| Wind power generation technology has become a research hotspot at home and abroad,and the traditional wind turbine has been connected to the three-phase balance voltage of large electrical network sucessfully.However,there are a series of difficulties in the phase locking and grid connected power generation in the case of three-phase unbalance voltage and asymmetric faults in the microgrid or small island network.In order to solve the mentioned-above problem,the control strategy of grid-side converter of wind turbine under the three-phase unbalance voltage and asymmetric faults is deeply studied in this thesis.Firstly,the grid-side converter of wind turbine and the three-phase unbalance voltage grid are modeled.The mathematical model of grid-side converter under three-phase balance voltage of power grid is studied,and the steady-state running characteristics of grid-side converter are analyzed.The mathematical model of grid-side converter which is under three-phase unbalance voltage is derived in order to research the operation characteristics of the grid-side converter under the mentioned-above situation.Secondly,a direct result of the traditional grid-side converter lock phase is not accurate due to the three-phase imbalance voltage of power grid.So the traditional method of lock phase has been improved.Based on the synchronous coordinate system software phase locked loop which is exsiting under the three-phase unbalance voltage,a norch filter is added,and the second-order generalized integrator is enhanced.The positive and negative sequence components are extracted from the unbalanced power grid,then the positive and negative sequence phase angle are calculated.Furthermore,the simulation model is built in MATLAB/Simulink,and the improved phase lock method is verified under the condition that the grid voltage unbalance is 10%.The simulation results show that the proposed method can extract the positive and negative voltage components and phase information quickly and accurately.Thirdly,the direct power control strategy is adopted in the grid-side converter control which has the advantages of simple algorithm and good response performance.However,the direct power control algorithm based on space vector modulation(SVM-DPC)which is based on PI control cannot meet the high frequency switch control of nonlinear system under voltage imbalance.In order to achieve the above-mentioned purpose,a SVM-DPC control based on sliding mode variable structure is proposed,which is controlled by the internal and external ring coordination control to suppress the two harmonic components of the DC bus voltage,and the traps are increased to achieve the reactive power without difference adjustment so that the influence of the two harmonic is eliminated,and the stability of the DC bus voltage is ensured.Then,the comparison of simulation between the traditional and the improved control strategy is carried out under the single phase-to-ground and two-phase current short-circuit fault conditions respectively.Acoording to the contrasting results,the correctness of the proposed control algorithm is verified.Finally,a hardware platform of grid-side converter based on DSP28335 is built.The main circuit,control circuit and peripheral protection circuit are designed,and the correctness of each circuit is tested.Based on the master control board of the hardware platform and the RTLAB semi-physical simulation platform,the comparion of voltages and currents of the traditional SVM-DPC based on PI control and the proposed SVM-DPC based on the sliding mode variable structure control strategy under different single-phase load mutation conditions is made,and the experimental results verify the effectiveness of the improved control strategy. |
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