There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

This paper presents a technology review of voltage-source-converter topologies for industrial medium-voltage drives. In this highly active area, different converter topologies and circuits have found their application in the market. This paper covers the high-power voltage-source inverter and the most used multilevel-inverter topologies, including the neutral-point-clamped, cascaded H-bridge, and flying-capacitor converters. This paper presents the operating principle of each topology and a review of the most relevant modulation methods, focused mainly on those used by industry. In addition, the latest advances and future trends of the technology are discussed. It is concluded that the topology and modulation-method selection are closely related to each particular application, leaving a space on the market for all the different solutions, depending on their unique features and limitations like power or voltage level, dynamic performance, reliability, costs, and other technical specifications.

Multilevel Voltage-Source-Converter Topologies for Industrial Medium-Voltage Drives

A thermal model of an electromechanical flywheel with a brushless DC machine is presented. Thermal analysis is particularly important during vacuum operation of the flywheel. Accurate prediction of temperature facilitates material selection, structural integrity and good performance. In addition, the choice of vacuum level, bearings, conductor sizes and system overload capability can be attributed to a good thermal model. A thermal model was developed and simulated on MATLAB Simulink. The simulation results were compared with experimental results and a good correlation was observed.

Thermal model of electromechanical flywheel with brushless DC machine

This paper presents a control algorithm for a synchronous reluctance machine with different windings. Due to the poor saliency ratio of the SynRM the use of field weakening alone limits the constant power speed range. In this work the machine uses two different windings, one with 18 turns and the other with 25 turns. The constant power speed range of the machine was extended by 76% when the windings were changed from 25 turns to 18 turns. The control algorithm implements field weakening before the windings are changed. When the machine winding configuration is changed constant power speed range can further be extended. Ideally the machine winding would be changed automatically by the use of power switches, however in this paper due to the unavailability of the power switches the machine windings were changed manually.

Control Strategy for a Variable Winding Synchronous Reluctance Machine for Traction Applications

This paper presents core loss prediction and measurement in active radial magnetic bearings found in flywheel systems. The methods and test results of two core loss measurement test benches (Clarkson test bench and commercial Donart test bench) are analyzed and compared when the materials are excited with sinusoidal flux density waveforms. The unique flux density waveforms found in magnetic bearing are analyzed and the relative core losses are predicted. Non-sinusoidal flux density waveforms are obtained in different sections of the active radial magnetic bearing. These waveforms are excited into the flexible Clarkson test bench and the core losses are measured. The measured core loss results are analyzed and compared with estimated losses and also losses obtained under sinusoidal waveforms.

Core loss prediction and measurement in magnetic bearing

Voltage in current out models are commonly used for induction machine (IM) emulation. This model does not provide the information of back electromotive force (EMF) and hence cannot be used for open-circuit fault emulation. A current in voltage out model (CIVO) is an alternate IM model where it provides back EMF information. However, the CIVO model cannot be used directly with the conventional emulator configuration. Hence, in this article, a new emulator configuration has been proposed to accommodate the CIVO model. The model is used in a new emulator configuration by incorporating physical emulator parameters. The article further discusses the upgrade of the model considering saturation nonlinearities. The emulator employing such a model mimics the real IM back EMF with high accuracy. The proposed emulator is tested for different gate driver faults, such as single switch and multiple switch open faults, starting and speed changing conditions. The experimental results are validated with a 5 hp machine. Different fault signatures are also identified with the help of the emulator to help with fault diagnosis and localization.

Induction Machine Emulation for Variable Frequency Drive Converter Faults

This paper presents the development of an equivalent electrical circuit model that represents the characteristics of a high temperature PEM fuel cell under various fault mechanisms. The three main degradation mechanisms that can reduce long term and short term performance are investigated and the specific electrochemical parameters that are affected are identified. A semi-empirical modelling approach based on captured experimental data is used to model the degradation mechanisms in order to reduce complexity. In particular, the effect of ripple current from the power electronics that accelerates degradation of the specific model parameters is discussed. The mechanism is also included in the model in order to increase functionality. Each voltage loss component and the associated degradation phenomena are equated to a circuit element and the final circuit model is presented. The simulated results are compared to experimental data to determine accuracy. The model is able to estimate performance loss based on operating time and conditions allowing for life cycle estimation and system optimization.

Pragasen Pillay

Papers

Thermal model of electromechanical flywheel with brushless DC machine

Control Strategy for a Variable Winding Synchronous Reluctance Machine for Traction Applications

Core loss prediction and measurement in magnetic bearing

Induction Machine Emulation for Variable Frequency Drive Converter Faults

Derivation of an equivalent electrical circuit model for degradation mechanisms in high temperature pem fuel cells in performance estimation