The Research On Control Technology For Modular Multilevel Converter HVDC Flexible

The further development of power electronic devices promotes the further development of flexible HVDC transmission capacity to increase further.Especially in power generation and online applications,flexible HVDC can show a greater advantage in the wind when conveying distance farther,because of this regulation has fast and accurate of the characteristics,making it easy for of the effective control of the tide for the entire system.In the field of transmission and distribution has a very wide range of applications,has attracted widespread attention at home and abroad in recent years,in the engineering and academia.This paper introduces the research background and significance of flexible HVDC technology,the latest developments and engineering applications,and analyzes the current situation of Flexible DC transmission technology,in-depth analysis the development of applied high voltage power transmission converter topology and carries out a detailed comparative analysis.Describes the combination of multi-level modules inverters,explaines how it works,in which the sub-module uses usually half-bridge structure.The upper and lower leg of each phase were an inductor in series,parallel capacitor sub-module DC side.To the good and stable operation of the system,given the bridge arm inductance,capacitance and sub-module series number,the number is equivalent to the level selection principle.Based on the work principle of modular multi-level module type flexible HVDC converter system works,the modulation strategy is proposed using triangular carrier wave phase shift-PWM switching frequency optimization method(PS-SFOPWM),which can optimizes and improves the utilization of a DC voltage,so that the modulation of 0.866 up to 1.15,the switching frequency is mainly concentrated in the vicinity of the peak,thereby reducing the switching frequency,thus reduce switching losses,conducted a comparative analysis of a variety of modulation methods.For the asymmetrical power failure,the paper proposed based on negative sequence voltage compensation control strategy to effectively suppress fault currents caused by negative sequence,which can be achieved in the positive sequence synchronous rotating coordinate non-negative sequence current differential control.Inhibition of asymmetrical fault current AC system caused the whole system running.Since no negative sequence currents carried decomposition,only one current controller simplifies the configuration parameters,the system still has a gooddynamic and static stability,without calculating the grid current positive and negative sequence components,which greatly simplifies the control system structure and parameters of stability.Finally,in order to verify the correctness of the above topology and control scheme,the paper built the simulation models in Matlab/simulink7.0 simulation software through time domain simulation model and control technology over the effectiveness and robustness.The simulation results show a triangular carrier wave phase shift-PWM switching frequency optimization method can improve the utilization of the DC voltage,while the capacitor voltage is within a predetermined range,between the three-phase circulation effectively suppressed by using the negative sequence voltage control compensation,effectively suppress the negative sequence current grid failure situations.