بررسی یک آنالوگ چند مرحله ای تک فاز مبتنی بر سری H / موازی اتصال H-پل Investigation of Single-Phase Multilevel Inverter Based on Series/Parallel-Connected H-Bridges

I. INTRODUCTION INDUSTRIAL applications require power converters with high-power levels. Semiconductor switches to process highlevel voltage or current are not always available in the trade or, if they are, they may be very expensive, increasing the system cost. In this context, multilevel converters have been greatly recognized as a solution to employ low-power-rating switches in medium- and high-power applications [1]–[5]. In technical literature, there are three types of classic multilevel topologies: (i) neutral-point-clampled (NPC) converter [6], (ii) flying capacitor (FC) converter [7], and (iii) cascaded H-bridge (CHB) converter [8], [9]. NPC and FC topologies are composed of a single dc-source and legs with series-connected semiconductor switches, while CHB converters provide a large number of levels by simply connecting multiple singlephase converter modules with multiple dc-sources [10], [11]. Modules with low-voltage-rating switches are typically more efficient and cheaper than the high-voltage ones. On the other hand, by connecting the modules in parallel, the system realiability and redundancy are increased [12]. However, the parallel connections create a path between the different modules and circulating currents appear. One simple way of eliminating these currents is making use of an isolation transformer, but the weight, size and cost associated with the transformer may be considered a drawback. Therefore, alternative solutions to mitigate the circulating currents based on control strategies and connection of inductors between the converters were discussed in [13]–[15]. Thus, by connecting switches (like NPC and FC topologies) or converter modules (like CHB topology) in series, it is possible to share the total dc-link voltage among them, reducing the voltage rating as well as the switching semiconductor losses. On the other hand, by connecting converter modules in parallel, it is possible to share the total current among them, reducing the current rating. So, series connections are indicated for medium- and high-voltage applications, whereas parallel connections are recommended for medium- and high-current applications. Multilevel voltages are generated with both types of connection [16]–[20], thus reducing the harmonic distortion when compared to conventional two-level converters. Phase-shifted pulsewidth modulation (PS-PWM) and levelshifted PWM (LS-PWM) strategies use high-frequency triangular carriers. In the paper, a space-vector PWM (SV-PWM) based on LS-PWM modulation is presented. The LS-PWM uses level-shifted carriers to determine the converter switching states. The LS-PWM is generally chosen for multilevel converters, where the number of level-shifted carriers is the number of possible levels minus one. The PS-PWM strategy is most commonly used in systems with parallelism. The number of phase-shifted carriers corresponds to the number of parallel paths of the converter. The triangular carriers are shifted from each other following the ratio 360◦/M (where M is the number of parallel paths of the converter) [1]–[4]. In this work, the PS-PWM and LS-PWM strategies are used together, since a multilevel system with series and parallel connections is proposed. This pape