Short Circuit Current Analysis And Calculation Of Doubly Fed Induction Generator

The insertion of large-scale wind generator has a great impact on the operation of Power system,especially when fault occurs.Doubly-fed induction generators(DFIG)are very sensitive to the disturbances of grid,so dynamic characteristics of its rotor current become very important under the rapid increase of DFIG-based wind generation.The implementations of low voltage ride through(LVRT)and short-circuit calculation of electric quantities depend on the accurate assessment of stator and rotor current.In this paper,research is divided into parts,firstly the research on the short current characteristics double-fed wind turbine unit with Crowbar or Chopper,secondly exploring the technology of LVRT with double-fed wind turbine unit.For the asymmetrical short-circuit fault,the short circuit current contribution of double-fed wind turbines(DFIG)has not received much attention so far.Usually,the stator resistance or the relation between the stator and rotor flux is neglected in traditional study,resulting in the presence of errors in transient characteristic analysis and calculation.Based on positive and negative sequence vector model of DFIG with space vector theory and sequence component method,the accurate mathematical expressions of the transient stator and rotor flux and short-circuit current are derived at the moment that asymmetric fault occurs,of which the transient characteristics are analyzed Simultaneously,Finally,the mathematical expressions of stator current are verified according to the standard DFIG model in MATLAB/SIMULINK.A chopper protection on the direct current(DC)link is applied to maintain the DC link capacitor voltage and limit the current of the rotor circuit with modern doubly fed induction generator wind turbines during the fault.Considering the analysis method of the Crowbar protection,The DC link system of such wind turbines is equivalent to an variable resistance.According to the relationship between the rotor of the short-circuit current and the variable resistance,the obtained equivalent variable resistance,getting through iterative method,is verified by the Simulink emulation of short-circuit current.And the transient characteristics of equivalent resistance,chopper resistance,port characteristics and electromagnetic torque are analyzed at the same time.In order to optimize the LVRT performance of doubly-fed wind turbine,on the basis of the existing LVRT technologies,this paper fully referencing the prominent advantages with Crowbar protection,Chopper protection and rotor converter control in the process of LVRT,put forward utilizing Crowbar protection to inhibit over-current of rotor circuit,Chopper protection to stable voltage of DC bus and RSC control to make timely output reactive power to the grid with the Crowbar resistance exited.And the transient characteristics are analyzed with Comprehensive protection and control strategycombined with the simulation.