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.

Swiss rectifier output voltage control with inner loop power flow programming (PFP)

Abstract: A novel control scheme is proposed for a three-phase buck-type SWISS rectifier. This control is based on an inner loop power flow programming that provides a well damped behavior of the output filter and decoupled input filter and converter dynamics. A DC/DC equivalent circuit of the SWISS rectifier is used for the analysis and evaluation of different control concepts for the switching stage of the converter. By using different feedforward loops for the calculation of the duty cycles the behavior of the converter can be modified to suit a desired characteristic. In this way, the converter can be programmed for constant voltage transfer ratio, constant output voltage, constant power transfer or other input/output relations. A comparative evaluation of different inner loop control schemes is presented. The control schemes are compared analytically using small-signal linear models. The comparison considers the dynamic behavior as well as the decoupling of the input and output variables. Simulation results using the DC/DC equivalent circuit model and the actual three-phase SWISS rectifier are presented. According to the results, the PFP control achieves an improved behavior of the converter in terms of damping of the resonances and decoupling of the input and output variables, compared to the other control schemes.

Electronically Controllable Impedance for Tuning of Active Metamaterials

Abstract: In recent years, passive metamaterials consisting of a geometrical arrangement of $LCR$ -resonator-type base units are proposed for the shielding of magnetic stray fields at low- and medium-frequencies. The screening factor of such passive metamaterial shields can be deteriorated by component value tolerances and variations with temperature and/or by aging. These issues can be overcome with an active metamaterial assembled by an array of engineered active base units. Each base unit contains an $LCR$ -resonator and a control unit called Electronically Controllable Impedance (ECI). The ECI consists of a four-quadrant power converter with integrated measurement and control units for emulating an arbitrary impedance and is used to tune the $LCR$ resonator such that the base unit exhibits an ideally purely electric inductive or capacitive behavior when excited by an external field. Thereby, each single $LCR$ -resonator’s resonant frequency is tuned to be close to the excitation frequency and the resonator’s quality factor $Q$ is optimally adjusted. This paper shows different realizations of an ECI and proposes an ECI control algorithm to tune the resonant frequency and to adjust the quality factor $Q$ of the resonators. Furthermore, experimental verifications of the ECI concept employing a linear power amplifier are provided.

New Series-Resonant Solid-State DC Transformer Providing Three Self-Stabilized Isolated Medium-Voltage Input Ports

Abstract: This work analyzes the properties of a new four-port, series-resonant, and bidirectional DC transformer topology in steady-state and transient conditions. The investigated topology evolves from the Integration of Three (3) Series-Resonant Converters (I3SRC), enables the realization of a converter with high efficiency (99.0%) and high power density (7.1 kW/dm3), due to the utilization of a three-phase High Frequency (HF) transformer and reduced capacitor currents in the secondary-side DC link, and features the use of a six-pack power module on the secondary side. The analysis is based on the results of detailed circuit simulations of an I3SRC, which takes the couplings between the phases, introduced by the three-phase transformer, into account and considers a DC transformer, i.e., DC-DC converter, with a total rated power of 15 kW, three primary-side DC voltages of 1.1 kV, and a secondary-side DC voltage of 700 V, which is a core part of a scaled demonstrator of a Solid-State Transformer (SST). The obtained findings clarify the self-stabilizing capabilities of the I3SRC for operation with Half-Cycle Discontinuous Conduction Mode (HC-DCM), even if the three isolated DC ports are subject to time-varying and substantially different power levels and different directions of energy transfer.

A new digital current control AC-DC converter for wind turbine

Abstract: In this paper, a new digital hysteresis current control for ac-dc converters is presented. The new control circuit has the peak and valley current detectors which are comprised of an RC integrator, comparator and R-S flip-flop. These current detectors can detect the peak and valley reactor currents in real time. The proposed control circuit structure and the operation principle are explained with the simulation result. As a result, it is confirmed that both peak and valley currents are detected instantaneously using the proposed method.

Design considerations for the drive system of an ultra-high speed spinning ball motor

Abstract: This paper presents considerations for the drive system of a magnetically levitated ultra-high speed spinning ball motor. The system shall provide rotation speeds in the range of several million rotations per minute in future. In a first approach, the functionality is analyzed and verified on a system, which is enlarged by a factor of four compared to the motor size which shall provide the targeted rotational speed of the rotor. Possibilities of improvement for the final drive design are investigated in detail by using 3D FEM analysis. Finally, a new coreless drive unit, which shall provide higher rotor speeds of smaller rotors, is developed.

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.