Study On The Calculation Method Of Short-Circuit Current For Large-Scale Wind Farm Groups

Wind power has developed rapidly in China, cluster development and centralized access to the power grid has become the main form of wind power development in our country. At present, China has builded 9 million kilowatts wind power bases. In addition, due to the different generator mechanism, network topology and operating conditions of wind turbins, it makes the transient characteristics of wind turbins quite different from the synchronous generators. A large number of wind farms connected to the grid in the form of clusters, which has brought great challenges to the traditional power system protection. The short circuit current calculation method of power system with wind turbines and wind farm groups is the basis of analyzing the influence of wind turbines access to grid and developing new protection principle. For the above research objectives, the main work of this dissertation is summarized as follows:(1) An accurate short circuit current calculation method of DFIG (doubly-fed induction generator) with Crowbar is proposed, which accounts the dynamic characteristics of the rotor current. Based on the analyzing of DFIG transient EMF variation process and dynamic rotor current influence, the DFIG stator and rotor flux is calculated accurately. And an improved calculation method for DFIG short-circuit current is put forward. The simulation results verify that the initial value and trajectory of proposed calculation method is more accurate then the calculation method without accounting the dynamic characteristics of the rotor current.(2) A short circuit current calculation method of DFIG accounting the influence mechanism of LVRT (low voltage ride through) control strategy is proposed. According to the input-output characteristic of current transformer, the equivalent model of current transformer and the transient model of DFIG are put forward. The influence mechanism of LVRT control strategy is analyzed. The short circuit current calculation model of DFIG is established, which accounts the influenceof LVRT control. Base on the fault steady state equivalent circuit of DFIG, the grid short circuit current calculation model with penetration of DFIG is proposed. Based on RTDS the experiment platform for grid with penetration of DFIG is founded, the proposed short-circuit current calculating method is validated.(3) A short circuit current calculation method of hybrid wind farm with DFIG and PMSG (permanent magnet synchronous generator) is put forward. The influence of the control strategy to the fault transient process is considered. The equivalent model of DFIG and PMSG are established. By the aggregation equivalent method, the wind farm equivalent model with multi type wind turbines is put forward, the change law of short circuit current during the fault is analyzed and the short-circuit current calculation method of wind farm is given. Base on the fault steady state equivalent circuit of DFIG and PMSG, the grid short circuit current calculation model with hybrid wind farm is proposed, which can calculate the short-circuit current of each branch under various fault conditions.(4) A short circuit current calculation method of wind farm groups is put forward. The coupling between the wind farm short circuit current and grid node voltage is analyzed. And the impact mechanism between short circuit current inter-field wind farm groups and short circuit current of gird is studied. The calculation method of wind farm groups short circuit current is put forward, and the current of each wind farm in groups is calculated accurately. Base on the fault steady state equivalent circuit of wind farm, the grid short circuit current calculation model with penetration of wind farm groups is proposed, which can calculate the short-circuit current of each branch under various fault conditions.