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

This paper presents a novel algorithm for in situ efficiency estimation for induction motors operating with distorted unbalanced voltages. The proposed technique utilizes the genetic algorithm, IEEE Form F2-Method F1 calculations, large motor test database and a new stray-load loss formula. The technique is evaluated by testing 3 small- and medium-sized induction motors with 11 different combinations of voltage unbalance and total harmonic distortion. The results showed acceptable accuracy. The technique may be used as a potential industrial tool that can help derate induction motors in the presence of voltage unbalance and harmonics distortion.

A novel in situ efficiency estimation algorithm for three-phase induction motors operating with distorted unbalanced voltages

A dynamic model for evaluating core losses in a hysteretic magnetic lamination is developed and then solved using a high-order finite element method that includes time-history effects. It is demonstrated that the dynamic hysteresis effect, previously used to explain the frequency dependence of B−H loops, is not a fundamental phenomenon of magnetic materials but originates from the skin effect. It arises because the measured flux density is an averaged value over the lamination thickness, and this value is influenced strongly by the skin effect. The study verifies that, unlike the observed dynamic hysteresis effect, the local B−H loop is in fact frequency independent. The developed dynamic core loss model is thus derived based on the frequency-independent B−H loop. It is shown that the developed model can accurately evaluate the losses for different frequencies and thicknesses based on only one set of inputs of an experimental B−H loop at one low frequency without a huge database of experimental losses.

High order finite element model for core loss assessment in a hysteresis magnetic lamination

Rotational core loss data improve the accuracy of estimating electric machine core losses. The losses are measured using magnetizing yokes, which generate a rotating field. The accuracy of the measured losses is dependent on the uniform distribution of the fields for all the flux-density levels and magnetization directions in an isotropic sample in simulations. Additionally, the field distribution patterns should be the same for all the flux-density vector directions. The measurement area where the field sensors are placed should be large enough for better representation of magnetic material properties. The design should also suppress the z -component of the magnetic field and the leakage flux above the sample. A two-dimensional magnetizing yoke design with a sample measurement area of 65 mm × 65 mm on a 136 mm diameter sample is proposed to meet these requirements. The design was prototyped, and both pulsating and rotational core losses of a 0.36-mm-thick “M-19” nonoriented electrical steel were measured at a range of flux densities and frequencies.

Rotational Core Loss Magnetizer: Design and Measurements

Temperature rise of an induction machine increases losses and decrease in the efficiency. The temperature rise depends on the machine design. It takes several hours after starting a machine to reach to thermal stability. In most of the in situ efficiency estimation methods, it is required to obtain the operating data of the machine at a thermally stable condition, which needs a long running time of the machine. In this paper, a method based on a particle swarm optimization (PSO) algorithm is proposed, which can estimate the machine efficiency at different loads with thermal stability. The machine operation data at the first 30 min after the start rather than data at a thermal stability condition are used in the method. The proposed algorithm utilizes two approaches to predict a full-load temperature at a thermally stable condition. The first approach is based on the insulation class of the machine and uses the equivalent circuit. The second approach is based on the trend of the temperature rise in the first 30 min of running the machine after the start. Furthermore, a method is proposed to narrow the parameters range, which helps the PSO to converge to the right answer. All results are validated by the experimental results.

Efficiency Estimation of the Induction Machine by Particle Swarm Optimization Using Rapid Test Data With Range Constraints

The switched reluctance motor is well known for its higher acoustic noise, caused by stator vibrations. Techniques for noise reduction require knowledge of the modal frequencies, which depend on mechanical shapes and dimensions as well as material properties, for example, Young's modulus, Poisson's ratio, mass density, etc. It is found that the generally accepted value of Young's modulus is not valid for a machine with laminations and no frame. This paper introduces a simple and nondestructive method for the measurement of Young's modulus; it is then used in a finite element program to determine the resonant frequencies of SRM stator vibration. The effects of mass density and Poisson's ratios are also discussed. The FE results are validated by vibration tests, which show good accuracy.

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Pragasen Pillay

Papers

A novel in situ efficiency estimation algorithm for three-phase induction motors operating with distorted unbalanced voltages

High order finite element model for core loss assessment in a hysteresis magnetic lamination

Rotational Core Loss Magnetizer: Design and Measurements

Efficiency Estimation of the Induction Machine by Particle Swarm Optimization Using Rapid Test Data With Range Constraints

Effects of material properties on switched reluctance motor vibration determination