Peter Barbosa

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

A zero-voltage and zero-current switching three-level DC/DC converter

This paper presents a novel zero-voltage and zero-current switching (ZVZCS) three-level DC/DC converter. This converter overcomes the drawbacks presented by the conventional zero-voltage switching (ZVS) three-level converter, such as high circulating energy, severe parasitic ringing on the rectifier diodes, and limited ZVS load range for the inner switches. The converter presented in this paper uses a phase-shift control with a flying capacitor in the primary side to achieve ZVS for the outer switches. Additionally, the converter uses an auxiliary circuit to reset the primary current during the freewheeling stage to achieve zero-current switching (ZCS) for the inner switches. The principle of operation and the DC characteristics of the new converter are analyzed and verified on a 6 kW, 100 kHz experimental prototype.

Active Neutral-Point-Clamped Multilevel Converters

This paper proposes a multilevel power conversion concept based on the combination of neutral-point-clamped (NPC) and floating capacitor converters. In the proposed scheme, the voltage balancing across the floating capacitors is achieved by using a proper selection of redundant switching states, and the neutral-point voltage is controlled by the classical dc offset injection. Experimental results are illustrated in the paper to demonstrate the system operation

Topologies and design considerations for distributed power system applications

This paper describes developments in the technology of converters primarily intended for server-type distributed power system (DPS) applications. The paper first addresses the single-phase two-stage DPS, discussing the available options for simple power factor correction and high-performance DC-DC converters. Next, the concept is extended to three-phase high-power front-end converters, which also use a two-stage approach. Finally the paper presents the developments in load converters, with emphasis on low-, medium-, and high-voltage regulator modules used to supply high-performance microprocessors. The discussion throughout the paper is supported by an extensive list of references and experimental results.

Active-neutral-point-clamped (ANPC) multilevel converter technology

This paper proposes a multilevel power conversion concept based on the combination of neutral-point-clamped (NPC) and floating capacitor converters. In the proposed scheme, the voltage balancing across the floating capacitors is achieved by using a proper selection of redundant switching states, and the neutral-point voltage is controlled by the classical dc offset injection. Experimental results are illustrated in the paper to demonstrate the system operation

Operation and Control of a Hybrid Seven-Level Converter

This paper introduces a novel hybrid seven-level converter that is based on the upgrade of the five-level active neutral-point-clamped converter concept and is suitable for high-power applications. The paper provides a comprehensive analysis for the operation of the converter. Based on the analysis, a space vector modulation (SVM)-based switching strategy that takes advantage of redundant switching vectors of the SVM strategy to counteract the voltage drift phenomenon of the proposed converter is proposed. The limit to the range of operation of the seven-level converter based on the proposed SVM strategy is also presented. It is shown that the ability to stabilize the dc-link capacitor voltages and the per-phase flying capacitors is a function of the converter operating indices, i.e., the load power factor and modulation index. The salient feature of the proposed SVM-based control strategy is that it enables proper operation of the converter with no requirements for additional controls or auxiliary power circuitry, within the specified range of operation. Performance of a converter under various operating conditions, based on the proposed SVM strategy, in the MATLAB/Simulink environment, is evaluated and experimentally demonstrated.

A three-level converter and its applicationto power factor correction

The first part of this paper introduces a novel three-level dc-dc converter topology. It is shown that a floating capacitor connected across the clamping diodes of a conventional three-level converter enables phase-shift control while retaining zero voltage switching. Design constraints and experimental results are shown for a 6-kW, 100-kHz dc-dc prototype. The second part of the paper discusses the same three-level topology operated as a single-stage front-end converter. In this application, the converter realizes power factor correction and dc output voltage regulation. To ensure low harmonic distortion, the input currents are operated in discontinuous conduction mode, and the performance, including soft switching, is also verified on a 3-kW, 50-kHz ac-dc converter.

A modified direct power control strategy allowing the connection of three-phase inverter to the grid through LCL filters

This paper proposes a novel approach to adapt a conventional direct power control (DPC) for high-power applications, where a third-order LCL filter is frequently required. The LCL filter can cause a strong resonance and requires additional effort for system control. The application of a DPC for the control of a three-phase voltage source inverter that is connected to the grid through an filter has not yet been considered. The addition of an active damping strategy, together with a harmonic rejection control loop, to the conventional DPC is proposed and analyzed in this paper. The steady-state, as well as the dynamic performance of the proposed system, is verified with simulation results and experimental measurements.

Five-level virtual-flux direct power control for the active neutral-point clamped multilevel inverter

Although multilevel inverters present numerous advantages such as high quality waveform, low switching losses, high voltage capability and low electromagnetic compatibility concerns, some drawbacks are evident. They require a higher number of semiconductors and either multiple isolated dc sources or a bank of series connected capacitors. Consequently, the control complexity increases considerably, since more switching devices normally result in a higher number of possible combinations and the balance of the capacitors has to be guaranteed. But on the other hand, multilevel inverters create an extra degree of freedom due to existing redundant voltage vectors, which produce the same output phase voltage level but with diverse effect on the dc-link and floating capacitors. Among the existing control techniques the virtual-flux direct power control (VF-DPC) has showed to be very suitable for grid connected systems since it controls the active and reactive powers directly without any internal current control loop or PWM modulator. However, in order to adapt the VF-DPC for multilevel systems, specifically for the recently proposed five-level Active Neutral-Point Clamped converter, additional features must be included and/or modified in the inner main control loop. In order to allow the controller to select a higher number of available voltage vectors, the active and reactive power hysteresis strategies are modified. Additionally, a method to balance the dc-link and floating capacitor voltages by applying available redundant states is implemented, based on the actual condition of the voltage across the lower dc-link and floating capacitors, as well output phase currents direction. The proposed five-level VF-DPC has been implemented using a 6 kW five-level prototype and has shown good static and dynamic performance.

A wide input voltage and load output variations fixed-frequency ZVS DC/DC LLC resonant converter for high-power applications

This paper presents a fixed-frequency zero-voltage-switching three-level DC/DC resonant converter. By applying phase-shift control between the primary and secondary sides of the transformer, the converter can operate at a fixed switching frequency. The converter can achieve ZVS operation in the entire load range by using the magnetizing inductance of the transformer. In addition, the converter can operate with wide input-voltage variations without penalizing the efficiency. Therefore, the converter is suitable for applications in which high efficiency and high power density are required. The principle of operation for the converter is analyzed and verified on a 2.75 kW, 775 kHz experimental prototype.