Johann W. Kolar

Bio: Johann W. Kolar is an academic researcher from ETH Zurich. The author has contributed to research in topics: Rectifier & Three-phase. The author has an hindex of 97, co-authored 965 publications receiving 36902 citations. Previous affiliations of Johann W. Kolar include Alstom & Infineon Technologies.

Evaluation of a delta-connection of three single-phase unity power factor rectifier modules (/spl Delta/-rectifier) in comparison to a direct three-phase rectifier realization

Abstract: In this paper, the voltage and current stress of a delta-connection of three single-phase boost-type unity power factor rectifier modules (Δ-Rectifier) is analyzed in order to perform a design comparable to a direct three-phase three-level six-switch unity power factor (conventional) rectifier system. The conduction losses of the power semiconductors are calculated using analytical approximations of the average and RMS values of the component currents, the switching losses are taken from previous experimental investigations. Based on this data an overview of the estimated power losses is given for the Δ-rectifier as well as for the conventional rectifier for 10 kW output power, 800 VDC output and 320 V/400 V/480 V/530 V (RMS, line-to-line) mains voltage. These investigations finally lead to efficiency and component count figures. Furthermore, special attention is paid to the control of the whole three-phase system, including the DC/DC converter output stages of the line-to-line modules of the Δ-rectifier. Finally, topics of the continuation of the research are identified as, e.g., the realization of a prototype in order to verify the theoretical results experimentally also for unbalanced mains voltage conditions and with nonideal components such as nonlinear input inductors employing iron powder cores and the analysis of a Δ-rectifier being formed by single-stage SEPIC-type line-to-line modules.

Evaluation of a delta-connection of three single-phase unity power factor rectifier modules (Δ-Rectifier) in comparison to a direct three-phase rectifier realization. Part II - component stress evaluation, efficiency, control

Abstract: In this paper the voltage and current stress of a delta-connection of three single-phase boost-type unity power factor rectifier modules (Δ-Rectifier) is analyzed in order to perform a design comparable to a direct three-phase three-level six-switch unity power factor (conventional) rectifier system. The conduction losses of the power semiconductors are calculated using analytical approximations of the average and rms values of the component currents, the switching losses are taken from previous experimental investigations. Based on this data an overview of the estimated power losses is given for the A-Rectifier as well as for the conventional rectifier for 10kW output power, 800VDC output and 320V/400V/ 480V/530V (rms, line-to-line) mains voltage. These investigations finally lead to efficiency and component count figures. Furthermore, special attention is paid to the control of the whole three-phase system including the DC/DC converter output stages of the line-to-line modules of the A-Rectifier. Finally, topics of the continuation of the research are identified as, e.g., the realization of a prototype in order to verify the theoretical results experimentally also for unbalanced mains voltage conditions and with nonideal components such as nonlinear input inductors employing iron powder cores and the analysis of a A-Rectifier being formed by single-stage SEPIC-type line-to-line modules.

Phase-oriented control of a modular 3-Phase 3-level 4-leg inverter AC power source supplying floating or grounded loads

Abstract: A control scheme for a high-performance three-phase AC power source is presented. The four-leg inverter output stage uses three bridge legs to generate the phase output voltages with reference to the neutral point potential, which is defined by the fourth bridge leg. The inverter is controlled using a phase-oriented control in order to achieve precise control of each output phase with unbalanced voltages and any kind of load. The neutral potential is controlled for maximum modulation range in case of floating load and controlled to zero for grounded load. The control scheme for the input PWM rectifier stage considers the control of the zero sequence voltage in order to prevent the appearance of circulating currents through the ground loop when the load star point is connected to ground. The control of the circulating currents also allows the safe parallel connection of two or more AC sources. Simulation results under balanced and unbalanced conditions demonstrate the performance of the AC source in terms of output voltage control and prevention of circulating ground currents.

Optimization and Comparative Evaluation of Multiloop Control Schemes for Controllable AC Sources With Two-Stage $LC$ Output Filters

Abstract: This study investigates the control system design and its small-signal properties for the output stage of a high-bandwidth four-quadrant three-phase switch-mode controllable AC voltage source (CVS) with an output power of $10\;\mathrm{kW}$ , a switching frequency of $48\;\mathrm{kHz}$ , and a two-stage $LC$ output filter. Each output phase of the CVS is operated individually, i.e., the phase voltages are generated with reference to the DC input-voltage midpoint, to allow maximum flexibility in the generation of the output-voltage waveforms to supply a wide range of different load types, such as DC, single-phase, and general three-phase loads including constant-power loads leading to negative small-signal load-resistance values. Three suitable multiloop control structures with inner-current- and outer-voltage-control loops are motivated, modeled, and are optimized with respect to different control performance indicators, e.g., small-signal control bandwidth, and for common boundary conditions, e.g., maximum overshoot of the output voltage in case of a reference voltage step. All structures employ conventional P and PI controllers, due to their simplicity and widespread use. Among the three structures, the capacitor–current feedback-control structure, which controls the two filter capacitor currents and the output voltage, is identified to be most competitive. The small-signal bandwidth determined for this structure is between $7.1\;\mathrm{kHz}$ and $15.5\;\mathrm{kHz}$ , depending on the value of the load resistance. This result, in combination with an excellent matching of calculated and measured step responses of the output voltage of a $10\;\mathrm{kW}$ hardware prototype, point out the effectiveness of the selected control structure and the usability of control structures that are composed of conventional P and PI controllers.

I and i

Abstract: 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 …

Multilevel inverters: a survey of topologies, controls, and applications

Abstract: 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.

Recent Advances and Industrial Applications of Multilevel Converters

Abstract: Multilevel converters have been under research and development for more than three decades and have found successful industrial application. However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers with a comprehensive and insightful review of where multilevel converter technology stands and is heading. This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry. In addition, new promising topologies are discussed. Recent advances made in modulation and control of multilevel converters are also addressed. A great part of this paper is devoted to show nontraditional applications powered by multilevel converters and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities.