and rotor each phase current through coordinate transformation; according to the LVRT requirement, DFIG must provide reactive power support to grid under the voltage sags, researched the transient characteristics of the electromagnetic torque and output electromagnetic power of DFIG under single-phase asymmetrical voltage drop. The results of the research show that, transient characteristics is related with rotor side excitation voltage of DFIG, which effect the transient characteristics mostly is the harmonic component. Therefore, established the state space equation of DFIG under single-phase asymmetrical voltage drop when the rotor excitation voltage can be adjusted, and researched the transient characteristics of current, electromagnetic torque and power. Using it as a base, proposed the LVRT control strategy by controlling the rotor side excitation voltage to suppressing the harmonic component, given the analytical results of the control voltage through the research on the transient characteristics of current. Then researched the stator and rotor current, electromagnetic torque and power of DFIG in the LVRT under this control strategy further.On the basis of theoretical research, based on Matlab simulation platform and the single-phase asymmetric LVRT mathematical model of DFIG, built the single-phase asymmetric LVRT simulation model of DFIG. Both of theory analysis and simulation results show that under single-phase asymmetrical voltage drop, the control strategy of this paper can not only effectively eliminating harmonic component of current, inhibiting the doubly-frequency component of electromagnetic torque and power, realizing the protection of the DFIG transmission system, but also can providing reactive power support for grid, meeting the requirements of LVRT.