Research On Low Voltage Ride Through Technology Of Doubly Fed Induction Generator Based On Sliding Mode Control

As a mature renewable power generation technology, wind power generation occupies a large proportion in china’s energy structure, and the doubly fed induction generators(DFIG) are widely used in the wind power system. The grid voltage dips can result in over-current in the rotor circuit and over voltage in the DC-link, which causes much harm to the DFIG. The widespread tripping of the DFIG could lead to transient instabilities of the power grid. Therefore, it is highly significant to study the low voltage ride through(LVRT) capability of the DFIG.In order to improve the LVRT capability of DFIG, this paper mainly studies the control strategy of DFIG during grid voltage dips. The main work and achievements of this paper are as follows:(1) In order to improve the control performance of rotor side converter(RSC)and the grid side converter(GSC), control strategies based on sliding mode control(SMC) are proposed for both RSC and GSC. Single power(voltage) control loop instead of cascaded current and power(voltage) control loops are adopted, which simplifies the design of the controllers.(2) The electromagnetic transient process of DFIG during both balanced and unbalanced grid voltage dip is analyzed based on the mathematical model of DFIG. It is found that current pulsation and over-current in the rotor circuit are caused by the electromotive force(EMF) induced by the stator DC and negative sequence flux.(3) A rotor voltage compensation control strategy was proposed for the RSC during grid voltage dips. Under the proposed control strategy, the AC terminal voltage of the RSC was controlled to compensate the DC and negative sequence components of the electromotive force. The proposed control strategy can eliminate the pulsation first and second order component in the rotor current under slight voltage dip and reduce the over-current in the rotor circuit under serious voltage dips.(4) For the GSC, an improved control strategy considering the power fluctuation in the reactor of the GSC is proposed. The proposed control strategy can accurately reflect the power balance between both sides of the capacitor, which can effectively minimize the fluctuation of the DC-link voltage.