The authors discuss high-voltage power conversion. Conventional series connection and three-level voltage source inverter techniques are reviewed and compared. A novel versatile multilevel commutation cell is introduced: it is shown that this topology is safer and more simple to control, and delivers purer output waveforms. The authors show how this technique can be applied to either choppers or voltage-source inverters and generalized to any number of switches.<<ETX>>

Multi-level conversion : high voltage choppers and voltage-source inverters

Multilevel inverters (MLIs) are being used in wide range of power electronic applications. These converters have attracted a lot of attention during recent years and exist in different topologies with similar basic concepts. This paper presents five main submodules (SMs) to be used as the basic structures of MLIs. The paper reviews the common MLI topologies from the structural point of view. The topologies are divided into the different SMs to show conventional MLI configurations and future topologies that can be created from the main SMs. A comparative study between different topologies is performed in detail. The MLIs are categorized and investigated with from different perspectives such as the number of components, the ability to create inherent negative voltage, working in regeneration mode and using single dc source.

A General Review of Multilevel Inverters Based on Main Submodules: Structural Point of View

In photovoltaic (PV) applications, a transformer is often used to provide galvanic isolation and voltage ratio transformations between input and output. However, these conventional iron- and copper-based transformers increase the weight/size and cost of the inverter while reducing the efficiency and power density. It is therefore desirable to avoid using transformers in the inverter. However, additional care must be taken to avoid safety hazards such as ground fault currents and leakage currents, e.g., via the parasitic capacitor between the PV panel and ground. Consequently, the grid connected transformerless PV inverters must comply with strict safety standards such as IEEE 1547.1, VDE0126-1-1, EN 50106, IEC61727, and $\text{A}S/N$ ZS 5033. Various transformerless inverters have been proposed recently to eliminate the leakage current using different techniques such as decoupling the dc from the ac side and/or clamping the common mode (CM) voltage (CMV) during the freewheeling period, or using common ground configurations. The permutations and combinations of various decoupling techniques with integrated voltage buck–boost for maximum power point tracking (MPPT) allow numerous new topologies and configurations which are often confusing and difficult to follow when seeking to select the right topology. Therefore, to present a clear picture on the development of transformerless inverters for the next-generation grid-connected PV systems, this paper aims to comprehensively review and classify various transformerless inverters with detailed analytical comparisons. To reinforce the findings and comparisons as well as to give more insight on the CM characteristics and leakage current, computer simulations of major transformerless inverter topologies have been performed in PLECS software. Moreover, the cost and size are analyzed properly and summarized in a table. Finally, efficiency and thermal analysis are provided with a general summary as well as a technology roadmap.

Transformerless Inverter Topologies for Single-Phase Photovoltaic Systems: A Comparative Review

This paper introduces a new module for asymmetrical multilevel inverters with a low number of components. The module is a square combination of two back-to-back T-type inverters and some other switches. A square T-type module produces 17 levels by 12 switches and 4 unequal dc sources (two 3 V DC and two 1 V DC). Also, it can be extended as a cascade connection in two strategies to achieve more levels. The module and its cascade connection are suitable for the applications in several dc sources systems such as photovoltaic farms, which lead to a modular topology with more voltage levels at higher voltages. Inherent creation of the negative voltage levels without any additional circuit (such as H-bridge circuit) is one of the main features of the proposed module. The low total harmonic distortion of the output voltage/current and low number of semiconductors are among the other advantages of the proposed module. A nearest level control method as a switching technique is used to produce high quality output voltage with lower harmonic contents. Simulations have been performed in MATLAB/Simulink and a prototype is implemented in the Power Electronics Laboratory; both the simulation and experimental results show a good performance.

