| In the doublely-fed VSCF WT, the rotor of DFIG is connected to the grid throughtwo back to back PWM converters, in which the converter connected to the grid iscalled grid-side converter. Based on Chinese government's 863 project "The Researchand Development of the Control System of MW VSCF Wind Turbine", this papergiven detailed result of theoretical analyses and practical validation about the keytechnology of grid-side converter. Usually the control-strategy of PWM converter is constructed on balanced threephase voltage which means that the grid's three phase voltage has the samefrequency,amplitude,phase difference(120 degree). When the three phase voltagesare unbalanced, which means the three phase voltage has same frequency,unequalamplitude,unfixed phase difference(maybe not 120 degree), the control system basedon the balanced grid voltage will result in 2 order fluctuation in DC voltage. Thefluctuation in DC bus can lead three effects. The first one is the ac-side currents have3 order harmonics, which may pollute the grid and influence other electro-equipments.The second one is the fluctuation result in the DC-side capacitances is frequent chargeand discharge energy, which may shorten its life;the last one is the fluctuation willincrease control pressure on rotor-side converter. this paper analyses the reasons of DC voltage's fluctuation of grid-side converter,and built the math model of grid-side converter in unbalanced voltage separatelybased on two phase stationary coordinate and rotated coordinate. Beyond the paperdeduces the control principle of restraining fluctuation of DC voltage, and put forwardtwo control methods: two stationary coordinate predicting-current control method andconstant power control method. Base on the test equipment of 60KW grid-side converter, the paper simulates thecontrol methods using Matlab. The results indicate that the method can effectivelyreduce DC voltage fluctuation of grid-side converter under unbalanced voltage.
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