Transient Characteristics Analysis And Simulation Of Grid-connected Dfig Wind Turbine Under Power Grid Failure

As the most important compound of smart grid construction, new energy power generation, especially wind power generation has got more and more attention. Since new power grid operation standards required that WTG should maintain certain time parallel operation during system fault, the short circuit current supplied by it would affect normal operation of protective relaying equipments and safety of electrical devices. Especially doubly-fed induction generator has been widely used at present. Because its operating characteristics depend highly on power grid, the short circuit current supplied by it would be influenced by the operation condition of distribution network and the short-circuit current characteristics of distribution network will become complicated. Based on these situations, this thesis has studied the transient characteristics of DFIG, and has finished the following results:Firstly, this thesis has introduced the characteristics of DFIG and its advantages in variable speed constant frequency wind power system, analyzed the basic structure and functions of main components, introduced the mathematical model and control strategy of DFIG, constructed a real-time simulation platform of DFIG on the basis of RTDS, and proved the effectiveness of model by DFIG operating characteristic simulation.Secondly, because the operational characteristic of grid-connected DFIG depends highly on power grid, the short circuit current supplied by DFIG would be directly influenced by the operation condition of distribution network while external failure happened. Thus, this thesis has deduced the expression of short circuit current supplied by grid-connection DFIG under symmetrical short-circuit and asymmetric short-circuit conditions on the basis of park generator model, and analyzed the short-circuit current characteristics of DFIG while grid broke down. Furthermore, calculation example has been simulated by DFIG simulation model on the basis of RTDS, and its correctness of short circuit current expression has been proved.Thirdly, distribution system would be converted from single-source network into multi-source network while interconnecting wind power generator with original system, which would possibly cause failure and false operation of some original relaying protection system. This thesis has analyzed the impact of DFIG on operation behavior of over-current protection when short circuit occurred at upstream and downstream of DFIG and feeders nearby by using real-time simulation model of DFIG on the basis of RTDS.In the end, to improve the voltage characteristic of WTG, SVC is proposed to be installed at the exit of wind turbine generator to regulate voltage. But there are few futher discussion on the transient characteristics of system with SVC connected. The impedance characteristic of SVC is firstly introduced in this paper, and the real-time transient model of Doubly-fed generator with SVC has been built on the basis of RTDS. Furthermore, the impact on terminal voltage and short-circuit current of DFIG by SVC under different power grid faults has been analyzed in the simulation.