A Square T-Type (ST-Type) Module for Asymmetrical Multilevel Inverters

Recently multilevel inverters (MLI) have attracted more attention in research and industry, as they are changing into a viable technology for several applications. The concept of MLI was introduced for high power and high/medium voltage applications as they can provide an effective interface with renewable energy sources. Developing of reduced switch MLI topology has been a rapid research topic since the past decade, which has not been reviewed so far. Therefore, this review article focuses on the different reduced switch MLI topologies under three categories such as symmetric, asymmetric and hybrid configurations. The important knowledge on these topologies is carefully tabulated based on the three categories in the comparison tables to understand the essential parameters of the MLI topologies. These configurations are not only generating higher voltage levels to improve the power quality but also to reduce the passive filter requirements. Also, this review includes a detailed perspective of various modulation techniques and control strategies for MLI topologies. In addition to that, the different performance parameters of MLI and its calculation methods are discussed with appropriate mathematical expression. This review will help in the selection of appropriate MLI topology for FACTS, motor drives and renewable applications.

A comprehensive review on reduced switch multilevel inverter topologies, modulation techniques and applications

This paper presents a new E-type module for asymmetrical multilevel inverters (MLIs) with reduced components. Each module produces 13 levels with four unequal dc sources and 10 switches. The design of the proposed module makes some preferable features with a better quality than similar modules such as the low number of semiconductors and dc sources and low switching frequency. Also, this module is able to create a negative level without any additional circuit such as an H-bridge, which causes reduction of voltage stress on switches. Cascade connection of the proposed structure leads to a modular topology with more levels and higher voltages. Selective harmonics elimination pulse width modulation (SHE-PWM) scheme is used to achieve high-quality output voltage with lower harmonics. MATLAB simulations and practical results are presented to validate the proposed module good performance. Module output voltage satisfies harmonics standard (IEEE519) without any filter in output.

An Envelope Type (E-Type) Module: Asymmetric Multilevel Inverters With Reduced Components

This paper presents a new reconfiguration module for asymmetrical multilevel inverters in which the capacitors are used as the dc links to create the levels for staircase waveforms. This configuration of the multilevel converter makes a reduction in dc sources. On the other hand, it is possible to generate 13 levels with lower dc sources. The proposed module of the multilevel inverter generates 13 levels with two unequal dc sources (2 V DC and 1 V DC). It also involves two chargeable capacitors and 14 semiconductor switches. The capacitors are self-charging without any extra circuit. The lower number of components makes it desirable to be used in wide range of applications. The module is schematized as two back-to-back T-type inverters and some other switches around it. Also, it can be connected as a cascade modular which leads to a modular topology with more voltage levels at higher voltages. The proposed module makes the inherent creation of the negative voltage levels without any additional circuit (such as H-bridge circuit). Nearest level control switching modulation scheme is applied to achieve high-quality sinusoidal output voltage. Simulations are executed in MATLAB/Simulink and a prototype is implemented in the power electronics laboratory in which the simulation and experimental results show a good performance.

A 13-Levels Module (K-Type) With Two DC Sources for Multilevel Inverters

This article presents a novel high step-up multilevel inverter (MLI) with a single dc source based on switched-capacitor modules in which it is suitable for the renewable energy applications with low-voltage dc source. The proposed topology has the ability of the voltage self-balancing for the capacitors without any external or closed-loop circuits. Another merit of the proposed structure is the production of bipolar staircase waveform without any end-side full-bridge in which it reduces the voltage stress on components remarkably. The circuit topology and its operation principle, capacitance calculations and comparison of the proposed circuit with other well-known single-source MLIs are discussed in detail. Finally, a 21-level prototype of the proposed inverter with 32 V input voltage and high gain 10 by using four modules was evaluated by both simulation and experimental tests.

Emad Samadaei

Papers

A General Review of Multilevel Inverters Based on Main Submodules: Structural Point of View

An Envelope Type (E-Type) Module: Asymmetric Multilevel Inverters With Reduced Components

A Square T-Type (ST-Type) Module for Asymmetrical Multilevel Inverters

A 13-Levels Module (K-Type) With Two DC Sources for Multilevel Inverters

A Novel High Step-Up Switched-Capacitor Multilevel Inverter With Self-Voltage Balancing