Low-voltage Operation And Control Of Doubly-fed Wind Tiirbine Generator Under Grid Fault

With wind power capacity increasingly, as well as the proportion of the capacityof wind power generation in the grid is on the rise, and the increasing influence ofwind power on the grid. Power system for a relatively high proportion of wind power,wind turbine generation sets splitting from the grid may result in the collapse of thegrid voltage and frequency, cause tremendous losses to the industrial production,thus limiting the large-scale development and application of wind power. Lowvoltage ride-through (LVRT) operation of wind power generation has become aninternational research focus on the grid fault. How to improve the control strategy ofdoubly fed induction generator (DFIG) sets in the fault grid is the key, from thedoubly-fed asynchronous wind turbine generator mathematical models and dualpulse-width modulation (PWM) converter control strategy study in the paper.Firstly, according to the tracking characteristics of the wind and the doubly-fedwind turbine operation mode, the mathematical model of the wind turbine, the drivetrain system, DFIG and grid-side converter have been established, a doubly-fed windpower generation system simulation model has been established in Matlab/simulinksoftware environment.Secondly, on the analysis of the characteristics of grid-side PWM converter, thegrid-side PWM converter mathematical model has been established on thethree-phase stationary coordinate system and two-phase synchronous rotatingcoordinate system, voltage and current dual-loop closed-loop control strategy of gridvoltage oriented has been proposed. On the grid fault, grid-side PWM converter bythe grid voltage fluctuations and load disturbance, the load current feed forwardcontrol strategy and the rotor instantaneous power fluctuations feedback controlstrategy have been proposed. On comparative analysis of the simulation, theproposed load current feed-forward control strategy in the case of the same circuitstructure, with anti-load disturbance; rotor instantaneous power fluctuationsfeedback control strategy can track the change of the rotor power, improved controlstrategy can rapidly suppress the fluctuations of the DC bus voltage and reduce thevalue of the DC bus capacitors.Thirdly, on the analysis of the basic characteristics of the doubly fed inductiongenerator, the dynamic mathematical model and the steady mathematical model ofDFIG have been established on the three-phase stationary coordinate system and two-phase synchronous rotating coordinate system. On the basis of the simplifiedmathematical model, the grid voltage oriented vector control strategy has beenproposed. The control strategy in the normal grid operation and generator stabilitystudies showed good performance. However, it can’t accurately reflect theparameters change of DFIG on the grid fault. Therefore, on the voltage dip,considerate the stator flux transient grid flux-oriented vector control strategy basedon the dynamic mathematical model of DFIG has been proposed. The simulationresults show that:1、the improved control strategy has good static characteristics onthe normal grid voltage stilly;2、compared with the traditional control strategy, theimproved control strategy has effectively improved the control of the rotor current inthe stator voltage sag;3、improved control strategy has reduced the electromagnetictorque dramatic changes and the impact of mechanical systems when the statorvoltage sag.Finally, in the Matlab/simulink software environment, the establishment ofvariable speed constant frequency (VSCF) doubly-fed wind power generation system.On the simulation and comparative analysis in the three–phase symmetry grid fault.The simulation results show that the improved control strategy of the stator and rotorconverters can effectively control the rotor current and DC bus voltage stability,improved the DFIG low voltage ride through capability and the stability of powersystem.