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 application of vector control to the PMSM (permanent-magnet synchronous motor) is described, and complete modeling, simulation, and analysis of the drive system are presented. State-space models of the motor and speed controller and real-time models of the inverter switches and vector controller are included. Performance differences due to the use of pulsewidth-modulation (PWM) and hysteresis current controllers are also examined. Particular attention is paid to the motor torque pulsations and speed response. Some experimental verification of the drive performance is also given. >

Modeling, simulation, and analysis of permanent-magnet motor drives. I. The permanent-magnet synchronous motor drive

For ptI see ibid, vol25, no2, p265-73 (1989) The authors develop a phase variable model of the BDCM (brushless DC motor) and use it to examine the performance of a BDCM speed servo drive system when fed by hysteresis and pulsewidth-modulated (PWM) current controllers Particular attention was paid to the motor large-signal and small-signal dynamics and motor torque pulsations The simulation included the state-space model of the motor and speed controller and real-time model of the inverter switches Every instance of a power device turning on or off was simulated to calculate the current oscillations and resulting torque pulsations The results indicate that the small- and large-signal responses are very similar This result is only true when the timing of the input phase currents with the back EMF (electromotive force) is correct The large-signal and small-signal speed response is the same whether PWM or hysteresis current controllers are used This is because, even though the torque pulsations may be different due to the use of different current controllers, the average value which determines the overall speed response is the same >

Modeling, simulation, and analysis of permanent-magnet motor drives. II. The brushless DC motor drive

Research has indicated that the permanent magnet motor drives, which include the permanent magnet synchronous motor (PMSM) and the brushless DC motor (BDCM) could become serious competitors to the induction motor for servo applications. The PMSM has a sinusoidal back EMF and requires sinusoidal stator currents to produce constant torque while the BDCM has a trapezoidal back EMF and requires rectangular stator currents to produce constant torque. The PMSM is very similar to the wound rotor synchronous machine except that the PMSM that is used for servo applications tends not to have any damper windings and excitation is provided by a permanent magnet instead of a field winding. Hence the d, q model of the PMSM can be derived from the well-known model of the synchronous machine with the equations of the damper windings and field current dynamics removed. Because of the nonsinusoidal variation of the mutual inductances between the stator and rotor in the BDCM, it is also shown that no particular advantage exists in transforming the abc equations of the BCDM to the d, q frame. Hence the solution of the original abc equations is proposed for the BDCM. >

Modeling of permanent magnet motor drives

The permanent magnet synchronous motor (PMSM) and the brushless DC motor (BDCM) have many similarities; they both have permanent magnets on the rotor and require alternating stator currents to produce constant torque. For application considerations, these two motor drives have to be differentiated on the basis of known engineering criteria. Some of the criteria used to assess these two machines include power density, torque per unit current, speed range, feedback devices, inverter rating, cogging torque, ripple torque, and parameter sensitivity. Guidelines for the appropriate machine to be used for a given application are given based on the results of the criteria. >

Application characteristics of permanent magnet synchronous and brushless DC motors for servo drives

Selective harmonic elimination pulsewidth modulation (SHEPWM) method is systematically applied for the first time to multilevel series-connected voltage-source PWM inverters. The method is implemented based on optimization techniques. The optimization starting point is obtained using a phase-shift harmonic suppression approach. Another less computationally demanding harmonic suppression technique, called a mirror surplus harmonic method, is proposed for double-cell (five-level) inverters. Theoretical results of both methods are verified by experiments and simulations for a double-cell inverter. Simulation results for a five-cell (11-level) inverter are also presented for the multilevel SHEPWM method.

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Pragasen Pillay

Papers

Modeling, simulation, and analysis of permanent-magnet motor drives. I. The permanent-magnet synchronous motor drive

Modeling, simulation, and analysis of permanent-magnet motor drives. II. The brushless DC motor drive

Modeling of permanent magnet motor drives

Application characteristics of permanent magnet synchronous and brushless DC motors for servo drives

Multilevel selective harmonic elimination PWM technique in series-connected voltage inverters