Voltage Balance Control Strategy Of Single-phase Diode Clamped Cascaded Rectifier

Due to its characteristics of high speed,comfort and punctuality,safety and environmental protection,high-speed railway is increasingly becoming one of the preferred means of transportation.With the increasing demand for high-speed railway transport capacity,the research of double-deck high-speed railway and the key technologies related to the reduction of weight and volume is imminent.Due to the rapid development of multi-level power electronics technology in recent years,power electronics transformers are born in operation.Compared with the low-frequency transformers in the traditional traction system,power electronics transformers have the advantages of small size,light weight.However,the existing industrial-grade power electronics transformers are complex in working conditions,which bring many challenges to the control strategy and modulation strategy of their front-end cascaded rectifier.In this paper,the single-phase NPC cascaded rectifier is taken as the research object.From theoretical derivation,simulation and experiment,the control strategy,modulation strategy,capacitor voltage balance strategy and DC-link voltage balance modulation strategy are studied.The main contents of this paper include:Firstly,according to the working characteristics of the single-phase NPC cascaded rectifier,the switching state and topology are modeled and analyzed.The state space mathematical model including grid-side current,grid-side voltage,DC-link voltage and other parameters is studied,and the control strategy based on transient direct current control is studied.Finally,the correctness of the theory is verified by simulation software.Secondly,in order to achieve DC-link voltage balance control,this paper studies the mechanism of DC-link voltage imbalance within and between modules of single-phase NPC cascaded rectifier,and studies the solution of DC-link voltage unbalance within and between modules.The implementation method of single-phase SVPWM modulation strategy is discussed in detail,and the effect of redundancy vector on DC-link voltage is discussed by taking three-module single-phase NPC cascaded rectifier as an example,which lays the foundation for the method of voltage balance between modules using redundancy vector.A Fixed Smooth Pulse Sequences Modulation(F-3SM)is studied to solve the the DC-link voltage unbalance between modules.The F-3SM effectively avoid the voltage level step,thus effectively reduce the frequency of the switch.The load unbalance degree was defined,and the stable control range of F-3SM is obtained by calculating the power balance and volt-second balance.On the basis of theoretical research,a single-phase NPC cascaded rectifier is built by MATLAB /Simulink simulation platform.The simulation results of grid-link current,DC-link voltage,capacitor voltage in the module and DC-link voltage between modules were analyzed under different working conditions,such as the system without F-3SM voltage balance strategy.The simulation results show that the system can work normally and stably under various working conditions,which verifies the correctness of the control strategy,the modulation strategy,the voltage balance strategy within the module and the voltage balance strategy between the modules.In order to verify the reliability of the research content furtherly,a low-power experimental platform was built to complete the design of experimental parameters and software programming.The single-phase NPC inverter experiment,single-phase NPC rectifier experiment and three-module single-phase NPC cascaded rectifier experiment were completed by using this platform.Experimental results prove the rationality of the main circuit structure,control strategy and modulation strategy of the system.The modulation and control strategies of cascaded rectifier studied in this paper can be extended to multi-modules.Besides,the cascaded rectifier could work normally and the DC-link voltage of each module is balanced when the loads of modules are extremely unbalanced,otherwise,the port input voltage level step is eliminated and the grid-link current does not distort.F-3SM can improve the reliability and safety of the vehicle power electronic transformer.