Transient Characteristic Analysis And Short-circuit Calculation Model Of Doubly Fed Induction Generator Under Grid Faults

The number of wind turbines connected to the grid is steadily increasing.The interaction between wind turbine and grid has been investigated extensively in recent times,especially under grid faults.The main types of wind turbines is a variable-speed wind turbines with doubly fed induction generator(DFIG).With the need of low voltage ride through(LVRT),Most protection schemes are based on a so-called crowbar.Due to a DFIG coupled to the grid directly,the voltage drop at the terminals will result in large,oscillatory currents in the stator windings of the DFIG.Because of the power electronic converter connected and the protection of the DFIG,The magnetic transient response will become complicated and the impact on the grid is a clearly critical issue.This thesis do some detailed research on the transient response and short-circuit current,the primary contents and original contributions are as follows:This thesis develops a general transient analysis model and a corrected transient analysis model of DFIG in which the crowbar protection is not neglected.The transient analysis model is obtained by solving the dynamic equations based on the space vector approach,and a correction factor ? is derived from characteristic roots of state-space equation which present the magnetic coupling between stator and rotor windings of DFIG.The study show that the crowbar resistance will increase the coupling of the rotor to stator,which change the dc decay of self-linking flux to the near-dc decay of flux.But the effect of the resistor on the coupling of the stator to rotor can be neglected.The contribution of crowbar to the rotor side can be considered to change the rotor side equivalent circuit.This thesis tries to help understand the dc-link voltage clamp caused by rotor side over voltage and analyses the behavior of the transient characteristic of DFIG.In the proposed analysis,the equivalent circuit and the resistance of rotor-side converter is derived from steady state response of the dc-link voltage clamp by means of simulations.Considering the duration of the dc-link voltage clamp,the transient-state equivalent circuit of the dc-link voltage clamp is determined by revise the steady-state equivalent circuit and the value of the integrated equivalent resistance is calculated.A more accurate sequence network of DFIG is derived in this thesis to aid in short-circuit calculations in wind-farm-based power system from a new understanding of short-circuit calculation.The transient equivalent circuit of DFIG is obtained and the behavior of the DFIG transient potential is rapidly attenuated which is clearly analyzed.Based on the external characteristics,A new method for deriving short-circuit calculation model is proposed through understanding the nature of traditional short circuit current.The sequence network of DFIG derived from the dynamic equations is depend on the difference of the slip speed.When the slip is large,DFIG can be equivalent to integrated impedance.When the slip is small,DFIG will contributes to the short-circuit current.In this thesis,simulation tests and experiments are proposed on Matlab/Simulink.The results show that the analysis of transient characteristic of DFIG under grid faults is correct.The corrected transient analysis model and sequence network of DFIG obtained in this thesis is verified with experimental evidence.