Multilevel inverter technology has emerged recently as a very important alternative in the area of high-power medium-voltage energy control. This paper presents the most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-clamped (flying capacitor), and cascaded multicell with separate DC sources. Emerging topologies like asymmetric hybrid cells and soft-switched multilevel inverters are also discussed. This paper also presents the most relevant control and modulation methods developed for this family of converters: multilevel sinusoidal pulsewidth modulation, multilevel selective harmonic elimination, and space-vector modulation. Special attention is dedicated to the latest and more relevant applications of these converters such as laminators, conveyor belts, and unified power-flow controllers. The need of an active front end at the input side for those inverters supplying regenerative loads is also discussed, and the circuit topology options are also presented. Finally, the peripherally developing areas such as high-voltage high-power devices and optical sensors and other opportunities for future development are addressed.

Multilevel inverters: a survey of topologies, controls, and applications

The use of distributed energy resources is increasingly being pursued as a supplement and an alternative to large conventional central power stations. The specification of a power-electronic interface is subject to requirements related not only to the renewable energy source itself but also to its effects on the power-system operation, especially where the intermittent energy source constitutes a significant part of the total system capacity. In this paper, new trends in power electronics for the integration of wind and photovoltaic (PV) power generators are presented. A review of the appropriate storage-system technology used for the integration of intermittent renewable energy sources is also introduced. Discussions about common and future trends in renewable energy systems based on reliability and maturity of each technology are presented

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Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey

Multilevel converters have been under research and development for more than three decades and have found successful industrial application. However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers with a comprehensive and insightful review of where multilevel converter technology stands and is heading. This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry. In addition, new promising topologies are discussed. Recent advances made in modulation and control of multilevel converters are also addressed. A great part of this paper is devoted to show nontraditional applications powered by multilevel converters and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities.

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Recent Advances and Industrial Applications of Multilevel Converters

Multilevel voltage source converters are emerging as a new breed of power converter options for high-power applications. The multilevel voltage source converters typically synthesize the staircase voltage wave from several levels of DC capacitor voltages. One of the major limitations of the multilevel converters is the voltage unbalance between different levels. The techniques to balance the voltage between different levels normally involve voltage clamping or capacitor charge control. There are several ways of implementing voltage balance in multilevel converters. Without considering the traditional magnetic coupled converters, this paper presents three recently developed multilevel voltage source converters: (1) diode-clamp, (2) flying-capacitors, and (3) cascaded-inverters with separate DC sources. The operating principle, features, constraints, and potential applications of these converters are discussed.

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Multilevel converters-a new breed of power converters

This paper presents a new multilevel converter topology suitable for very high voltage applications, especially network interties in power generation and transmission. The fundamental concept and the applied control scheme is introduced. Simulation results of a 36 MW-network intertie illustrate the efficient operating characteristics. A suitable structure of the converter-control is proposed.

An innovative modular multilevel converter topology suitable for a wide power range

This paper presents a control strategy for fuel cell vehicle (FCV) traction drive systems using the Z-source inverter. The Z-source inverter utilizes an inductor-capacitor (LC) network (impedance-source) to link the main inverter circuit to the fuel cell. One of the Z-source network capacitors can be replaced with a battery. By controlling the shoot-through duty cycle and modulation index, we can control the battery state of charge (SOC), produce any output AC voltage to the traction motor, and control the fuel cell output power, all independently and simultaneously. There is no need for any DC-DC converters to manage and control battery SOC and fuel cell output power. With the impedance network, the Z-source inverter can advantageously use the shoot through state to boost voltage. This one-stage power conversion and control system has higher reliability, higher efficiency, and lower cost. These facts make the Z-source inverter desirable for use in hybrid electric vehicles, as the cost and complexity is reduced when compared to the traditional inverters. This paper explores a control strategy to regulate battery SOC, fuel cell output power, and produce desired AC voltage to the traction motor.

