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.

Analysis and design of fixed voltage transfer ratio DC/DC converter cells for phase-modular solid-state transformers

Abstract: Many Solid-State Transformer (SST) concepts employ cascaded AC/DC converter cells to handle the comparatively high medium-voltage (MV) grid voltages, resulting in a phase-modular structure. Accordingly, each cell — formed by an AC/DC input stage and an isolated DC/DC output stage — processes a power fluctuating with twice the grid frequency. The series resonant converter (SRC) operated in the half-cycle discontinuous-conduction-mode (HC-DCM) is a highly attractive choice for the isolated DC/DC converter because of its high efficiency. However, this converter does not offer any control possibilities; instead, it couples the two DC voltages through certain dynamics with fixed voltage transfer ratio. This leads to a propagation of the input side power fluctuations through the SRC to the common LV bus, which has certain consequences on the converter design. The paper therefore re-derives a dynamic model of the SRC's terminal behavior in a generic way, which also covers the case of comparatively small DC link capacitors. The experimentally verified dynamic model is then used to discuss and optimize the choice of the input and output side capacitances of the DC/DC converter cell with respect to the placement of the converter's system level resonances, such as to obtain minimum volume and losses. Finally, aspects related to the design of a scaled demonstrator system featuring similar dynamic behavior as the full-scale system are addressed and first measurement results are presented.

Analysis of Theoretical Limits of Forced-Air Cooling Using Advanced Composite Materials With High Thermal Conductivities

Abstract: Cooling systems take a significant portion of the total mass and/or volume of power electronic systems. In order to design a converter with high power density, it is necessary to minimize the converter's cooling system volume for a given maximum tolerable thermal resistance. This paper theoretically investigates whether the cooling system volume can be significantly reduced by employing new advanced composite materials like isotropic aluminum/diamond composites or anisotropic highly orientated pyrolytic graphite. Another strategy to improve the power density of the cooling system is to increase the rotating speed and/or the diameter of the fan, which is limited by increasing power consumption of the fan. Fan scaling laws are employed in order to describe volume and thermal resistance of an optimized cooling system (fan plus heat sink), resulting in a single compact equation dependent on just two design parameters. Based on this equation, a deep insight into different design strategies and their general potentials is possible. The theory of the design process is verified experimentally for cooling a 10 kW converter. Further experimental results showing the result of the operation of the optimized heat sink are also presented.

A Comparative Study of Multicell Amplifiers for AC-Power-Source Applications

Abstract: AC test power sources are essential for testing electric equipment that is ac-mains connected. Typically, linear power amplifiers (LPAs) are mainly employed as ac test sources because of their high fidelity and excellent dynamic behavior. However, these LPAs have very high losses in their output stages, which make the systems bulky and expensive due to the large heatsinks that are required. In recent years, two approaches have attracted popular interest to improve the efficiency of ac power sources. These are hybrid operation, i.e., combination of linear and switch mode, and multilevel switch-mode inverters. In this paper, three types of multilevel power amplifiers, based on hybrid or switching technology, are presented, namely: hybrid multicell amplifier (H-MCA), amplitude-modulated (AM) + pulsewidth-modulated (PWM) MCA (AP-MCA), and PWM MCA (P-MCA). For each of these topologies, the operating principle and control scheme are introduced. Finally, a 1-kVA laboratory prototype, which can realize all the three topologies, is built, and the measured performance based on this universal prototype are compared.

A Miniature 500 000-r/min Electrically Driven Turbocompressor

Abstract: The trend in compressors for fuel cells, heat pumps, aerospace, and automotive heating, ventilation, and air-conditioning systems is toward ultracompact size and high efficiency. This can be achieved by using turbocompressors instead of scroll, lobe, or screw compressors, increasing the rotational speed and employing new electrical drive system technology and materials. This paper presents a miniature electrically driven two-stage turbocompressor system running at a rated speed of 500 000 r/min. The design includes the thermodynamic analysis, the electric motor, the inverter, the control, and the system integration with rotor dynamics and thermal considerations. Experimental measurements such as the compressor map are presented for air under laboratory conditions. The two-stage turbocompressor has been tested up to a speed of 600 000 r/min, where a maximal pressure ratio of 2.3 at a mass flow of 0.5 g/s has been reached. To the authors' knowledge, this is the highest rotational speed achieved with an electrically driven turbocompressor.

Comparative Life Cycle Cost Analysis of Si and SiC PV Converter Systems Based on Advanced $\eta$- $\rho$-$\sigma$ Multiobjective Optimization Techniques

Abstract: This paper presents a novel virtual prototyping routine for power electronic converter systems. The approach facilitates a comprehensive and systematic benchmarking of different converter concepts based on a multiobjective optimization regarding the efficiency, power density, and costs. The underlying modeling framework is based on detailed and experimentally verified models. In particular, novel cost data as well as unpublished switching loss and core loss measurements are incorporated. The proposed virtual prototyping routine is employed to carry out a comparative study of the potential of Si and SiC semiconductors in a 10-kW residential three-phase photovoltaic inverter application. For this purpose, a state-of-the-art hard-switched three-level Si insulated-gate bipolar transistor (IGBT) system is compared to a hard-switched and to a soft-switched two-level SiC MOSFET system. The candidate systems for each concept are selected among the $\boldsymbol {\eta}$ - $\boldsymbol {\rho}$ - $\boldsymbol {\sigma}$ Pareto-optimized designs based on the life cycle costs. The hard-switched two-level SiC candidate system is found to be the most attractive solution featuring the lowest life cycle costs. When compared to the Si-based candidate system, not only a better power density and efficiency result. At the same time, besides the lower life cycle costs ( $-$ 22%), lower component costs ( $-$ 5%) can also be attained. The attractiveness of the found SiC solution is underlined by the simple control and the lowest component count among all concepts.

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.