Joao P. R. A. Mello

Dynamic Voltage Restorer Based on Three-Phase Inverters Cascaded Through an Open-End Winding Transformer

This paper investigates a dynamic voltage restorer (DVR) composed of two conventional three-phase inverters series cascaded through an open-end winding (OEW) transformer, denominated here DVR-OEW. The DVR-OEW operating with either equal or different dc-link voltages is examined. The proposed topology aims to regulate the voltage at the load side in the case of voltage sags/swells, distortion, or unbalance at the grid voltage. A suitable control strategy is developed, including space-vector analysis, level-shifted PWM and its equivalent optimized single-carrier PWM, as well as the operating principles and characteristics of the DVR. Comparisons among the DVR-OEW and conventional configurations, including a neutral-point clamped converter-based DVR, are furnished. The main advantages of the DVR-OEW compared to the conventional topologies lie on: 1) reduced harmonic distortion, 2) reduced converter losses, and 3) reduced voltage rating of the power switches. Simulated and experimental results are presented to validate the theoretical studies.

Shunt active power filter based on cascaded transformers coupled with three-phase bridge converters

This paper proposes a multilevel shunt active power filter (SAPF) to deal with either harmonic currents compensation or reactive power compensation. Such a device can reduce the harmonic distortion at the grid currents provided by non-linear loads located in stiff systems. The proposed SAPF is based on three-phase bridge (TPB) converters connected to cascaded single-phase transformers. The transformers arrangement permits the compensator to use a single dc-link unit which simplifies the control strategy and number of sensors. The multilevel waveforms are generated by using a suitable PWM strategy associated with the transformers turns ratio. Modularity and simple maintenance make the proposed SAPF an attractive solution compared to some conventional configurations. The model, PWM technique and control strategy, are presented as well as studies considering harmonic distortion and semiconductor losses estimation. Simulation and experimental results are presented in order to validate theoretical approaches.

AC/DC converters with open-end grid for AC machine conversion systems

This paper presents five configurations of AC/DC converters for AC machine conversion systems. They interconnect a three-phase open-end winding grid supplier with DC/AC converters and an AC machine. These configurations are divided into two groups. Group A is composed of configurations with two DC-links, and Group B has configurations with only one DC-link. The discussion is focused on the AC/DC converters of the grid-side, since the arrangements for the machine-side are standard in each case. Group A contains two configurations, and the machine-side is composed of a six-phase AC machine with two conventional three-phase DC/AC converters. Group B has three configurations, and the machine-side can be composed of a six-phase AC machine with one six-phase DC/AC converter or of a three-phase AC machine with one three-phase DC/AC converter. The proposed configurations are employed to reduce the harmonic currents of the grid-side or the switching frequency of the converters. They also guarantee feasibility when highpower applications are considered. Therefore, the analyses of the systems are presented, including the PWM technique and the control strategy. Simulation and experimental results are also presented and compared.

Multilevel Reduced Controlled Switches AC–DC Power Conversion Cells

This paper presents multilevel ac–dc single-phase power conversion cells for applications in which the regenerative operation is either not required or prohibited. The ac primary power source is a single phase from a permanent magnet generator or from an electrical grid. The cells are composed of two h-bridges in cascade: 1) an unidirectional high-voltage h-bridge with two controlled switches; and 2) two power diodes, processing the active power, and a low-voltage h-bridge with four controlled switches and a floating capacitor. These cells can be part of multiphase systems, in which it is required one cell for each phase. Some examples of application for three-phase systems are given, exploring different ways of connection between the single-phase cells. Simulation and experimental results regarding the proposed cells and conventional h-bridge based ac–dc converters are shown for validation and comparison purposes.

Shunt Active Power Filter Based on Cascaded Transformers Coupled with Three-Phase Bridge Converters

This paper proposes a multilevel shunt active power filter (SAPF) to deal with either harmonic current compensation or reactive power compensation. Such a device can reduce the harmonic distortion at the grid currents provided by nonlinear loads located in stiff systems. The proposed SAPF is based on three-phase bridge converters connected to cascaded single-phase transformers. The transformer arrangement permits the compensator to use a single dc-link unit, which simplifies the control strategy and the number of sensors. The multilevel waveforms are generated by using a suitable pulse-width modulation (PWM) strategy associated with the transformer turns ratio. Modularity and simple maintenance make the proposed SAPF an attractive solution compared with some conventional configurations. The model, the PWM technique, and the control strategy are presented, as well as studies considering harmonic distortion and semiconductor losses estimation. Simulation and experimental results are presented in order to validate theoretical approaches.

