Research On Sub-module Capacitance Optimization Strategy For CHB-SST Based On Virtual Zero Impedance

Due to the access of distributed energy and renewable energy,the form of distribution network has gradually developed into the intelligent distribution system,the cascaded H-bridge-based solid state transformer(CHB-SST)applied in intelligent distribution network has been widely applied and studied due to the advantages of uncomplicated layout,high-degree modularization and so on.However,the current characteristics of the sub-modules(SMs)of CHB stage will lead to the second-frequency voltage ripple on the DC side,which requires large SM capacitance to suppress.The large capacitors of SMs restrict the improvement of the system power density,and become an inherent problem of CHB-SST.In view of the inherent problem,control strategies,operating characteristics and SM capacitance optimization methods of CHB-SST are studied in this paper.Firstly,the inherent SM large capacitance problem of CHB-SST is analyzed,and the principle of SM capacitance optimization based on virtual zero impedance is proposed.The generation mechanism of second-order frequency ripple current(SFRC)of CHB stage is analyzed,and the SM current distribution model is established.The equivalent impedance of SM capacitance and QAB under traditional single voltage closed-loop control is derived,and the impedance channel through which SFRC flows is analyzed by the impedance model.The generation mechanism of ripple voltage is analyzed,and the constraint of SM capacitance under traditional control is given.According to the three-phase symmetry of SM second-harmonic ripple power,the idea of establishing low-frequency harmonic impedance channel based on virtual zero impedance(VZI)to optimize SM capacitance is proposed,and the new SM capacitance constraint is given.Secondly,a SM capacitance optimization method based on current-mode virtual zero impedance(CMVZI)paralleled is proposed.The establishment method of low-frequency harmonic impedance channel based on CMVZI is introduced,and a voltage and current double-loop control strategy for QAB stage is proposed,which makes SFRC transfer to QAB stage through current open-loop control.The equivalent input impedance of QAB is derived with the proposed current-mode virtual zero impedance-based capacitance optimization(CMVZI-CO)control strategy,the actual distribution of SFRC is given according to the SM capacitance impedance and the equivalent input impedance of QAB,which verifies the feasibility of the proposed CMVZI-CO control strategy by theoretical analysis.A 1.8MW simulation system is built in PLECS,and a reduced-scale experimental platform is built based on TMS320F28335.The feasibility of the proposed CMVZI-CO control strategy is verified by both simulation and experiment.And then,a SM capacitance optimization method based on voltage-mode virtual zero impedance(VMVZI)paralleled is proposed.The establishment method of low-frequency harmonic impedance channel based on VMVZI is introduced,and a double voltage closed-loop control strategy for QAB stage is proposed.An improved proportional integral and resonant(IPIR)controller is designed to track the second-frequency ripple voltage of SM and make SFRC decouple with SM capacitor.The equivalent input impedance of QAB is derived with the proposed voltage-mode virtual zero impedance-based capacitance optimization(VMVZI-CO)control strategy,the actual distribution of SFRC is given according to the SM capacitance impedance and the equivalent input impedance of QAB,which verifies the feasibility of the proposed VMVZI-CO control strategy by theoretical analysis.A 1.8MW simulation system is built in PLECS to verify the feasibility of the proposed VMVZI-CO control strategy.And the feasibility of the proposed VMVZI-CO control strategy is also verified based on the built reduced-scale experimental platform.Finally,under the traditional control strategy and the proposed virtual zero impedance-based capacitance optimization(VZI-CO)control strategy proposed in this paper,the current stress and efficiency of QAB stage are evaluated,the switches of CHB stage and QAB stage,as well as the capacitance of SMs are selected,the inductance on primary side and the high-frequency transformer of QAB stage are designed and the 3D engineering model is given by SOLIDWORKS.The value of the proposed VZI-CO control strategy in practical engineering application is evaluated from the perspective of the volume of system.