Study On Transient Model And Protection Improvements For Power System With DFIG Wind Turbines

Doubly fed induction generator(DFIG)wind turbines are widely used in wind power systems,due to this,It takes more and more Challenges to the stability and security of distribution network.The rotor side of DFIG is connected to power grid by back-to-back converters,and the structure and control strategies of DFIG are ob-viously different from those of the conventional synchronous generator,so the fault characteristics of DFIG wind power system is also different from those of conventional system.It brings a lot difficult to transient modeling of distributed wind farm.Therefore,it is of great significance to develop transient modeJing of distributed wind farm,to investigate the traditional relay protection performance in wind power system and provide countermeasures so as to ensure the safety and reliability of wind power system.Research works in this thesis focus on the power system connected with DFIG wind turbines.Firstly,the differential equation of DFIG flux is used to describe the DFIG electromagnetic transient process,Output short-circuit current under two typical control modes of low voltage crossing,and the factors affecting transient current characteristics under different control modes are analyzed.The analysis results show that the short circuit current characteristics of DFIG under three typical LVRT control strategies are very different.Further,based on the analysis of fault characteristics and influencing factors of DFIG units,the mapping relationship between fault voltage and fault current eigenvalues(frequency,phase and RMS)of a single DFIG unit port is studied,and the transient simplifi ed model of a single DFIG unit is established.Taking a DFIG single machine as an example,the fault experiments of random voltage drop of two three-phase faults and random transition resistance of two two-phase faults are carried out.Secondly,based on the study of transie nt equivalent model of DFIG single unit and the influence factors of rotor fault current,a simplified transient clustering modeling method for DFIG multi-unit integration into distribution lines in decentralized wind farms is studied,taking wind speed and voltage sag degree as inputs at the moment before fault.Finally,through the dynamic characteristics of the feed-out current of the decentralized wind farm,the current adaptive protection setting principle and the improved protection scheme for the line downstream of the grid-connected point are studied.The applicability and reliability of the research conclusions are illustrated by simulation software,field experiment data and fault recorder data.At the same time,the scheme has practical popularization value.