Research Of Bidirectional AC-DC Converter Based On Improved Digital One-cycle Control

An ideal model of with resistant load in AC side was built in conventional one-cycle control(OCC)to make the grid current follow the shape of grid voltage.Under this control method,the grid current would be distorted when grid voltage harmonics existed.Moreover,the current phase lag also existed in OCC,due to the neglecting of voltage of the filter inductor.And the current distortion would become more serious if the filter inductor was nonlinear.The defects above made OCC hardly be applicated in high performance AC-DC converter.In this thesis,research is carried out on a single phase full-bridge AC-DC converter,which is applied to a microgrid storage system.Firstly,an analog bipolar OCC based on peak current modulation is discussed,and its stable range is analyzed.The result reveals that with a lifting current,one-cycle controlled AC-DC converter could be stable both in rectification and inverting mode.Secondly,a digital unipolar OCC based on average current sampling is proposed and its performance compared with analog OCC is analyzed.Moreover the accurate equivalent model of one-cycle controlled single phase AC-DC converter is built and analyzed in detail.By comparing tow lifting forms of with constant R_p and with constant R_f,the result draws that the lifting form with constant R_p can obtain higher and constant power fact.Thirdly,an improved digital OCC is presented to raise its ability of grid voltage harmonics immunity.The original grid voltage is replaced by the voltage handled by a second order generalized integrator(SOGI)as the AC source of the ideal equipment model and then the impedance characteristic of OCC is changed after adding SOGI.The gird current follows the shape of output voltage of SOGI and its distortion is suppressed by this way.Fourthly,the current distortion caused by the saturation of the inductor is discussed in detail,and a calculation method of the voltage of a nonlinear filter inductor in software is proposed.Finally,experimental verification with a 2kW prototype is presented to confirm the effectiveness of the proposed scheme.Simulation and experimental results show that the advantages exhibited by the conventional OCC,such as high dynamic performance and easy implementation,are reserved in the improved OCC,meanwhile the current distortion caused by grid voltage harmonics have a sizable suppression with the method of SOGI.And,the accurate voltage compensation of a nonlinear filter inductor not only mitigates the current distortion caused by the saturation of the inductor,but also solves the problem of grid current phase lag and make the unity power realized both in rectification mode and inverting mode.