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) have gained increasing interest for advanced energy-conversion systems due to their features of high-quality produced waveforms, modularity, transformerless operation, voltage, and current scalability, and fault-tolerant operation. However, these merits usually come with the cost of a high number of components. Over the past few years, proposing new MLIs with a lower component count has been one of the most active topics in power electronics. The first aim of this article is to update and summarize the recently developed multilevel topologies with a reduced component count, based on their advantages, disadvantages, construction, and specific applications. Within the framework, both single-phase and three-phase topologies with symmetrical and asymmetrical operations are taken into consideration via a detailed comparison in terms of the used component count and type. The second objective is to propose a comparative method with novel factors to take component ratings into account. The effectiveness of the proposed method is verified by a comparative study.

Voltage Source Multilevel Inverters With Reduced Device Count: Topological Review and Novel Comparative Factors

The multilevel converter, (MC) had been recommended for high- and medium-level power applications for decades. It is one of the most popular converters due to its ability to decrease the harmonic distortion in the output waveform without decreasing the converter power output. This paper presents the basic concept of four different types of multilevel converter: the neutral-point clamped, flying capacitor, cascaded H-bridge and modular. This paper also reviews five different modulation techniques for multilevel converters: pulse width modulation, space vector pulse width modulation, phase shifted carrier pulse width modulation, sinusoidal pulse width modulation and selective harmonic elimination pulse width modulation. In addition, recent publications regarding developments and improvements of fundamental concern to MCs and modulation techniques are reviewed in this paper. The implementation of MCs within renewable energy systems is also discussed here.

Reviews on multilevel converter and modulation techniques

This paper proposes a modular multilevel converter (MMC) based switched reluctance motor (SRM) drive with decentralized battery energy storage system for hybrid electric vehicle applications. In the proposed drive, a battery cell and a half-bridge converter is connected as a submodule (SM), and multiple SMs are connected together for the MMC. The modular full-bridge converter is employed to drive the motor. Flexible charging and discharging functions for each SM are obtained by controlling switches in SMs. Multiple working modes and functions are achieved. Compared to conventional and existing SRM drives, there are several advantages for the proposed topology. A lower dc-bus voltage can be flexibly achieved by selecting SM operation states, which can dramatically reduce the voltage stress on the switches. Multilevel phase voltage is obtained to improve the torque capability. Battery state-of-charge balance can be achieved by independently controlling each SM. Flexible fault-tolerance ability for battery cells is equipped. The battery can be flexibly charged under both running and standstill conditions. Furthermore, a completely modular structure is achieved by using standard half-bridge modules, which is beneficial for market mass production. Experiments carried out on a three-phase 12/8 SRM confirm the effectiveness of the proposed SRM drive.

MMC-Based SRM Drives With Decentralized Battery Energy Storage System for Hybrid Electric Vehicles

Model predictive control (MPC) has emerged as a promising approach to control a modular multilevel converter (MMC). With the help of a cost function, the control objectives of an MMC are achieved easily by using an MPC approach. However, the MPC has several technical challenges and issues including the need of accurate system models, computational complexity, and variable switching frequency operation and weighting factor selection, when it comes to the control of an MMC. In the past few years, several research studies are conducted to address some of the challenges and issues in an MPC and developed several model predictive algorithms for an MMC. In this paper, the importance of each challenge and its impact on the system performance is discussed. Also, the MMC mathematical models used in the implementation of MPC are presented. Furthermore, some of the popular MPC algorithms are discussed briefly, and their features and performance are highlighted through case studies. Finally, summary and future trends of MPC for an MMC are presented.

Model Predictive Control of High-Power Modular Multilevel Converters—An Overview

A new static synchronous compensator (STATCOM) based on the diode-clamped modular multilevel converter (DCM2 C) is proposed in this paper. In this converter topology, the capacitor voltage is clamped by using a low power rating diode in each submodule. The quantity of voltage sensors is significantly reduced and is free from the number of voltage levels. Furthermore, the voltage balancing control method becomes very simple and the capacitor voltage balance speed is fast. Based on the structure of modular multilevel converter, the DCM2C-STATCOM has the capability of Var compensation and negative-sequence current compensation. The topology characteristics and compensation control method of DCM2C-STATCOM are investigated in this paper. Experimental results obtained from a laboratory prototype validate that the capacitor voltage of the proposed DCM2C-STATCOM can be well balanced and the Var and negative-sequence current compensations are effective.

