Research On Fault Characteristic And Protection Strategy Of Wind Power Generaton With Doubly Fed Induction Generator

With doubly-fed induction generator of large scale connected to the grid, its fault transient characteristics are different from traditional synchronous machine, which produces some kind of threat to the safe operation of the grid. Therefore, it is very necessary to study the fault mechanism of DFIG and the influence to relay protection. Firstly, based on the research of mathematical model and power generation characteristics of DFIG, this paper proposes short circuit characteristic calculation methods of DFIG considering Crowbar not accessed, and flux transient characteristics and short-circuit current calculation method of DFIG considering Crowbar accessed. Then, the stator and rotor short-circuit current expressions under grid faults are deduced. On this basis, by analyzing wind farms fault characteristic to the impact of collector line relay protection, a new adaptive distance protection scheme is proposed in this paper and its effect is verified by simulation model.In the case of Crowbar not operation, a calculation method for short-circuit current of DFIG considering grid side and rotor side converter transient adjustment is proposed in this paper. First, DFIG fault equivalent network considering the rotor side converter control is established. On this basis, with the dc bus voltage fluctuations, double loop transient control process of the gird converter in the short circuit process is described quantitatively. Thus, the mechanism of the stator short circuit current with double frequency harmonic component is revealed, and the analytic expressions for stator and rotor full current are obtained. Then, based on the stator external characteristics of DFIG under grid fault, three-phase symmetrical fault analysis model of DFIG is studied. Finally, the simulation experiments verify the correctness of the derived expressions and related theories.In the case of Crowbar operation, a calculation method of the transient flux and short circuit current of DFIG considering Crowbar accessed is proposed in this paper. First, the positive and negative sequence equivalent network of DFIG accounting Crowbar is established, by using space vector method and coordinate transformation, the stator and rotor flux analytic formula which is almost no approximation is obtained. Then the variation law of characteristic root flux with the change of Crowbar resistance and the characteristics of flux amplitude phase with the change of motor speed is analyzed through theoretical calculation to explore the transient variation of each component of flux. On this basis, the stator short-circuit expression is further derived under grid symmetrical and asymmetrical. Finally, the time-domain simulation results verify correctness and effectiveness of the derived expression.Based on wind farms fault characteristic, its impact on relay protection and problems of existing protection configuration of the collector line are analyzed, and a new adaptive distance protection for wind farm collector lines is proposed. First, according to the fault voltage & current vector distribution in the system, the voltage drop equation between the fault point and the relay is established and the adaptive setting coefficient is calculated. Then, the fault location is determined by the phase relationship between the fault current and the measured current at the relay. Finally, an adaptive distance protection criterion is built. Simulation results show that the proposed method is able to modify the protection setting value adaptively, and less affected by transition resistance. Besides, it is simple information and easy to implement.