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

The conventional current-source inverter has two major problems: unidirectional power flow and voltage boost operation, which make it impossible to be used in many applications, such as hybrid electric vehicles and general-purpose variable-speed motor drives. Z-source inverters (ZSIs) can solve both problems. Quasi-ZSIs (qZSIs) were recently proposed as an important improvement to traditional ZSIs. Besides the advantages inherited from ZSIs, qZSIs also have several of their own merits. This paper presents a comprehensive study on the new features of current-fed qZSI, including the advantageous buck-boost function, improved reliability, reduced passive component ratings, and unique regeneration capability. The current-fed qZSIs are bidirectional with an additional diode, unlike the voltage-fed ZSI that needs a switch to achieve bidirectional power flow. A modified space vector pulse-width-modulation method is proposed, and the available operating regions for motoring and regeneration operation are analyzed in this paper. Since the current-fed qZSI has the same operation as the current-fed ZSI, many results of this paper are also applicable to the current-fed ZSI. A reverse-blocking insulated-gate bipolar-transistor-based current-fed qZSI prototype was developed in the laboratory. Simulation and experimental results are shown to verify the theoretical analysis.

Current-Fed Quasi-Z-Source Inverter With Voltage Buck–Boost and Regeneration Capability

To overcome the drawbacks of the conventional gate drive, in this paper a closed-loop gate control method is proposed. In this novel method, the switching speed, specifically di/dt, is measured from the voltage across the parasitic inductance of the IGBT module and a closed-loop control is employed to adaptively adjust the gate drive voltage to control the switching speed according to a preset control reference. As a result, both the voltage overshoot and current overshoot can be effectively controlled. This new gate drive method enables programmable control of switching speed and allows full use of the capability of the power devices. The oscillations during IGBT switching are also reduced and associated EMI problems can be mitigated. The relationships between the controlled voltage overshoot, current overshoot and associated energy losses are derived and can provide guidelines for practical design. Also, the proposed gate drive fully utilizes gate drive signal information of amplitude and duty cycle, and provides a new capability for the system modulator to effectively control both the power electronic circuit's steady-state and transient behaviors simultaneously.

Closed-Loop Gate Drive for High Power IGBTs

Z-source inverters are suited for applications which require a large range of gain, such as in renewable energy. Recently proposed Trans-Z-source inverters (TZSIs) and T-source inverters (TSIs) characterize improved voltage gain thanks to the application of coupled inductors with turns ratio higher than one. The present paper extends the concept of Trans-Z-source (T-source) inverters. Two novel impedance networks: the LCCT (inductor-capacitor-capacitor-transformer) Z-source impedance network and the LCCT-qZ-source impedance network are presented. The former contains two built-in DC-current-blocking capacitors connected in series with transformer windings and therefore prevent the transformer core from saturation while the latter contains one built-in DC-current-blocking capacitor and combines the features of qZ-source and T-source impedance networks. The novel topologies of LCCT-impedance-source inverters: LCCT-Z-source inverter (LCCT-ZSI) and LCCT-quasi-Z-source inverter (LCCT-qZSI) proposed in the paper characterize available continuous input current which is the advantage compared to TZSIs and TSIs. Simulations and experimental results are provided to verify the analysis of two proposed topologies.

New type LCCT-Z-source inverters

A novel harmonic power filter system consisting of passive LC filters and voltage source pulsewidth-modulated (PWM) converters is described. This filter system has the merits of both conventional passive LC filters and conventional active power filters. The operating principle and a control scheme for the system are described. Compensation characteristics are discussed from both a theoretical and experimental point of view. The practicability and validity of compensation for large-capacity thyristor converters is experimentally demonstrated. >

A novel harmonic power filter

A new scheme for grid-connected photovoltaic (PV) interface by combination of a quasi-Z source inverter (qZSI) into cascaded H-bridge (CHB) is proposed in this paper. The proposed scheme enables PV string voltage boost to a higher level, and solves the imbalance problem of DC-link voltage in traditional CHB inverters. A multilevel voltage waveform of inverter output is generated by an improved phase shifted sinusoidal pulse width modulation (PS-SPWM) algorithm, which introduces shoot-through states into the conventional zero states to control qZS-CHB module. The effective control schemes are proposed to regulate the maximum power point tracking (MPPT) of each string, and control the DC-link voltage of each H-bridge, respectively. Grid injected power is controlled corresponding to the proportionality factors of each PV string output power. A 1.5 kW system is built in MATLAB/SIMULINK, and the simulation results verify the proposed novel multilevel PV interface inverter and its control principles.

Fang Zheng Peng

Papers

Current-Fed Quasi-Z-Source Inverter With Voltage Buck–Boost and Regeneration Capability

Closed-Loop Gate Drive for High Power IGBTs

New type LCCT-Z-source inverters

A novel harmonic power filter

A new grid-connected PV system based on cascaded H-bridge quasi-Z source inverter