Research On Pulse Width Modulation Strategy For NPC/H-Bridge Five-Level Inverter

Comparing with traditional two-level inverter, multilevel inverter has the merits of lower harmonic content, lower swithing loss, small EMI and so on.This paper took NPC/H-Bridge five-level inverter as the research object. In order to improve the output performance of the inverter, a comprehensive study of its PWM strategies was carried out. The main research contents are as follows:1. The structural features and working principle of NPC/H-Bridge five-level inverter were introduced. Aming at the shortcomings of the unbalance device power between and in the three-level bridge arms and uneasy applied to NPC/H-Bridge cascaded multilevel inverter when applied PWM strategies based on vertical distribution of carriers to the topology, three hibrid carrier based PWM methods were analysized. Theoretical analysis and simulative results showed that these three methods can preferably balance the device power between and in the bridge arms, increase the equivalent carrier switching frequency, and are easy applied to NPC/H-Bridge cascaded multilevel inverter. For the sake of improve the DC voltage utilization of the above carrier based PWM methods and decrease the switching loss, this paper adopted the switching frequency optimal PWM method, which can improve the DC voltage utilization about 15%, reaching the effect of SVPWM algorithm. To reduce the carrier number, a minimum carrier PWM strategy was studied both by simulation and experimentation. Finally, a comparison of harmonic performance and fundamental wave content of output line voltage of all the above modulation strategies was carried out under different modulation index.2. The specific implementation steps when applied traditional SVPWM algorithm to NPC/H-Bridge five-level inverter was detailedly introduced and verified by simulation and experimentation. Aming at the problem of large amount calculation and bad real time in this algorithm, three generalized and fast multilevel simplified SVPWM algorithms were studied. Simulative and experimental results showed that these three algorithms have the advantages of small amount calculation, uncomplicated to realization and easy to be applied to higher level.According to the feature of the NPC/H-Bridge topology, a phase shift SVPWM algorithm suitable for NPC/H-Bridge cascaded multilevel inverter was proposed by combination of three-level SVPWM and phase shift technology. This method can overcome the shortcomings of carrier based PWM methods, such as low DC voltage utilization, large harmonic content, uneasy to optimize the output performance of the inverter and so on. Taking two-level and three-level for instance, the essence connection between SPWM method and SVPWM method were deeply investigated.3. The common-mode voltage generation mechanism of NPC/H-Bridge five-level inverter was analysized. Aming at the problem of less obvious common-mode voltage reduction effect of the traditional 7-segent type SVPWM, this paper started from the modulation strategies, a 5-segent type SVPWM was designed which can not only restrain the maximum value to a lesser extent, but also reduce switching loss. Meanwhile, two modulation strategies which can basically eliminate the common-mode voltage were studied, and all the above reduce or eliminate common-mode voltage strategies were verified by simulation.4. A five-level experimental prototype which takes TMS320F28335 DSP and XC3S400 FPGA as the core controller was presented. All the modulation strategies were verified on this experimental prototype.