Cascaded open-end winding transformer based DVR

This paper introduces and generalizes a class of multilevel dynamic voltage restorer (DVR) for voltage sags/swells compensation of high-power sensitive loads. Such a device can improve the power quality of sensitive loads located in stiff systems. The proposed DVR is based on three-phase bridge converters series-connected by means of cascaded transformers using the concept of open-end winding (OEW). Hence, two DC-links can provide either symmetrical (i.e., equal DC-link voltages) or asymmetrical (i.e., different DC-link voltages) operation of the DVR converters. Generalization for K-stages is presented as well. The proposed configuration is named as DVR-COEW (i.e., cascaded open-end winding). Such a topology permits to generate a maximized number of voltage levels per converter leg. The multilevel waveforms at the output voltages of the converter are generated by using a suitable PWM strategy associated with both: i) DC-link voltages ratio and ii) transformers turns ratio. Modularity and simple maintenance make the proposed DVR-COEW an attractive solution compared to some conventional configurations. The model and PWM control are addressed in this paper. Simulation and experimental results are presented.

Three-phase four-wire inverters based on cascaded three-phase converters with four and three legs

In this paper, a generalization for three-phase four-wire inverters is proposed. In this generalization, an arbitrary number of three-phase four-wire dc–ac converters are cascaded by means of injection transformers, in order to compose a resultant inverter with a single dc link. Its converters may have four or three legs composed by a pair of the complementary insulated-gate bipolar transistor. The injection transformers have different turns ratios calculated in order to maximize the number of levels produced in the output voltage. In this way, the converter cells within the inverter are asymmetrical in terms of power rating. In this paper, the proposed generalization is applied to a specific case, where two four-wire converters are cascaded, which generates four different proposed configurations. These configurations are analyzed in detail and compared with the conventional four-leg inverter by means of data acquired from simulation and experimental results. In these results, the proposed and conventional inverters feed an unbalanced three-phase load accordingly to the same reference voltage, and the load current total harmonic distortion, voltage weighted total harmonic distortion, transformers and semiconductors losses, switching frequency, and power rating are analyzed in order to conclude that proposed configurations present better performance, even though they have more semiconductors and demand injection transformers.

Multilevel reduced controlled switches AC-DC power conversion cells

This paper presents multilevel ac–dc single-phase power conversion cells for applications in which the regenerative operation is either not required or prohibited. The ac primary power source is a single phase from a permanent magnet generator or from an electrical grid. The cells are composed of two h-bridges in cascade: 1) an unidirectional high-voltage h-bridge with two controlled switches; and 2) two power diodes, processing the active power, and a low-voltage h-bridge with four controlled switches and a floating capacitor. These cells can be part of multiphase systems, in which it is required one cell for each phase. Some examples of application for three-phase systems are given, exploring different ways of connection between the single-phase cells. Simulation and experimental results regarding the proposed cells and conventional h-bridge based ac–dc converters are shown for validation and comparison purposes.

Four-wired dynamic voltage restorers based on cascade open-end winding transformers

This work studies three-phase four-wire dynamic voltage restorers (DVRs) based on a cascade of open-end winding transformers. The possible proposed topologies can be observed if each transformer set is connected to the conventional two-level (2L) three-phase four leg converters and two dc-links per stage. However, in this paper particular/intermediate solutions are also considered by means of the number of fourth-legs connected at each cell and the possibility to reduce the number of dc-links without issues associated with common mode currents (CMC). The configurations are generalized for K stages. In this way, the proposed topologies can have K + 1 or 2K (the focus of this paper) dc-links connected to the converter pairs. The fourth-leg features in the converters improve the zero-sequence voltage compensation capability for the DVR and extra dc-links improves the multilevel performance. Additionally, an optimized multilevel performance is achieved by considering the transformers turns ratios. Symmetrical (i.e., equal dc-link voltages) and Asymmetrical operation (i.e., unequal dc-link voltages) cases are also considered. Modeling, PWM and control strategy are presented. Simulation and experimental results are provided to validate theoretical approaches.

Investigation of power rectifier under non-sinusoidal input based on hybrid multilevel converter

This paper discusses a power rectifier based on a cascaded topology composed of a three-leg cell and three H-bridge cells. Each leg of the three-leg cell is cascaded connected to an H-bridge cell. Since different types of converter are series-connected, the topology is assumed to be hybrid. The capacitors of the H-bridge cells operate as floating capacitors. It is shown that the discussed topology permits the operation even if the grid voltages present harmonic components without the need to increase the capacitor voltages. Control and Pulse Width Modulation strategies are explained in order to adjust the capacitor voltages and grid currents. Simulation and experimental results demonstrate the validity of the discussed strategies and the feasibility of the system. The hybrid topology has its harmonic distortion and semiconductor losses values compared to the ones obtained with the conventional three-leg converter.