| This paper has analyzed the influence of wind power connection on the grid. To be more specific, the normal operation of wind farm (WF) should absorb the reactive power from the grid, leading to the decline in the system voltage. When the grid is subject to disturbance or breakdown, the wind-power generator set should also absorb large-capacity reactive power from the gird, thus undermining the voltage stability and the LVRT realization. In the case of variable wind speed, the WF output power will be easily fluctuated, resulting in the voltage fluctuation of the system. If the reactive power compensation capacity of the wind farm is not sufficient, the voltage at the connecting point will be fluctuated, leading to voltage flicker.The differentiation of the equivalent exciting current at the DFIG rotor side is not equal to zero under grid disturbance, which is the basis for the establishment of the dynamic mathematic model for double-fed wind power generator rotor side converter. This model can lay a foundation for the control strategy of DFIG rotor side converter, when DFIG generator connected to an unbalanced electrical grid. For the traditional design of DFIG rotor-side controller, it is based on the prerequisite that the voltage and flux linkage in the grid should be stable. In addition, the transient process of the voltage and flux linkage should be neglected. When the DFIG generator connected to an unbalanced electrical grid, the voltage and flux linkage will be subject to fluctuation, leading to the poor performance of traditional control system. Based on the dynamic mathematic model for DFIG rotor side and the directional vector control of stator voltage, this paper has built the control strategy for DFIG rotor side under unbalanced Grid voltage condition.The contribution of this paper is proposed using the stator flux observer, to replace the traditional stator flux estimation scheme. The improved control scheme is suitable for the DFIG behaviors during external network fault. The stator flux observer based on combined voltage and current models are investigated. The correction loop employs a proportional-integral (PI) correction element based on the current error, and guarantees the observer stability.This paper presents a new control strategy, this is a DC Voltage Feed-forward Compensation Control strategy for DFIG rotor-side converter and Grid side converter. In the traditional model of the DFIG, it is assumed that:Vdc=Vdc=const, Grid side converter and Rotor side converter is idealized by a gain G*=V*dc/2.Vp. In actually the DC bus voltage can be changed, so converter gain coefficient (G=Vdc/2.Vp) also changes. If value of the G increases, the control signal increases, endangering the safety of the Grid side converter and the Rotor side converter. To ensure stable of the converter gain coefficient under changing conditions of the DC bus voltage, the GSC and RSC controller model must change. In order to strengthen the feed forward control to offset the disturbance, it is necessary to join the compensation in the feed forward control channel, to further improve the feed forward control affect. The comparison of simulation verifies the correctness and effectiveness of the control strategy.By taking the 2 MW double-fed wind power generation system scheduled for Ninh Thuan grid in 2018 as an example, this paper has conducted a simulated research into the generator speed change and response characteristic during grid disturbance. It is found that the improved control strategy for DFIG rotor side proposed in this paper has outperformed the traditional control strategy in terms of control performance and dynamic response speed when the grid is subject to disturbance. Hence, it has enhanced the stability of the system.At the present stage, the wind farm mainly adopts the switched capacitors for reactive power compensation. The compensation capacity of the capacitors is discrete. In addition, it has the disadvantage of slow adjustment and weak ability to adjust the voltage. Hence, it fails to satisfy the dynamic need of wind farm for reactive power. Aimed at those problems above, this paper present a solution to the problem of reactive power compensation and harmonic compensation so as to improve the stability of grid-connected wind farm and LVRT ability by using STATCOM. What is more, this paper has also compared the performance of STATCOM and SVC under the condition of the same compensation capacity in the same installation location. According to the simulation result, STATCOM has outperformed SVC.Wind power generation system is a strongly nonlinear system with uncertain parameters. The traditional linear control method can only ensure the control effect in the vicinity of stable working point, thus failing to satisfy the actual requirement of controlling the wind power generation system. With the aid of backstepping method and Lyapunov's theory of stability, this paper has designed the controller for wind power generation system. The thesis researches coordination between STATCOM and the simplified Backstepping Controller for DFIG to improve voltage stability of wind power system in Ninh Thuan, Viet Nam. The backstepping controller for double-fed wind turbine rotor-side converter can improve the compatibility of generator with grid stability. In the case of breakdown, STATCOM and the backstepping controller of DFIG wind turbine rotor-side converter will join forces to overcome the voltage collapse and enhance the low-voltage ride-through capability of the wind farm. |
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