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Publications

Publications by Nayara Brandão de Freitas

2020

Multilevel Single-Phase Converter With Two DC Links

Authors
de Freitas, NB; Jacobina, CB; Cunha, MF;

Publication
IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS

Abstract
In this article, a multilevel single-phase converter is proposed and investigated. Its structure is based on cascaded-transformer systems, which are very interesting in applications in which a single dc source is available. The features of well-known multilevel cascaded H-bridge and transformer-based solutions are integrated into the proposed converter. As a result, the number of synthesized voltage levels can be optimized without excessively increasing the number of transformers. The basic configuration has six two-level insulated-gate bipolar transistor legs, two injection transformers, and two dc links (the lowest voltage dc link may be a floating capacitor or be connected to a small dc power source). The configuration is generalized and the calculation of the transformer's turns ratios as well as the dc-link voltages to maximize the number of voltage levels is provided. The proposed configuration is compared with cascaded H-bridge and a single-phase shared leg converter, which are also cascaded by means of transformers. Compared with the conventional converters, the proposed one has lower switching losses and higher conduction losses. Thus, the proposed configuration is more interesting in terms of semiconductor losses for high-voltage and low-current applications. Experimental and simulation results are shown to demonstrate the feasibility of the system.

2019

Transformer-Based Single-Phase AC-DC-AC Topology for Grid Issues Mitigation

Authors
de Freitas, NB; Jacobina, CB; Gehrke, BS; Cunha, MF;

Publication
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS

Abstract
This paper proposes a single-phase ac-dc-ac converter composed of a transformer and two three-leg converters connected in series. The proposed converter allows to supply the load with a voltage with constant amplitude and frequency, and to operate with the grid current with low harmonic distortion and high power factor. In addition, the converter can be used to mitigate fundamental overvoltage and voltage harmonics at the grid. The system model, regions of operation, a power flow analysis, and a method to regulate the dc-link voltages using the voltage vector redundancies are given. In the first version of this paper, two pulsewidth modulation (PWM) techniques (including space vector PWM) and a control system were provided. The proposed topology is compared with two conventional systems in terms of operation range, harmonic distortions, and semiconductor losses. Simulation and experimental results are provided to validate the feasibility of the system.

2020

Two Novel Open-End Winding Multilevel Unidirectional Six-Phase Rectifiers With Reduced Switch Count

Authors
da Silva, I; Jacobina, CB; de Freitas, IS; Sousa, RPR; Maia, ACN; de Freitas, NB;

Publication
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS

Abstract
In this article, two new unidirectional multilevel six-phase power rectifier topologies are proposed and investigated. Such topologies may be applied to ac-dc systems such as wind energy conversions systems, aerospace generator drives, telecommunications or any other application where regenerative operation is either not required or prohibited. The first topology is composed of two three-level neutral-point-clamped (NPC) converters connected to the open-end windings of a six-phase permanent magnet synchronous generator, and a three-phase noncontrolled diode bridge converter. The second topology is composed of three modified NPC converters with the substitution of two controlled switches by diodes in each leg. The use of a noncontrolled converter with diodes aims to reduce the controlled switches count, the system complexity, and the costs. However, it makes both systems nonreversible. The system models, operating principles, the space vector pulsewidth modulation strategies, the dc-links balance, and the control system to ensure the elimination of the zero-crossover distortion caused by the use of the diodes are presented. Due to the high number of levels generated, the systems are suitable for high power applications with voltage and current ratings restrictions. A study concerning harmonic distortion and semiconductor losses for both systems is performed, in order to be compared with a standard configuration. The feasibility of the system is demonstrated by simulation and experimental results.

2020

Capacitor Voltage Balancing for Single-Phase Asymmetric Cascaded H-Bridge Inverters

Authors
Monteiro, AP; Jacobina, CB; Mello, JPRA; de Freitas, NB; Matias, RR;

Publication
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS

Abstract
This article presents an analysis of conventional single-phase cascaded H-bridge multilevel inverters composed of two and three cells. The dc-link can be a dc source or a floating capacitor. Two methods are given to regulate the floating capacitor voltage. In this way, according to the capacitor voltage balancing method used, operating regions and power distribution on the floating capacitor are presented for different dc-link voltage ratios and several ranges of modulation index and load power factor. A mathematical analysis is done in order to demonstrate the influence of the modulation index and load power factor over the power distribution. Finally, simulation and experimental results are provided to validate the theoretical considerations.

2021

Grid-Connected Induction Motor Using a Floating DC-Link Converter Under Unbalanced Voltage Sag

Authors
Cunha, MF; Jacobina, CB; de Freitas, NB;

Publication
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS

Abstract
This article proposes a series compensator with unbalanced voltage sag ride-through capability applied to grid-connected induction motors. A conventional three-phase voltage source inverter (VSI) is intended to regulate the motor voltages. The VSI is connected in series with the grid and a three-phase machine with open-ended windings. The proposed system is suitable for applications in which no frequency variation is required, like large pumps or fans. The VSI dc-link voltage operates as a floating capacitor through the energy minimized compensation (EMC) technique, in which there is no dc source or injection transformer. The motor load condition determines the minimum grid voltage positive component (sag severity) to keep EMC operation. Meanwhile, a voltage unbalance may increase the dc-link voltage requirements. A 1.5-hp four-pole induction motor has been used to verify the ride-through capability of the proposed compensator under grid voltage disturbances. A total harmonic distortion (THD) analysis of grid currents demonstrates that the proposed system provides low THD even if no passive filter is used. The operating principle, converter output voltage analysis, pulsewidth modulation technique, control strategy, and components ratings are discussed as well. Simulation and experimental results are presented to demonstrate the feasibility of the system.

2020

Single-Phase Cascaded H-Bridge Inverters Without Power Regeneration

Authors
Monteiro, AP; Jacobina, CB; Mello, JPRA; de Freitas, NB;

Publication
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS

Abstract
Cascaded H-bridge (CHB) inverters may have some of its cells with a bidirectional power flow between the dc source and the load. Considering this, switching strategies for multilevel CHB inverters are proposed for applications in which the regenerative mode is not desired, forcing all the cells to have either null or positive power flow toward the load. In order to validate the theoretical considerations, simulation and experimental analysis are made, where it is possible to maintain the high voltage quality and avoid the regenerative mode at any operating point.

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