Research On Dc Side Voltage Equalization Control Strategy Of Cascaded H-Bridge AC/DC Converter

Cascaded AC/DC converters are widely used in new energy and microgrid systems,such as power electronic transformers,STATCOM,etc.Among the cascaded AC/DC converters,the cascaded H-Bridge multilevel converter can maintain the DC side voltage stability and achieve unity power factor because of its simple control,high redundancy,easy modularization and independent control.However,the cascaded H-Bridge multilevel converter has DC side voltage balance problem,which affects the system stability.Therefore,maintain the balance on the DC side voltage has become the focus of scholars’ research.This thesis analyzes the reason of voltage imbalance by studying the mathematical model and topology of cascaded H-Bridge,and proposes a new I-P droop equalization control scheme based on the single-stage frequency-doubling carrier phase-shift modulation strategy.Comparing the traditional balance control scheme with the quasi-distributed droop control scheme in the thesis,clarified the shortcomings of the traditional control scheme due to the increase of the complexity of the algorithm when the number of modules increases,which leads to longer system voltage balance time and lower efficiency.The balanced control scheme adds a droop control module on the basis of the double closed loop,without the need for information interconnection between modules,realizes the independent control of the modules,greatly reduces the time required for voltage balance,improves the operating speed of the system,and achieves unity power factor.And build a two-module model for open and closed loop simulation.This simulation results verify the feasibility and effectiveness of the new I-P droop balance control strategy.On the basis of simulation verification,this paper set up two-module and three-module cascaded H-bridge experimental platform,expounds the design of the main circuit,device selection and control.This experiment using a new quasi-distributed droop equalization control scheme,experiments were carried out in active and reactive power(STATCOM)respectively,and the equalization of the capacitor voltage of each module and the unity power factor of the grid side were realized.The experimental results proved the effectiveness of the proposed control scheme.