| Doubly-fed wind power generation system has been widely used in wind power generation for its excellent control performance and economy. At the same time, due to the system structure of stator direct connection to the grid, doubly fed induction generator (DFIG) is susceptible to the grid dips, whchi cause rotor-side over-voltage and over-current in the process of grid failure, resulting in DFIG wind turbine off-grid operation. With the proportion of wind power capacity in the power system increasing, specification for wind farm grid integration requires that wind turbine must have low voltage ride through (LVRT) capability in the case of grid failure, ensuring the safe and stable operation of the power system. Therefore, the study of how to improve the low voltage ride through capability of DFIG wind power generation system is an urgent and challenging issue. This paper focuses on this topic.The operation and control of DFIG is studied. The mathematical models of DFIG and back-to-back converter are built. The DFIG vector control strategy under stator voltage orientation coordinate is built based on the decoupling control of DFIG active power and reactive power, and the rotor side current control. The DFIG vector control performance shows poor damping characteristics of the system, which will cause poordamped fluctuation when accounting grid fault.The rotor side overcurrent and the converter DC link overvoltage are the main reasons cause the off-grid of the DFIG. A detailed analysis of the stator flux is carried out for the in-depth understanding of the DFIG electromagnetic transient process under grid fault. A DFIG LVRT scheme based on the back EMF negative sequence component suppression is proposed to deal with the power impact of grid flux negative sequence component on the rotor side under unbalanced grid fault. The power impact to the rotor and the fluctuation of the DC link during unbalanced grid fault are reduced through the back EMF negative sequence component suppression on the rotor side voltage control.An important requirement of the wind farm LVRT is dynamic reactive power support. The traditional reactive power compensation equipments can not provide dynamic reactive power support due to the poor switching ability. This paper presents DFIG LVRT scheme based on supercapacitor energy storage. During a grid fault, the grid side converter provides dynamic reactive power to support grid voltage, supercapacitors stable DC voltage. The coordination control strategy of the DC-DC converter and the grid side converter is established achieving the stable DC link voltage and the grid dynamic reactive support during grid fault. In order to verify the effectiveness of the proposed DFIG LVRT control strategy, the physical experimental model of DFIG system and supercapacitor energy storage system are build in the laboratory. The operation and control of DFIG is carried out to verify the correctness of the DFIG mathematical model its control system established in this paper. The coordinated operation experiment of the supercapacitor energy storage system, the DC side and the grid converter is studied. Experimental results show that the system can effectively control the DC voltage, verifying the DFIG LVRT control strategy based on supercapacitor. |
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