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.

Design and Experimental Analysis of 166 kW Medium-Voltage Medium-Frequency Air-Core Transformer for 1:1-DCX Applications

Abstract: The galvanic isolation of solid-state transformers (SSTs) is typically realized with a medium-frequency (MF) magnetic-core transformer (MCT). Previous demonstrations indicate that achieving highly power dense and lightweight MCTs imposes several challenges on the design because of stringent requirements related to insulation and cooling. This work investigates the achievable efficiency of an optimized 166kW / 7 kV air-core transformer (ACT), which is a core part of a DC Transformer (DCX), i.e., an unregulated DC-DC SST with a voltage scaling defined by the transformer turns ratio. The ACT features relatively low complexity of the construction, comparably high coupling values, and high efficiency. Modeling, optimization, and construction of the realized ACT are explained and guidelines regarding insulation, cooling, and shielding of the magnetic stray flux are discussed in detail. Furthermore, the prototype is experimentally validated to demonstrate its full functionality. In the investigated DCX, which is based on a series resonant converter (SRC) topology, the realized ACT is found to achieve a full-load efficiency of 99.5% and an unprecedented gravimetric power density of 16.5 kW/kg. With the use of 10 kV SiC MOSFETs, the complete DCX is estimated to reach an efficiency of 99% at 166kW output power.

Part II: experimental analysis of the very sparse matrix converter

Abstract: For the sake of simplicity no LC input filter was considered but the input voltage has been assumed to be directly applied to the converter input. There, the time behavior of the mains phase currents iN,i, i.e. of the input filter inductor currents of a practical system was obtained by averaging the discontinuous rectifier input currents over one pulse half period. The simulation results are compiled in Fig.19, where (d) shows the purely sinusoidal behavior of input and output quantities. The local average a i of the rectifier stage input phase current ia is lying in phase with the corresponding mains phase voltage ua , accordingly the system shows ohmic fundamental mains behavior (cf. Eq.(4)) As Fig.19(a) clearly shows, each power transistor is clamped within a π/3-wide interval of an input and/or output fundamental period what is indicated by a voltage across the power transistors of bpb S u = 0 and/or Bn S u = 0. In Fig.19(f) the time behavior of the DC link current i is given where i>0, and/or i=i+, i− is valid (cf. Eq.(50)), according to the output power factor of cos Φ2=0.961 (cf. Fig.19(d), load phase displacement of Φ2 < π/6). VIII. PRACTICAL REALIZATION

Behavior of the flying capacitor converter under critical operating conditions

Abstract: The Flying Capacitor Converter (FCC) offers an attractive alternative to conventional 2-IeveI converter topologies due to the easily acquired high number of voltage levels and the increased effective switching frequency. However, balancing of the flying capacitor (FC) voltages is crucial in practice since a deviation from the nominal voltage levels increases harmonics in the output voltage and, more importantly, jeopardizes the integrity of the converter due to overvoltages across the power transistors. Modulation inherent FC balancing techniques (termed natural/passive balancing) have been thoroughly analyzed in literature, however only for stationary operating conditions. In this paper, the behavior of the FCC and the effectiveness of passive balancing will be analyzed in detail regarding specific operating conditions present in typical industry applications such as converter start-up, shut-down, standby and operation under fault conditions. The basis for the analysis is a 5-level, 2 kW FCC embedded in two typical industry applications: single-phase PV inverter and single-phase PFC rectifier.

Modelling for the lifetime prediction of power semiconductor modules

Abstract: Reliability engineering has emerged as a relatively new branch of power electronics (PE) supporting the fast progress towards advanced power electronic converter systems (PECS) with significantly improved reliability ratings. PECS operate under increasingly severe temperature profiles, i.e., fast temperature cycling (TC) between extreme temperature levels. Accordingly, the reliability requirements for power semiconductor modules as fundamental components of PECS are significantly increased. Power module manufacturers have been working on new power module designs and packaging technologies in order to increase endurance and prolong the lifetime of power modules in the future, and subsequently enable high performance of the PECS also concerning reliability [1]. In the future, the reliability aspects have to be included into novel multi-domain optimization tools that will further improve the design of PECS. The first step towards this goal is to allow the integration of lifetime models of the system components into the design process.

High-Bandwidth High-Temperature (250 °C/500 °F) Isolated DC and AC Current Measurement: Bidirectionally Saturated Current Transformer

Abstract: In an increasing number of application areas and industry sectors, such as the automotive, aerospace, military or oil and gas industry, a trend towards higher ambient temperature rating from 85 °C upwards for electrical machines and power electronic converters can be observed. To reduce the impact of high ambient temperatures on the power density, the interest in power electronic converters with SiC power semiconductors operated up to a junction temperature of 250 °C rises. The control of power electronic converters typically requires a precise, fast, and robust current measurement. However, analyzing current measurement concepts from the literature reveals that there is a lack of measurement systems, that are galvanically isolated and able to measure dc and higher frequency ac currents fast at high ambient temperatures of 250 °C. In this paper, a current measurement concept of a bidirectionally saturated current transformer is presented, that is able to measure dc and sinusoidal ac up to 1 kHz and 50 A at an ambient temperature of 250 °C with an relative error of 2.6% and less than 0.5% error after initial calibration. Furthermore, a prototype is designed, built, and used for the experimental verification of the concept.

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.