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 power transfer of a dual active bridge (DAB) dc–dc converter is not linearly proportional to the phase shift between the two active bridges. When the DAB converter controller receives the power reference, the phase shift needs to be obtained by either solving a quadratic equation online or looking up from a table. The online calculation method is computationally inefficient. The lookup table method has only limited operating points. In addition, when using interpolation to find missing operating points from the table, the control accuracy will be reduced. This letter proposes a simple phase-shift modulation using a parabolic carrier. The proposed method converts the power command into phase shift by modulation. Therefore, the solving of the quadratic equation can be eliminated. The Verilog results are provided to verify the proposed parabolic carrier. A case study of DAB with sinusoidal ripple current charging concept is provided to demonstrate the advantage of the proposed method.

A Simple Phase-Shift Modulation Using Parabolic Carrier for Dual Active Bridge DC–DC Converter

The modular feature of modular multilevel converter (MMC) makes it stand out for medium/high voltage applications. However, as the number of sub-modules increases, the control complexity of voltage balance of each sub-module sharply increases. Conventionally, the MMC sub-module voltage cannot be balanced without voltage monitoring and control. This paper mathematically proofs that four-level MMC has the merit of self voltage balancing by nature. This merit eliminates the voltage monitoring and control for MMC. The four-level MMC simulations are provided for verification purpose. In addition, since the sub-module capacitor voltage tends to converge to nominal value, the low frequency voltage ripple is greatly reduced, which allows the sub-module capacitance to be extremely small. The proposed four-level MMC is 1/8th –1/6th the capacitance compared to conventional MMCs.

A Four-Level Modular Multilevel Converter with Self Voltage Balancing and Extremely Small DC Capacitor

A new carrier-based predictive current controlled pulse width modulation (PCCPWM) method is proposed to modulate matrix converters. The PCCPWM controller utilizes a discrete-time model to predict the future values of output currents and generates proper duty ratios to minimize the output current errors. The modulation algorithm and the related equations are derived by using average concept over one switching period. Also, this paper presents various matrix converter topologies, such as single-phase and 2-phase matrix converters and 3-phase matrix converter with the proposed PCCPWM strategy. The feasibility and validity of the proposed strategy are verified by experimental results.

Carrier-based predictive current controlled pulse width modulation for matrix converters

An effective control solution for speed sensorless vector control of induction motor fed by a cascaded neutral point clamped (NPC) inverter is proposed, where the novel SPWM pulse rotation control approach provides a simple way to implement vector control for induction motor when the cascaded NPC inverter is employed. The single carrier is used in the novel pulse control, instead of the multi-carriers, which makes the voltage modulation simpler and the hardware burden decrease. It is of great benefits to the cascaded NPC inverter, i.e., the output voltages and powers of all inverter modules, and two series-capacitor dc voltages of each inverter module are perfectly balanced, and a low switch frequency of all inverter modules provides a synthesized high frequency PWM phase voltage. A rotor flux-oriented vector control is combined with back EMF-based MRAS speed estimation, which results in a speed closed-loop control. The voltage sensors together with the filters of changeable parameters ensure the precision of speed estimation for the whole frequency range. The proposed scheme is applied to control an 800kW 4160V induction motor fed by the 1MVA 6000V 17-level cascaded NPC inverter, where the controller mainly consists of one DSP and one CPLD. The experimental results verify the proposed scheme.

Speed Sensorless Vector Control Induction Motor Drives Fed by Cascaded Neutral Point Clamped Inverter

An electric bus system has been operating in the downtown area of Chattanooga, TN. The buses use traditional hard-switched IGBT inverters driving special induction motors with a speed sensor (tachometer) and two embedded flux-sensing windings to provide rotor speed and flux information to the motor controller for implementation of high performance field oriented control (vector control). The induction motor is oil-cooled and equipped with an internal planar gear reduction. The system has experienced failures in both speed sensors and flux sensors due to their unreliable and weak EMI-immunity feature and hostile (oil-leakage) environment. A speed- and flux- sensorless induction motor drive system with a new 100 kW soft-switching inverter has been implemented to replace the existing system, targeted at the use of commercially-off-the-shelf induction motors instead of the customer-designed special motors. The project was supported under the US Department of Energy funding and local industry partnership.

Fang Zheng Peng

Papers

A Simple Phase-Shift Modulation Using Parabolic Carrier for Dual Active Bridge DC–DC Converter

A Four-Level Modular Multilevel Converter with Self Voltage Balancing and Extremely Small DC Capacitor

Carrier-based predictive current controlled pulse width modulation for matrix converters

Speed Sensorless Vector Control Induction Motor Drives Fed by Cascaded Neutral Point Clamped Inverter

Speed and flux sensorless field oriented control of induction motors for electric vehicles