| With the dramatic increase in installed capacity of wind power, wind farms have a significant affect on the power grid when wind velocity has a fluctuation or wind farm inner and outer failure occurs. To guarantee the security and stability of the power grid, it is urgent to analyse the impact of wind farm to the grid, therefore, the generator model and wind farm model should be elaborated and research in-depth.DFIG (Doubly Fed Induction Generator) has a dominating position in wind power transformation. The model and control configuration of DFIG formulated specifically. Based on these, the formulas of controller parameters, both rotor side and grid side, have been achieved according to zero-pole regulation and frequency domain analysis. These formulas can provide theoretical foundation for control parameters calculation that belongs to generator unit and equivalent unit.Based on the generator model, the research about wind farm modeling and its equivalence method have been launched. The change of parameters of DFIG after equivalence has been analysed, and the transfer formulas for parameters from generator unit to equivalent unit have been provided. For electric and mechanical parameters, capacity weighting method has been used. For control parameters, according to the obtainded calculation formulas, the transfer formulas for control parameters have been provided based on actual value and per unit value, respectively. The simulation result justify the validty of the aggregation method.The LVRT(Low Voltage Ride Through) conrol stratgy combined with active Crowbar and demagnitizing control has been payed great interests and got widely used, because it can not only minimum the duration that Crowbar put into operation, but also can make the generator to generate reactive power during the failure. Based on the theoretical analysis for the dynamic behavior of DFIG during the failure, the impact of positive, negative and zero sequence has also been considered, then, the model of DFIG demagnitizing control has been established. The rotor side controller has been protected according to the compensation, which is based on measureing the component of positive, negative and zero sequence. The criterion for Crowbar switching has been elucidated in detail, which can provide foundation for group division that must be conducted in subsequent procedure.The regional and multiple wind farm cascading trip-off induced by wind farm inner failure have caused serious impacts on the grid. Based on the demagnetizing control of LVRT, and the criterion for the group dividing was the switching of Crowbar circuit after the fault. First, using generator state equation to calculate the stator flux at the time that fault occurs but before the Crowbar switching. The impact of rotor side converter has been taken into consideration, and the rotor current was not ignored. And then, combined with the switch criterion of demagnetizing control strategy, the demagnetizing rotor current was obtained. Finally, the group division was done by estimating whether the Crowbar was acted. After analyzing the characteristics of wind farm inner failure, the original collector network did not need to be changed, but using nonlinear programming to calculate the access point of equivalent generator. The example and simulation demonstrated that the method described in this thesis can accurately reflect the characteristics of the wind farm before and after the fault. Providing support for stability analysis of power system including wind farm, especially wind farm inner failure. |
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