Multilevel Converters: An Enabling Technology for High-Power Applications

| Multilevel converters are considered today as the state-of-the-art power-conversion systems for high-power and power-quality demanding applications. This paper presents a tutorial on this technology, covering the operating principle and the different power circuit topologies, modulation methods, technical issues and industry applications. Special attention is given to established technology already found in industry with more in-depth and self-contained information, while recent advances and state-of-the-art contributions are addressed with useful references. This paper serves as an introduction to the subject for the not-familiarized reader, as well as an update or reference for academics and practicing engineers working in the field of industrial and power electronics.Ministerio de Ciencia y Tecnología DPI2001-3089Ministerio de Eduación y Ciencia d TEC2006-0386

THD Analysis of a Seven, Nine, and Eleven Level Cascaded H-Bridge Multilevel Inverter for Different Loads

A multilevel inverter is implemented for generating the required staircase AC voltage of output from various steps of voltages of DC sources. The multilevel inverter gives a better harmonic spectrum and a compatible quality of output. This article delves into an analytical analysis of the total harmonic distortion (THD) of different multilevel inverters which employ a multicarrier PWM technique. This technique is implemented for operating the switches at their respective angle of conduction. This paper deals with various cascaded H-Bridge multilevel inverters (CMI) with various loads that are modelled by implementing the MATLAB/Simulink platform. The output gives a better result of the proposed model in terms that it is helpful towards reducing the THD and the losses of switching

Grid-Connected Single-Star Bridge-Cells Modular Multilevel Cascaded Converter with Selective Harmonic Elimination Techniques

Nowadays, Renewable Energy Sources (RESs) are receiving enormous attention due to the noticeable exhaustion of fossil fuel and its emission of greenhouse gases. DC-AC converters have attracted the attention of the researchers, as they are entailed to integrate RESs to the grid to comply with the grid frequency and voltage requirements. Due to the high penetration of RESs, especially with elevated power levels, high-power converters are needed, which necessitates higher voltage and current ratings of the semiconductor devices. The unavailability of high voltage semiconductor devices has directed the attention to the use of either series connection of semiconductor devices or Multilevel Inverters (MLIs). MLIs allow using several low rated semiconductors to hold the high output power of the inverter. The MLI output waveform is close to sinusoidal in nature, therefore it may require a small filter to enhance the output power quality. There are many types of MLIs, where the most common MLIs are Flying Capacitor, Diode Clamped, and Modular Multilevel Cascaded Converter (MMCC). The MMCC can be classified into three main formations, the Single-Star Bridge-Cells MMCC (SSBC-MMCC), the Double-Star Bridge-Cells MMCC (DSBC-MMCC), and the Double-Star Chopper-Cells MMCC (DSCC-MMCC). The main advantage of the MMCC is the modularity and scalability. In addition, the MMCC does not require any clamping diodes or flying capacitors for clamping the voltage across the switches. In this thesis, the MMCC will be used to integrate high-power RESs to Grid. Nevertheless, the high-power applications necessitate low switching frequency operations. One of the most common controlling techniques of MLI with low frequency operation is the Selective Harmonic Elimination (SHE). SHE insures also the output current Total Harmonic Distortion (THD) to be minimized. One disadvantage of the SHE method is that the complexity of the algorithm along with the equations used is increased by the increase of the MMCC number of levels. Therefore, other alternatives of SHE techniques will be studied in this work to overcome this complexity. This thesis focuses typically on MMCC, particularly the SSBC-MMCC. In this work, a high-power grid-connected SSBC-MMCC is controlled with three different SHE techniques, complying with low switching frequency operation limitation in high-power applications. In addition to the Conventional SHE (C-SHE) technique, Quasi-SHE (Q-SHE) and Asymmetrical-SHE (A-SHE) approaches are proposed and assessed. Q-SHE and A-SHE approaches are based on eliminating selected low order harmonics (for instance, eliminating the fifth and seventh order harmonics), irrelevant to the number of employed levels provided that the number of levels allows for the required harmonic elimination. Compared with the C-SHE approach, the Q-SHE and A-SHE require less computational burden in solving the required equation groups, especially when a high number of levels and/or multiple switching angles for each voltage level are needed, while maintaining the same dv/dt of the output voltage. A 5MW, 17-level, grid-connected SSBC-MMCC, controlled in the synchronous rotating reference frame, is employed for assessing the addressed SHE techniques. The assessment is validated through simulation results using Matlab/Simulink platform

Real-Time Selective Harmonic Minimization for Multilevel Inverters Using Genetic Algorithm and Artificial Neural Network Angle Generation