Control strategy for fuel cell vehicle traction drive systems using the Z-source inverter

This paper presents a novel boost-half-bridge micro inverter and its control implementations for single-phase grid-connected photovoltaic systems. The proposed topology consists of a transformer isolated boost-half-bridge DC-DC converter and a full-bridge pulse-width-modulated inverter. The boost-half-bridge converter integrates the conventional boost converter and the half-bridge converter by using only two active devices. The promising features such as circuit simplicity, low cost, high efficiency and high reliability are obtained. Moreover, a high performance plug-in repetitive controller is proposed to regulate the grid current. High power factor (> 0.99) and very low total harmonic distortions (0.9% ∼ 2.87%) are guaranteed under both heavy and light load conditions. Dynamic stiffness is also achieved under load step change conditions. In addition, a variable step size MPPT method is adopted such that fast tracking speed and high MPPT efficiency are both guaranteed. A 210W prototype was fabricated and tested. Simulation and experimental results are provided to verify the validity and performance of the circuit operations, current control and MPPT algorithm.

Grid-connected boost-half-bridge photovoltaic micro inverter system using repetitive current control and maximum power point tracking

A new PV power generation topology, based on the quasi-Z source inverter (QZSI) with battery, is proposed in this paper With a battery paralleled with one of the capacitors, this system can smooth the grid-injected/load power when the PV panel outputs a variable power with fluctuations The battery can be charged or discharged without any extra circuit, because of the unique impedance network of QZSI The operating principle and power flow of this system are analyzed The closed-loop control strategy is used to achieve Maximum Power Point Tracking (MPPT) and keep the output voltage stable Three operating modes of the proposed PV power generation system are simulated in the MATLAB/Simulink to verify its principle and theoretical analysis A prototype is built in the lab, and the experimental results validate the whole system

Quasi-Z source inverter with battery based PV power generation system

Z-source energy conversion is a recent concept introduced for adding voltage-boost functionality to the otherwise buck-only dc-ac inverter. The same concept can equally be extended to other converter topologies including the indirect ac-ac matrix converter, where only a single Z-source impedance network needs to be inserted to its intermediate dc-link. Because of such a simple insertion, topological extension of the indirect Z-source matrix converter is viewed as trivial with its operating states easily deducible from its existing dc-ac counterpart. In contrast, modulation of the extended indirect ac-ac entity is judged as nontrivial and not at all obvious if advantages like buck-boost flexibility, minimum commutation count and ease of implementation are to be attained simultaneously, in addition to sinusoidal input and output quantities commonly set as objectives for ac-ac energy conversion. Attaining these advantages simultaneously is no doubt important since any shortfalls or compromises would make it even harder to justify for those passive components introduced to the otherwise “all semiconductor” indirect matrix converter. Bringing forth these advantages by unveiling related optimal modulation theories is therefore a relevant investigation that is pursued now to better present the indirect Z-source matrix converter as an attractive alternative for ac-ac buck-boost energy conversion. Findings presented here on have already been verified in simulation or experimentally using a laboratory-constructed prototype.

Optimal modulation of indirect Z-source matrix converter

Recently, there is a strong demand for high power conversion system in power electronics area especially in photovoltaic (PV) power conditioning system (PCS), wind power generation, and fuel cell power generation system. However, there is a limit in increasing power level mainly due to the limitations in switching device's voltage and current rating. One way to solve this problem is to connect power converters in parallel. Parallel operation of inverter has many advantages such as modularity, ease of maintenance, (n+1) redundancy, high reliability, etc. This paper presents parallel operation of voltage-fed-trans-Z-source inverter. A 2.6 kW prototype inverter is built and tested to verify performances of the proposed inverter.

Fang Zheng Peng

Papers

Control strategy for fuel cell vehicle traction drive systems using the Z-source inverter

Grid-connected boost-half-bridge photovoltaic micro inverter system using repetitive current control and maximum power point tracking

Quasi-Z source inverter with battery based PV power generation system

Optimal modulation of indirect Z-source matrix converter

Parallel operation of trans-Z-source inverter