A Novel STATCOM Based on Diode-Clamped Modular Multilevel Converters

Multilevel converters have become very attractive for high-voltage-level power conversion in renewable power generation applications. The converter topology is an important issue in the studies of multilevel converter. Many multilevel topologies have been developed, but few of them are qualified with capacitor voltage self-balancing capability. This paper proposes a novel diode-clamped modular multilevel converter with simplified capacitor voltage-balancing control. In this topology, low-power rating diodes are used to clamp the capacitor voltages of the converter. Only the top submodule in each arm of the converter requires capacitor voltage control. Consequently, very few voltage sensors are needed for voltage control and the control computation burden is reduced greatly when the quantity of the submodules is high. A simple voltage-balancing control method with carrier phase-shifted modulation strategy is developed for this topology. Experiments based on a laboratory prototype were carried out and the results validated the capacitor-balancing performance of the proposed topology.

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A Novel Diode-Clamped Modular Multilevel Converter With Simplified Capacitor Voltage-Balancing Control

Modular multilevel converter (MMC) is very popular in medium-voltage applications, such as high-voltage direct current transmission, static Var generator, and motor drives. The arm capacitor voltage of MMC is usually constant and the output voltage is adjusted by regulating the pulse width modulation (PWM) index for switches in previous studies. In this paper, a novel flexible capacitor voltage control strategy for MMC as motor drives was proposed, with which the arm capacitor voltage was controlled flexibly according to the motor speed, by regulating the dc component and ac component of the PWM reference signal, and unsymmetrical control was used for the control of the upper arm and the lower arm in MMC. With the proposed control strategy, the circuit efficiency and the precision of the output voltage of the converter could be improved when the motor speed was low, and d i /d t and d v /d t of the motor windings could be decreased. These proposals were implemented on a laboratory prototype, and experiments were carried out. The validity of the control strategy under both steady and dynamic states was verified.

A Novel Flexible Capacitor Voltage Control Strategy for Variable-Speed Drives With Modular Multilevel Converters

The modular multilevel converter (MMC) has been proven to be an effective solution for grid-tied applications with medium/high voltage ratings. The MMC contains a large amount of floating capacitors, which require the voltage balancing control. The voltage balancing control complexity is normally increased with the number of floating capacitors. As a result, the computation burden of the controller as well as the corresponding hardware cost will be high. This article proposed a novel Z-type MMC, which can achieve capacitor voltage self-balancing without any balancing control. Therefore, the overall control of the converter is significantly simplified. Both mathematical analysis and experimental results have confirmed the effectiveness of the proposed method.

A Novel Z-Type Modular Multilevel Converter With Capacitor Voltage Self-Balancing for Grid-Tied Applications

This article proposes a voltage-adjustable capacitor isolated solid-state transformer (ACISST) to link the medium-voltage direct current distribution and low-voltage direct current distribution. Compared with traditional isolated bidirectional dc–dc converter, the proposed ACISST does not contain high-frequency transformer in base modules (BMs). The galvanic isolation between the primary side and the secondary side of the ACISST is realized by the isolation capacitor as quasi-isolation. The elimination of the high-frequency transformer eliminates the core losses of magnetic elements, improves the conversion efficiency to a certain extent, and reduces the weight of the ACISST. The voltage-adjustable unit realizes the secondary voltage regulation of stable control when the primary-side voltage fluctuates in an acceptable range or load power varies. Bidirectional power flowing is one of the features of ACISST, which is based on the bidirectional power flowing characteristic of BM and simplified dual active bridge. Simulation and experimental results are presented to validate the theoretical analysis.

Zhen Chen

Papers

A Novel STATCOM Based on Diode-Clamped Modular Multilevel Converters

A Novel Diode-Clamped Modular Multilevel Converter With Simplified Capacitor Voltage-Balancing Control

A Novel Flexible Capacitor Voltage Control Strategy for Variable-Speed Drives With Modular Multilevel Converters

A Novel Z-Type Modular Multilevel Converter With Capacitor Voltage Self-Balancing for Grid-Tied Applications

Voltage-Adjustable Capacitor Isolated Solid-State Transformer