Analysis Of The Voltage Dip Fault For Doubly Fed Induction Generator

Based on the flexible regulation capability of active power and reactive power,smaller capacity of rotor converter, stronger capability of leading phase operation, widerspeed range and so on, the doubly fed induction generator(DFIG) has become themainstream generator in wind power plants, but due to the direct connection of statorwinding to the electricity grid and the connection of rotor winding through smallcapacity converter to the electricity grid,the DFIG is much too sensitive to theelectricity grid fault. With the continuous growing of doubly fed induction generatorsused in the wind power plants, the relation between generators and power system isbecoming more and more close. The transmission of electricity grid fault through thetransformer and power line to the DFIG could lead to the generator voltage dip andcause a series of electromagnetic impulse, and when the situation is much worse, itcould make the large capacity wind farm disconnect from the electricity grid which maydestroy the stability of power system. Hence, it is of great necessity to study DFIG’stransient behavior after fault and the impact of the commonly used Crowbar protectioncircuit on the LVRT performance characteristic. This paper mainly includes:Firstly，the paper introduces the fundamental theory of doubly fed machine，including steady state phasor diagram，the equivalent circuit and fundamental equations.On the basis of above theory, it analyzes the principle of variable speed constantfrequency operation of doubly fed machine，and elaborates doubly fed machine’stransient mathematical model base on the synchronous rotating coordinate system andits commonly used coordinate transformation. Secondly, through constructing a finiteelement model of a2MW DFIG, this paper simulates the generator’s rated behavior andcalculates the generator’s rated parameters.Thirdly, this paper simulates the transientprocess of DFIG’s stator voltage dip by use of finite element method. Based on theanalysis of flux path, flux density and permeability for the generator’s stator side threephase short circuit, the paper compares the inductance parameters and flux linkagebefore and after the fault and explains the fundamental cause of fault generator’selectromagnetic impulse. Then, the paper simulates the generator’s magnetic field andtransient process for four different kinds of stator side voltage dips. At last, the paperintroduces the domestic standard of wind power plants LVRT, analyzes the principle ofCrowbar protection circuit and compares DFIG’s transient process simulation differences of traditional mathematical model and finite element model. Based on aboveanalysis, this paper analyzes the influence of different factors on Crowbar protectioncircuit LVRT performance.The results show that on account of flux linkage conservation，DFIG’s magneticlines of flux mainly pass the leakage magnetic circuit and generator’s teeth are saturatedheavily which makes the generator’s inductance parameters decrease obviously andcauses the great electromagnetic impulse. This paper uses the finite element methodwhich can consider the impact of magnetic circuit saturation on fault process, so itsconclusions can provide related theory for the analysis of DFIG’s transient process andLVRT control strategy.