Research On Low Voltage Ride Through Technology Of Doubly Fed Induction Wind Power Generation System And Voltage Sag Detection

When the power system voltage drop fault, the DFIG (Doubly Fed InductionGenerator,) wind power system without LVRT (Low Voltage Ride-through)technology will be off-grid, which will make the gird failure worse. For DFIG windpower system, the LVRT technology is becoming more and more important. For thecontrol method of DFIG wind power system, it’s a critical point to detect the voltagedrop quickly and accurately for the LVRT.Firstly, the mathematical model of the doubly-fed induction motors is established.The derived mathematical model of doubly-fed induction motor is also establishedwith the translation of three-phase steady-state coordinate system and two-phaserotating coordinate system.According to the network side converter topology analysis,the mathematical model is made through d, q coordinate transformation which makesthe model simplified. Mathematical model of DC power is made by calculating thestorage of capacitor. The voltage drop detection algorithms of Double-fed wind powergeneration system is provided and mathematical models of DFIG wind power systemLVRT capability is studied.Secondly, several different types of voltage drop, which contain single-phasefault, two-phase ground fault, phase failure and three-phase fault, is analyzed.According to the basic structure of the PLL, dq transformation of three-phase voltagegrid is made, and the d-axis component is taken as an error, which could get the phaseinformation through PI adjusting and integrating. By using of PSIM and MATLABsimulation, the simulation program is made and proved through the ways of settingthe sampling frequency, real-time sampling, phase lock calculation, output of the lockphase, etc. The test results of the three phase, two-phase and single phase voltage dropcondition through simulation, is proved that the detection algorithm can improved thespeed and accuracy in detection of grid voltage drop.Subsequently, Crowbar Circuit of the DFIG wind power generation systems inLVRT technology is studied, which contains the main analysis of the active, passiveCrowbar topology and the advantages, disadvantages of Crowbar. The resistance andswitching strategies of the Crowbar circuit is optimal designed theoretically by thefault current of the grid voltage drop.Finally, a test is achieved by2MW doubly-fed wind turbine using Crowbar technology in different situations grid voltage drop, which shows the validity andfeasibility of the LVRT methods.