| Multilevel inverters(MLIs)have become increasingly important due to the increasing demand of industrial applications.Cascaded H-bridge MLIs(CHBMLIs)play a role in medium and high voltage appliances because of their superiority such as simple structure,easy modularization and expansion etc.The CHBMLIs can be classified into symmetrical and asymmetric according to different DC voltage ratios.In this paper,CHBMLIs are taken as the research object,and these symmetric and asymmetric topology and corresponding modulation technology are studied in depth.The specific work is as follows:(1)Phase Disposition(PD)method presents excellent harmonic elimination performance when applied to the symmetric CHBMLIs.However,the power of each cell cannot be evenly distributed.Thus,a novel modulation scheme is proposed after a profound analysis of degree of freedom on the basis of PD scheme presented,so as to ensure power sharing among all modules and excellent harmonic profile of line-line voltage.The proposed scheme utilizes all carriers to achieve rotation in a carrier interval among different carrier layers.This method not only achieve optimal harmonic elimination and power balance,but also accelerate the process of power sharing among all cells.Finally,simulation and experimental verification are carried out.(2)When hybrid frequency modulation strategy is applied to the asymmetric CHB with DC voltage in a ratio 1:1:2,power sharing among all modules is not achieved.Thus,a novel modulation scheme is proposed.Part of the high voltage cell is unchanged,while the low voltage cells are replaced by improved level shifted modulation,and then dual-layer power balance control is carried out based on it.The upper layer is balanced control between high and low voltage cells,and the output voltage is redistributed by adjusting the conduction angle of the high voltage cell.The lower layer is the balance control among the low voltage cell,and the improved LS-PWM achieves carrier rotation to evenly distribute the output voltage between the low voltage cells.Compared to hybrid frequency modulation method,the proposed method can not only achieve the even power distribution among operating cells,but also evenly distribute the switching loss among the low voltage cells,at the same time,it can reduce the overall switching loss.Finally,the results are verified by simulation and experiment.(3)For symmetric CHBMLIs,1/4 periodic symmetric nonlinear harmonic elimination equations are established.This paper presents the solving principles of Newton(NR)and particle swarm optimization(PSO),and analyzes their shortcomings,that is,NR method has good local optimization ability,but extremely depends on initial value,while PSO algorithm does not depend on initial value,but local convergence is slow.On this basis,a PSO-NR algorithm is proposed,which consists of two parts.The global optimization is carried out by using the optimized PSO,and NR is responsible for local optimization by using the local extreme value obtained by PSO.The method improves the success rate of solving SHEPWM harmonic elimination equations,and expands the range of solution.The obtained switching angle can effectively eliminate the target sub-harmonics,which is verified by simulation and experiment.Figure 49 Table 18 Reference 74... |
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