Reuben P. R. Sousa

Comparative analysis of cascaded inverters based on 5-level and 3-level H-bridges

This paper investigates two converters composed of a five-level H-bridge cell cascaded with a three-level one. Two types of 5-level H-bridges are used, the first one is composed of a NPC leg and a 2-level leg (in the first topology), and the second one is composed of NPC legs only (in the second configuration). In both topologies the 5-level cell is cascaded with a 3-level H-bridge. Both converters were analyzed for the DC-link voltage ratios 1:1, 1:4 and 1:6 between the two power cells. These cases have been chosen due to the synthesis of equally spaced converter voltage levels. A generic PWM strategy for both configurations that covers any DC-link voltage ratio is presented. In all cases, both converters were compared in therms of harmonic distortion and semiconductor power losses, as well as in terms of redundant switch states. Experimental results are presented in order to show the feasibility of the topologies and the validity of the discussed PWM strategies.

An unidirectional single-phase AC-DC-AC three-level three-leg converter

An unidirectional AC-DC-AC three-level three-leg converter is proposed in this paper. This converter is composed of an unidirectional rectifier and a bidirectional inverter, using two bidirectional NPC legs and an unidirectional NPC leg. A bidirectional NPC leg is shared between rectifier and inverter. When compared with the two-level leg, the three-level leg presents voltage stress reduction on switches for the same DC-link voltage level and also allows a reduction in harmonic distortion for the same switching frequency. Suitable modeling, PWM and control strategies of the system are presented. As the converter is unidirectional, the grid current and the generated voltage at load side must be synchronized with generated voltage at grid side. That eliminates the zero-crossover distortion in the grid current caused by the use of diodes without the hysteresis control. Simulation and experimental results are also presented.

Hybrid Dual and NPC Six-Phase Drive Systems

This paper presents two hybrid six-phase machine drive systems. The topologies are considered to be hybrid because two three-phase groups of the machine are fed by different types of converters. The purpose of these topologies is to be alternative configurations, being midway between the conventional six-leg two-level (2L) inverter, composed of 12 switches, and more complex structures, such as the six-leg three-level neutral point clamped (3L-NPC) inverter, composed of 24 switches and 12 clamping diodes, or the conventional 12-leg dual inverter, composed of 24 switches. One of the hybrid topologies is composed of 18 switches and the other is composed of 18 switches and 6 clamping diodes. A comparative study of harmonic distortion, semiconductor losses, and machine torque ripple is performed. Compared with the conventional 2L inverter, the proposed systems provide better performance in terms of harmonic distortion, overall semiconductor losses, and machine torque ripple and are less complex than dual and NPC inverters. Pulse width modulation (PWM) strategy is discussed and simulated. A modification in PWM strategy of the hybrid configurations is performed in order to obtain similar torque ripple to the cases when dual and 3L-NPC converters are used, even with lower number of semiconductor devices. Experimental results are also presented in order to validate the PWM strategy.

Three-phase/four-wire dc/ac converters based on three- and two-level legs

In this paper, four three-phase/four-wire dc/ac converter topologies assembled with three-level and two-level legs are proposed. Several relevant characteristics of the converters are addressed, such as voltage capability, harmonic distortion, and power losses. Experimental and simulated results are presented.

AC drive systems based on six-phase machine with three neutrals

This paper presents an AC drive system based on six-phase machine with three neutrals. The machine side converter has nine power legs and can be connected to the grid through one single DC-link or three isolated DC-links. Compared with the conventional back-to-back AC six-phase machine drive system, the proposed drive system permits reducing the DC-link voltages, eliminating step-up transformer, and the voltage at the grid side converter has higher amplitude than that of the machine side, reducing the transmission I2R losses. The system model and PWM strategy are presented and experimental results demonstrate the feasibility of the system.

Hybrid asymmetric cascaded multilevel inverters based on three- and nine-level H-bridges

This paper proposes two hybrid asymmetric cascaded multilevel inverters. Both inverters consist of the series connection of a three-level (3L) H-bridge cell and a nine-level (9L) H-bridge cell with an unequal dc voltage ratio of 1:8. The topologies of the 9L H-bridge cell are based on modular multilevel cascade converter (MMCC) with double-star chopper cells (DSCC). Both the proposed inverters can produce 19 output voltage levels. To accomplish this, a modulation strategy is developed. Compared to 19-level conventional MMCC, the proposed topologies have fewer components and less losses in the semiconductor devices. Additionally, a voltage balancing algorithm and circulating current control are discussed in order to maintain the stability of the system. At last, to validate the proposed modulation and control strategy, simulation and experimental results are presented.

Series connected three-phase AC-DC power converters

This paper presents three three-phase AC-DC configurations in which two are unidirectional and have reduced switch count. The investigated topologies have one DC-link and are composed by two converters in series connection through transformers, that process part of the total power, while the conventional converters have two DC-links and a transformer processing all the power. The replacement of switches by diodes aims to reduce the system complexity and costs, so that less driver circuitry is necessary. The proposed converters presented equal or less power losses in relation to the conventional ones. The model and control strategy of the systems, as well as simulation and experimental results are presented.

Unidirectional single-phase AC-DC-AC three-level and two-level three-leg converters

Four unidirectional AC-DC-AC three-level and two-level three-leg converters are proposed in this paper. This converters are composed of an unidirectional rectifier and a bidirectional inverter, using three-level and two-level legs and an unidirectional NPC leg at rectifier side. When compared with the two-level leg, the three-level leg presents voltage stress reduction on switches for the same DC-link voltage level and also allows a reduction in harmonic distortion for the same switching frequency. Suitable modeling, PWM and control strategies of the systems are presented. As the converters are unidirectional, the grid current and the generated voltage at load side must be synchronized with generated voltage at grid side. That eliminates the zero-crossover distortion in the grid current caused by the use of diodes without the hysteresis control. Simulation and experimental results are also presented.

Delta interconnected hybrid three-leg converters

This paper proposes possibilities of interconnected hybrid three-leg converters, that are comprise of combinations of two-level and/or three-level legs. In this way, four topologies are presented and compared in aspects such as harmonic distortion of grid currents, semiconductor losses and rating of power switches. The hybrid solution allows improvements in quality of the output converter voltages, reducing the harmonic distortion of grid current. This advantage can be turned into switching frequency reduction and consequently semiconductor losses reduction. Circuit model equations, simulation and experimental results (acting as shunt compensator and inverter) are presented as well.

Open-end unidirectional topologies with reduced controlled switch count

In this work, two three-phase unidirectional open-end AC-DC configurations with reduced switch count were investigated. Both topologies have two DC-links and arise from the replacement of controlled switches by diodes, in order to reduce system complexity and costs. The first topology is composed by two converters: a three-level NPC (Neutral-Point Clamped) and a three-phase diode rectifier. The second configuration consists in two three-phase three-level NPC converters with some switches replaced by diodes. The comparison between proposed and conventional topologies in terms of harmonic distortion and power losses is presented. The control strategy of the systems as well as a detailed model, simulation and experimental results are presented.