Research On DC Voltage Balance Control Strategy Of Cascaded H-bridge Rectifier

With the development of the smart grid,more and more distributed generation systems are connected to the power grid,which imposes higher requirements on power quality,stability and flexibility of the power system.In the field of electric transmission,the contradiction between the light weight requirement of the electric locomotive traction drive system and the bulky and large volume of power frequency traction transformer is very prominent.The power electronic transformer has the characteristics of high power density,good controllability,strong compatibility and intelligence,which can solve the problems of power system,locomotive traction and distributed generation.The cascaded power electronic transformer is very suitable for the actual industry,so it has great potential value and development prospects.In this paper,based on the input stage of the cascade power electronic transformer—cascaded H-bridge rectifier as the research object,from two aspects of theory analysis and computer simulation research of single-phase and three-phase cascaded H-bridge rectifier working principle,control strategy and dc side capacitor voltage imbalance problem.Firstly,the working principle of single-phase and three-phase cascaded H-bridge rectifier is analyzed and the corresponding mathematical model is established.On this basis,the double closed-loop control strategy based on quasi-proportioned resonant controller is introduced into the single-phase cascaded H-bridge rectifier,and the control performance of the quasi-proportional resonant controller is better than that of the proportional resonant controller under the voltage frequency deviation,and the parametric characteristics of the quasi-proportional resonant controller are analyzed in detail.Then the decoupling control strategy based on dq coordinate system is introduced into the control of three-phase cascaded H-bridge rectifier,and the design method of the controller is given.At the same time,the modulation technology of cascaded multilevel converters is introduced.The application of carrier phase-shifting modulation technique in cascaded H-bridge rectifier is analyzed emphatically,and the conclusion is that the carrier phase-shift modulation technology can reduce the output harmonic by increasing the equivalent switching frequency and offsetting each other by the low-order harmonics.Then,the influence of equivalent load,capacitance parameter and carrier phase shift modulation on the DC side capacitor voltage in cascaded H-bridge rectifier is analyzed.And an improved DC-based voltage balance algorithm based on modulation ratio reconfiguration control strategy is proposed,which can solve the problem of DC voltage imbalance of cascaded H-bridge rectifier under the condition of reducing a large number of PI controllers.Finally,in order to verify the feasibility and correctness of the control strategy?modulation technique and voltage balance algorithm adopted in this paper,the main circuit parameters are designed and the simulation model is established by MATLAB/Simulink.The simulation results strongly validate the correctness,feasibility and validity of the proposed theory.