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

Uprating of hydro generators requires studies into the potential areas of failure like hotspots within the machine core. Rotational core losses cause localized heating of the core therefore understanding the distribution and behavior of rotational flux in the machine is key. This paper studies the distribution of rotational flux in a hydro generator as the core material is saturated. The percentage of rotational flux in the stator increases as the material is saturated especially for aspect ratios above 0.6. This can affect the loss distribution in the machine. Moreover experimental core loss data of non-sinusoidal flux densities in various parts of the machine at no load and full load operation are also presented.

Effect of saturation on rotational flux distribution in hydro generators

This paper considers the concept of very advanced technologies being developed in the West, to solve rural electrification problems in the developing world. The myth that poor quality is cheaper and therefore more affordable for rural electrification is dispelled. On the contrary this paper points to instances when problems of the rural poor have been solved by using cutting edge technologies where standard solutions had proved unworkable. In this paper a futuristic technology, the white light emitting diode (LED), for general lighting, is examined as a solution to African rural electrification. Comparisons are made with conventional technologies.

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White LEDS for rural lighting

This article presents the effect of different magnetization pulse widths and methods on the back electromotive force (EMF), no-load losses, and minimum inverter rating at different operating conditions of a variable flux permanent magnet synchronous machine (VF-PMSM/VFM). Precise information of the back-EMF helps to estimate the cogging torque and torque ripple. An accurate determination of the core loss leads to a better efficiency estimation and machine design. An experimentally determined minimum inverter rating reduces the cost of the motor drive system. An adaptive nonlinear filter is used to estimate the VFM back-EMF during the motoring mode. The VFM is often demagnetized and remagnetized by injecting a current pulse to achieve the high-speed and high-torque requirements. The sudden injection of a d -axis current pulse will change the magnet flux linkage and back-EMF harmonics. In this article, harmonics present in the machine back-EMF due to different magnetization and demagnetization current pulse widths and magnetization methods are analyzed. Besides, the quality of the back-EMF for different speeds and machine no-load losses are presented for different magnetization levels. The minimum inverter current rating is determined for magnetizing current and magnetizing current at full load considering the minimum pulse amplitude and width to magnetize the VF-PMSM.

Back-EMF Analysis of a Variable Flux Machine for Different Magnetization States

This paper presents the electromechanical flywheel energy storage system designed to enhance rural electrification in Sub-Saharan Africa. Most non-grid connected areas, mostly rural, are powered with solar home systems whose sustainability is limited by the short life span, low power density and low storage efficiency of the battery storage. The electromechanical flywheel rotor is made from glass fiber and epoxy composite and designed using novel shape profiles based on Berger, Porat and Stodola's design. Dr. Stodola's design utilizes a stress based solution by introducing a central hole for a shaft inclusion. An Axial flux brushless DC machine was considered because of the low cost, ease of construction and simple control. The system stores 1,080kJ and supplies 100W with an operating speed range of 8,000–25,000 rpm. Stress, modal and thermal analysis are performed on the flywheel system using finite elements and analytical methods. The brushless DC drive together with the DC-DC converter were constructed and tested. The various components were simulated, prototyped using locally available material and the results presented.

High speed electromechanical flywheel design for rural electrification in Sub Saharan Africa

Steady state analysis techniques (ie Motor Current Signature Analysis) cannot be applied to variable speed wind generators since their operation is predominately in the transient A new non-stationary fault detection technique is proposed to detect inter-turn stator faults in doubly-fed wind generators The technique is a combination of the Extended Park's Vector Approach and a new adaptive algorithm It will be shown that the new technique can unambiguously detect inter-turn stator faults under transient conditions while providing insight into the degree of severity of the fault

Pragasen Pillay

Papers

Effect of saturation on rotational flux distribution in hydro generators

White LEDS for rural lighting

Back-EMF Analysis of a Variable Flux Machine for Different Magnetization States

High speed electromechanical flywheel design for rural electrification in Sub Saharan Africa

A New Algorithm for the Detection of Inter-Turn Stator Faults in Doubly-Fed Wind Generators