Research On Low Voltage Ride Through Capability Of DFIG

With the rapid development of wind power industry, the proportion of windpower in the power system is increasing year by year, large-scale wind power has animpact on the stability of power grid and the power sector has put forward higherrequirements on low voltage ride-through capability of wind turbines.Therefore, theresearch on low voltage operation capacity of the wind turbine is very necessary andmeaningful. This paper, by using MATLAB/SIMULINK simulation platform, hasdone research on the low voltage operation ability of doubly-fed wind turbine.The main contents of the research are about the analysis of low voltage ride-through capability of DFIG by using MATLAB/SIMULINK simulation tool. Themain work and conclusions focus on three aspects as follows：(1)Firstly,analysis is conducted on the reasonable value range of Crowbarresistance, combined with simulation examples to discuss the influence of Crowbarvalue on the performance of DFIG LVRT;Then, the Crowbar circuit switching controlstrategy is discussed detailedly.Three schemes of Crowbar circuit disconnection,before, when and after the removal of the fault, are researched in-depth respectivelythrough simulation.Analysis shows the scheme of quitting Crowbar before faultremoval is relatively good for the operation and recovery of the DFIG wind powersystem.On the basis of above,choosed the time as the switching criterion of Crowbarcontrol strategy;Finally, based on hysteresis control as the DC side unloading circuitcontrol strategy is proposed.(2)Constructing grid simulation examples.The factors that affect the DFIGcapability of low voltage ride through is discussed, analysis is conducted on the gridside converter current maximum limiting value and the influence of its wattless powercompensation moment on the stability of DFIG. Obtaining a reasonable limiting valuecan stabilize the DC bus voltage, and timely control wattless power compensation ofgrid side converter can reduce the transient oscillation of DFIG system. Base on aboveanalysis, two kinds of low voltage ride through schemes are proposed. For the gridvoltage drop of85%,two low-voltage schemes are compared, one is the rotor sideCrowbar circuit, the other is with additional DC side unloading circuit, and thesimulation results show that by increasing the Crowbar resistance value, the latter canabsorb the excess energy, which makes it more effective than the former to suppress the transient inrush current in the stator and rotor windings as well as to stabilize theDC bus voltage.(3)We add the Crowbar circuit and STATCOM dynamic reactive powercompensation device to the wind farm with DFIG in that the STATCOM device cansatisfy the DFIG’s demand of reactive power from the power grid after switching onthe Crowbar circuit. By simulation of the DFIG dynamic responses under differentdegrees of voltage drop, especial in the case of85%.We discover that when the gridvoltage drop is serious, the effect of STATCOM on reactive power compensation isobvious and provide a little active power to the grid, and compared with turning onthe Crowbar circuit only, connecting both STATCOM device and Crowbar circuit donot have an impact on DFIG’s components when providing a bit of active power and alot of reactive power; when the voltage drop is mild, DFIG can achieve low voltageride through by its own capability, the effect of adding external crowbar circuit andSTATCOM device is not obvious; a large voltage drop has deeper impact to DFIGsystem than its recovery, and a small voltage drop is just the reverse.