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

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

The aim of this paper is to present a review of current control techniques for three-phase voltage-source pulsewidth modulated converters. Various techniques, different in concept, have been described in two main groups: linear and nonlinear. The first includes proportional integral (stationary and synchronous) and state feedback controllers, and predictive techniques with constant switching frequency. The second comprises bang-bang (hysteresis, delta modulation) controllers and predictive controllers with on-line optimization. New trends in current control-neural networks and fuzzy-logic-based controllers-are discussed, as well. Selected oscillograms accompany the presentation in order to illustrate properties of the described controller groups.

Current control techniques for three-phase voltage-source PWM converters: a survey

The matrix converter is an array of controlled semiconductor switches that connects directly the three-phase source to the three-phase load. This converter has several attractive features that have been investigated in the last two decades. In the last few years, an increase in research work has been observed, bringing this topology closer to the industrial application. This paper presents the state-of-the-art view in the development of this converter, starting with a brief historical review. An important part of the paper is dedicated to a discussion of the most important modulation and control strategies developed recently. Special attention is given to present modern methods developed to solve the commutation problem. Some new arrays of power bidirectional switches integrated in a single module are also presented. Finally, this paper includes some practical issues related to the practical application of this technology, like overvoltage protection, use of filters and ride-through capability.

Matrix converters: a technology review

This paper presents a review of advanced control techniques for microgrids. This paper covers decentralized, distributed, and hierarchical control of grid-connected and islanded microgrids. At first, decentralized control techniques for microgrids are reviewed. Then, the recent developments in the stability analysis of decentralized controlled microgrids are discussed. Finally, hierarchical control for microgrids that mimic the behavior of the mains grid is reviewed.

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Advanced Control Architectures for Intelligent Microgrids—Part I: Decentralized and Hierarchical Control

An electrical control system includes multiple frames of reference in the error path of a controller. In each such frame, a particular chosen component of the error vector appears as a constant vector and is passed through a pure integrator thus acquiring infinite gain. In the steady state, the net error vector can then be forced to have zero content at each of the targeted frequencies (as set by the rotational velocity of the frames of reference). Both parallel and series configuration of the controller are provided, as well as the control methods used by the controllers.

Multiple reference frame controller for active filters and power line conditioners

An inductio motor torque/flux control system the motor flux is forced down while accelerating and force up while decelerating to maintain a required flux level at all speeds. This is achieved through vectory control by generating a direct current component in relation to the flux demand while imposing limits thereto corresponding to the maximum desirable resultant current vector, and by generating a quadrature current component which is held between limits defined by the sum of the squres of the two current components. The three-phase currents of the motor generated by vector control transformation are used to control a voltage-source inverter in a bang-bang fashion. The conventional bang-bang technique is improved by tying up one pole while controlling in the bang-bang mode the two other poles. To this effect the motor back emf is detected when the poles are simultaneously tied to one of the DC link terminals, and the tied-up pole is selected by the emf so detected.

Induction motor torque/flux control system

Adaptive control is used in an asynchronous motor drive. A reference model based on the model equation of the motor is combined with an adjustable model responsive to the direct and quadrature components of the current and to an estimated speed. An adaptive mechanism counting in a P-I amplifier expands to the direct deviation between the direct and quadrature components of the flux outputed by the two models and generates a feedback signal representing the estimated speed. The adjustable model reacts to such feedback signal so that the estimated speed readily becomes the actual speed of the motor. As a result, a tacho-less AC motor drive is obtained in which under any running conditions the motor immediately takes the required speed.

Tacho-less vector control adaptive system for motor drive

A bridge between the inductor and the capacitor of the DC-Link of a voltage-source inverter AC motor drive includes a GTO (or transistor) device used for forced-commutation in the regenerating mode, while the current is controlled ON and OFF according to a duty-cycle defined by an hysteresis circuit to hold the current flowing into the inductor to a magnitude which establishes a boost on the capacitor voltage, so that in the motoring, as well as the regenerating mode of operation, the voltage of the capacitor is kept constant and higher than upon the converter output.

Ac motor drive with improved voltage-source inverter

In a direct AC to AC converter system the normal rectifier assistance functions are phase modulated by a 30° lag and by a 30° lead, and switching between lagging and leading function is effected at a high frequency according to a duty cycle: ##EQU1## and where θ is the electrical angle of the AC power supply, thereby insuring a sinusoidal input current in the system.

Colin D. Schauder

Papers

Multiple reference frame controller for active filters and power line conditioners

Induction motor torque/flux control system

Tacho-less vector control adaptive system for motor drive

Ac motor drive with improved voltage-source inverter

Direct AC/AC converter system