In order to study voltage sag adaptability of doubly-fed induction generators (DFIG) in power system and achieve low voltage ride-through capability, this paper analyzes the electromagnetic transients within DFIG through approximated analytical expression of stator fault current and rotor fault current obtained by the superposition between transient current based on vector method and steady state current based on equivalent circuit method under the following circumstances: stator terminal three-phase symmetrical voltage dip, the rotor side converter cut off. and the crowbar circuit in operation. On the base of the theory analysis, a 2 MW DFIG model is established by PSCAD and its experiment on the prototype validates the effectiveness of approximated analytical expression of fault current based on the superposition method between transient current and steady state current on a 7.5 kW DFIG platform, which could present the transient changes of the DFIG flux and current correctly.%为了研究双馈感应发电机对电网电压跌落的适应能力,以及其实现低电压穿越的功能,文章通过将由向量法求出的瞬态电流与由等效电路法求出的稳态电流进行叠加而得出的定子、转子故障电流的近似解析式,来分析在定子端三相对称电压跌落、转子侧变换器断开、投入Crowbar电路情况下的双馈感应发电机内部的电磁关系变化过程.此外,在理论分析的基础上,文中建立了2 MW双馈感应发电机的PSCAD模型,且在7.5 kW双馈风力发电测试平台上进行了实验验证.仿真和实验结果表明,这种通过瞬态电流和稳态电流进行叠加的方法而求得的双馈感应发电机故障电流的近似解析表达式可以准确地反映出双馈感应发电机磁链和电流的瞬态变化.
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