This work approximates the selective harmonic elimination problem using Artificial Neural Networks (ANN) to generate the switching angles in an 11-level full bridge cascade inverter powered by five varying DC input sources. Each of the five full bridges of the cascade inverter was connected to a separate 195W solar panel. The angles were chosen such that the fundamental was kept constant and the low order harmonics were minimized or eliminated. A non-deterministic method is used to solve the system for the angles and to obtain the data set for the ANN training. The method also provides a set of acceptable solutions in the space where solutions do not exist by analytical methods. The trained ANN is a suitable tool that brings a small generalization effect on the angles\u27 precision and is able to perform in real time (50/60Hz time window)

The Age of Multilevel Converters Arrives

This work is devoted to review and analyze the most relevant characteristics of multilevel converters, to motivate possible solutions, and to show that we are in a decisive instant in which energy companies have to bet on these converters as a good solution compared with classic two-level converters. This article presents a brief overview of the actual applications of multilevel converters and provides an introduction of the modeling techniques and the most common modulation strategies. It also addresses the operational and technological issues

Advanced control of grid-connected multilevel power electronic rectifiers

Multilevel power electronic converters have been gaining attention due to their ability to supply high amounts of power and to handle high voltage levels. In this dissertation, grid connected AC-DC rectifier application is investigated with different topologies and control scheme. At first, neutral point clamped (NPC) rectifier is employed to transfer power from the grid to the load. The NPC rectifier has two capacitors in order to build multilevel output voltage. However, it causes voltage unbalancing problem. Therefore, the new method has been proposed to regulate each capacitor voltage at the same voltage level. Experimental results show that it is effective to balance capacitor voltages of the NPC and it can improve total harmonic distortion (THD) of the grid current as a result. Furthermore, 7 voltage levels can be achieved by using hybrid multilevel rectifier which consists of an NPC and cascaded H-bridges (CHB). Because the hybrid multilevel rectifier has total 8 capacitors which are completely discharged at first, large inrush currents from the grid might cause hazards. Therefore, the paper develops a pre-charge routine for building it up to steady state operation in which unity power factor control (PFC) and load voltage control are achieved. Finally, multiple reference frame theory (MRF) is used to improve THD of the grid currents when the hybrid multilevel rectifier is connected with distorted grid voltage source. After calculating 5th harmonic of the grid current in real time, the voltage reference for the hybrid multilevel rectifier will be compensated in a feedback loop. Experimental results show validity in improving THD of the grid currents. --Abstract, page iv

Application of the cascaded multilevel inverter as a shunt active power filter

Abstract unavailable please refer to PD

Investigation on Cascade Multilevel inverter for Medium and High-Power Applications

It is hard to connect a single power semiconductor switch directly to medium voltage grids (2.3, 3.3, 4.16, or 6.9 kV). For these reasons, a new family of multilevel inverters has emerged as the solution for working with higher voltage levels. Multilevel inverters have received more attention in industrial application, such as motor drives, static VAR compensators and renewable energy systems, etc. Primarily multilevel inverters are known to have output voltages with more than two levels. As a result, the inverter output voltages have reduced harmonic distortions and high quality of waveforms. Additionally, the devices are confined to fraction of dc-link voltage. These characteristics make multilevel inverter to adopt for high-power and high-voltage applications. A good number of multilevel inverter topologies have been proposed during the last two decades. Contemporary research has engaged novel converter topologies and unique modulation schemes. Moreover, four major multilevel inverter structures have been reported in the literature these are as follows: cascaded H-bridges inverter (CHB) with separate dc sources, diode clamped (neutral-clamped), and flying capacitors (capacitor clamped), P2 Multilevel inverters. Although different multilevel inverter exists, Cascade Multilevel Inverter (CMI) is one of the productive topology from multilevel family. In reality, on comparing with other multilevel based topologies, CMI feature a high modularity degree because each inverter can be seen as a module with similar circuit topology, control structure, and modulation. Therefore, in the case of a fault in one of these modules, it is possible to replace it quickly and easily. Moreover, with an appropriated control strategy, it is possible to bypass the faulty module without stopping the load, bringing an almost continuous overall availability. All this features make CMI an outstanding power converter. However, one of the greatest limitations of CMI is utilization of separate DC source for each H-Bridge cell. This not only increases cost but also affects the reliability of the system. This is the key motivation for this dissertation. In the present work, we have investigated different CMI based topologies with separate and single DC sources and finally proposed a new CMI based configuration with single dc source by using three-phase transformers. The proposed CMI based inverter presented in this thesis is well defined with logical and mathematical approach. Additionally to illustrate the merits, it is compared with traditional multilevel inverters. The feasibility of proposed inverter is demonstrated with different illustrations and confirmed by experimental results. The proposed CMI is well suited for grid / photovoltaic and FACTS systems. To elevate the application of proposed CMI a shunt active power filter (APF) design is demonstrated. In this case, the goal is to inject, in parallel with the load, compensation current to get a sinusoidal source current. The proposed APF is verified through Matlabsimulation. Finally, Opal-RT verifications are performed to verify the final design