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

The self-excited synchronous reluctance generator (SynRG) represents an alternative to supply electric power to remote communities. It is robust, simple, and less expensive compared with other types of brushless generators. However, for reliable operation in stand-alone mode, the self-excitation of the machine must be assured, which demands that certain starting conditions are met. It is observed through experiments that the existence of a minimum residual flux in the core, though necessary, is not sufficient to trigger self-excitation. The start-up acceleration also plays an important role since it may create transients in the q- axis flux linkage that can lead to demagnetization of the core and consequent failure in the terminal voltage build-up. This paper studies the effects of these starting conditions on the self-excitation phenomenon and presents criteria in terms of minimum residual flux and maximum start-up acceleration to trigger self-excitation without the risk of core demagnetization. The criteria to ensure the self-excitation for synchronous reluctance machines is developed using the dq -model of the machine and the energetic model. The developed concept is also applicable to other SynRGs operating in stand-alone mode.

Self-Excitation Criteria of the Synchronous Reluctance Generator in Stand-Alone Mode of Operation

Flywheel rotor manufacture for rural energy storage in sub-Saharan Africa

This paper examines the required energy for the magnetization and demagnetization of magnets in a spoke type AlNiCo-based variable-flux machine and studies the torque characteristics of this machine at different magnetization levels. The low coercivity magnet in this machine can be magnetized or demagnetized using a short-time current pulse with negligible Ohmic loss. An advanced method is proposed to estimate the required energy for magnet demagnetization or magnetization to a specific level. A test procedure is developed to measure the energy that is injected to the variable-flux machine during the demagnetization and magnetization procedures. Since this machine has the ability to operate at various magnetization levels, it is of great importance to obtain the torque characteristics such as torque mean value, the peak to peak value of the torque, as well as the torque ripple, at different operating conditions. A static torque measurement test procedure using a variable speed drive system is developed to measure the torque waveform of the variable-flux machine at different magnetization levels. The verified finite element model of the variable-flux machine is used to analyze the harmonic content of the back-emf and the no-load air gap flux density at different levels of magnetization.

Magnetization and Demagnetization Energy Estimation and Torque Characterization of a Variable-Flux Machine

Most condition monitoring techniques are based on steady state analysis. However, machines typically operate under transient conditions thus prompting the development of non-stationary fault detection techniques. In this paper, the steady state analysis technique (Motor Current Signature Analysis) is extended to a new non-stationary fault detection technique to detect several different faults in a low-voltage high current PMSM. It will be shown that the Motor Current Signature Analysis (MCSA) technique cannot be applied directly to machines operating under transient conditions. The new technique, which is based on an adaptive algorithm capable of extracting non-stationary sinusoids, is able to extract fault information during transient operation of the machine.

A New Algorithm for the Detection of Faults in Permanent Magnet Machines

Direct-drive wind energy conversion systems (WECS) are required to operate at low speeds determined by that of the turbine. This paper addresses the problem of adapting a small, standard, readily available PM synchronous machine for direct coupling to a small wind turbine. In particular, it investigates several minor changes to the machine design, which will ensure that rated power can be delivered to its load, whilst operating at the reduced speeds required by a direct-drive WECS.

Pragasen Pillay

Papers

Self-Excitation Criteria of the Synchronous Reluctance Generator in Stand-Alone Mode of Operation

Flywheel rotor manufacture for rural energy storage in sub-Saharan Africa

Magnetization and Demagnetization Energy Estimation and Torque Characterization of a Variable-Flux Machine

A New Algorithm for the Detection of Faults in Permanent Magnet Machines

Impact of direct-drive WEC Systems on the design of a small PM wind generator