Study On Grid-connected Technology Of Large Capacity Doubly-fed Induction Generator

With the rapidly increasing capacity of grid-connected wind power and the development of offshore wind power, the operating characteristics of wind turbine during the fault have drawn more concern. In this dissertation, a detail model of5MW doubly fed induction generator (DFIG) based on dual PWM control is built. A relatively simple low voltage ride through (LVRT) method is proposed based on the existing LVRT methods. A thyristor controlled voltage regulator (TCVR) is installed in the low-voltage side of the wind farm substation. When voltage dip occurs, the grid-side voltage is reasonably compensated according to the degree it drops to improve the stator voltage of DFIG, thereby reducing the flux changes in amplitude, so that the rotor current is restricted under the limit and at the same time the over-voltage of DC bus during the fault is suppressed. The proposed control strategy is add into the established5MW DFIG wind power system model to simulate the operating characteristics of wind turbine under severe symmetrical fault to verify the validity of the proposed method.In addition, an assessment method of voltage fluctuation and flicker of wind farm considering wind resource influence is presented in this dissertation. Voltage fluctuation and flicker level induced by grid-connected wind power may exceed the national standard, resulting in serious power quality problems, which makes it necessary to assess these two items before wind power connection. This paper adopts a new assessment method. Different from the calculation methods of voltage fluctuation and flicker presented in international electrotechnical standards (IEC61400-21), it takes the influence of wind resources on these two indexes into account. The voltage fluctuations induced by gust wind under different wind power outputs is calculated and short-term flicker value Pst is got from IEC flickermeter. A new wind farm in China is assessed by the proposed method and IEC standards respectively. The results show that the proposed method can not only carry out assessment of voltage fluctuation and flicker effectively, and can also consider the potential impact of wind farm caused by wind speed changes.