Modulation Strategy Of Type-D1 Of Hybrid CHB Inverter And Optimization Of Its Power Balance

With the wide application of high-voltage and high-power inverters in the industrial field,cascaded H-Bridge(CHB)multilevel inverters and corresponding modulation technologies have rapidly developed.CHB multi-level inverter has become a research hotspot because of its advantages such as simple control,low output voltage harmonic content,high reliability,and easy modularization.This article takes the hybrid CHB multi-level inverter as the research object,and the specific content is as follows:Starting from the topology and modulation strategy of CHB multilevel inverter,this article focuses on the research of D1-2 hybrid CHB nine level inverter,and expounds the generation mechanism and elimination method of current backflow.In the traditional CBPWM modulation strategy,LS-PWM modulation strategy improves the waveform quality,but there is the problem of power imbalance.PS-PWM modulation strategy has the characteristics of frequency doubling effect and self equalization,but its high-voltage unit works in high frequency,which increases the switching loss and reduces the service life.Aiming at the problem of power imbalance between cascaded units,four modulation strategies are proposed for D1-2 hybrid CHB nine level inverter.One is the modulation strategy based on 1/4 carrier cycle,and on this basis,an optimization scheme is proposed to reconstruct the modulation wave,so that the number of carrier layers is greatly reduced,and the power balance of two low-voltage units is realized.Second,based on the idea of logic control,the driving signal is directly logically calculated,which greatly reduces the difficulty of digital design and implementation,and then realizes the cyclic distribution of output voltage and symmetrical output waveform of all levels of units,and finally realizes the power balance of two low-voltage units.Thrid,based on the high-frequency CBPWM modulation strategy to achieve the balance of the two low-voltage units,the power is distributed by calculating the switching angle of the high-voltage unit,and the power balance of the whole unit is realized.Fourth,based on the low-frequency SHEPWM modulation strategy,the power balance of the whole unit is achieved by using the strategy of equivalent splitting of fundamental amplitude without increasing the switching frequency of the device.The simulation model and experimental platform are built to verify the correctness and feasibility of the proposed strategy.The topology law is further extended,and the topology of P low-voltage units and Q high-voltage units is analyzed.It is found that with the increase of cascade number,how to simplify modulation strategy and eliminate the problem of reverse current is found.How to reduce the switching frequency of high-voltage units and optimize the waveform quality become a contradiction point.Aiming at this problem,a hybrid modulation strategy for the high and low voltage units is proposed.The high-voltage units adopt a ladder wave and the low-voltage units adopt a CBPWM modulation strategy based on carrier cycle,which can obtain more levels and better output waveform quality while eliminating the problem of reverse current.At the same time,it can prevent the high-voltage units from working in a high-frequency state,and use a logic control strategy to exchange the high-voltage original drive signal for 1/4 cycle to achieve power balance.Finally,a simulation model and experimental platform of the D1-D2-type hybrid CHB thirteen-level inverter was built,and its effectiveness and feasibility were verified through simulation analysis and experiments.