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

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

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

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.

/pdf/recent-advances-and-industrial-applications-of-multilevel-26fluutoc9.pdf

Recent Advances and Industrial Applications of Multilevel Converters

http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

This work presents a comprehensive cost-aware comparison of isolated bidirectional Si and SiC dual-active-bridge (DAB) concepts for a 5-kW 100–700-V input voltage range dc microgrid application. A conventional three-level DAB (3LDAB) is compared to an advanced five-level DAB (5LDAB) topology, where the latter enables reduced rms currents within the given voltage range. Both concepts employ a loss-optimized modulation scheme enabling zero-voltage switching. A multiobjective optimization routine is proposed to systematically assess the concepts with respect to the efficiency, power density, and the costs. A novel waveform model and advanced component models are considered, which are verified using a hardware prototype. The calculated Pareto fronts show that SiC MOSFETs enable significantly higher efficiencies and power densities than Si IGBTs, while similar costs can be achieved. The performance comparison between the SiC MOSFET-based 3LDAB and 5LDAB reveals a fundamental superiority of the 3LDAB, which is mainly due to the higher chip area utilization and the lower component count of this concept. Finally, the calculations and the hardware prototype prove that despite the galvanic isolation and wide voltage range, efficiencies above 98 % in a wide operating range are possible, which was previously not seen in the literature.

Comparative $\eta$– $\rho$– $\sigma$ Pareto Optimization of Si and SiC Multilevel Dual-Active-Bridge Topologies With Wide Input Voltage Range

Switching devices based on SiC offer outstanding performance with respect to operating frequency, junction temperature, and conduction losses enabling significant improvement of the performance of converter systems. There, the cascode consisting of a MOSFET and a JFET has additionally the advantage of being a normally off device and offering a simple control via the gate of the MOSFET. Without dv /dt-control, however, the transients for hard commutation reach values of up to 45kV/μs, which could lead to electromagnetic interference problems. Especially in drive systems, problems could occur, which are related to earth currents (bearing currents) due to parasitic capacitances. Therefore, new dv/d t-control methods for the SiC JFET/Si MOSFET cascode as well as measurement results are presented in this paper. Based on this new concepts, the outstanding performance of the SiC devices can be fully utilized without impairing electromagnetic compatibility.

https://www.hpe.ee.ethz.ch/uploads/tx_ethpublications/06365336.pdf

D $v$ /D $t$ -Control Methods for the SiC JFET/Si MOSFET Cascode

In this study, the power density of a two-stage, i.e., LCLCL, electromagnetic interference (EMI) filter for a 10 kW three-phase, T-type three-level, boost-type PWM rectifier system is optimized concerning construction volume. It is found that reasonable boundary conditions limit or define most component values and only few degrees of freedom are remaining. In this context, the ratio of the first filter stage's common to differential mode inductance, k
 L
 = L
 cm,1
/L
 dm,1
, is identified to particularly affect the total filter volume. For given switching frequency and DC-link voltage of f
 s
 = 48 kHz and V
 dc
 = 700 V, a flat minimum of the overall filter volume results for 10 <; k
 L
 <; 40. The boxed volume of the filter realized for k
 L
 = 18 is 763 cm
 3
 (46.6 in
 3
), which corresponds to a filter power density of 13.1 kW / dm
 3
 (215 W / in
 3
). Measurements of conducted EMI confirm that the rectifier system complies with the considered limits of conducted EMI (CISPR 11, class A).

EMI Filter Volume Minimization of a Three-Phase, Three-Level T-Type PWM Converter System

Solid-state transformers (SSTs) are a promising technology as they provide new functionalities and services enabling future smart grids. An SST establishes the interface between an MV ac grid and an LV (ac or dc) grid or load. The SST must provide high reliability even in the event of certain grid faults, where the SST is subject to exceptional electrical stresses. In addition, the MV and LV grids are exposed to failures of the SSTs. Spurred by such challenging requirements, this paper defines the different relevant stresses, identifies the corresponding protection needs, and proposes adequate protection circuitries and devices. The protection mechanisms that are currently used for low-frequency transformers are analyzed and an adapted version is proposed for SSTs. MV short circuits and overvoltages are identified as the most critical situations and are analyzed in detail. From the presented in-depth investigation, guidelines are extracted for designing robust SSTs and are applied to a 1 MVA, 10 kV SST.

Protection of MV Converters in the Grid: The Case of MV/LV Solid-State Transformers

A high power density 10 kW three-phase 12-pulse rectifier is analyzed for applications in future more electric aircrafts. The experimental results, which are in good accordance with the theory, show high efficiency and low input current harmonics for a wide operating range. Furthermore, two novel rectifier topologies, which are formed by combining the passive 12-pulse rectifier with a boost stage on the DC side are proposed. This allows to guarantee a constant output voltage and/or to overcome the problem of the dependency of output voltage on the mains voltage amplitude and output power level.

Johann W. Kolar

Papers

Comparative $\eta$– $\rho$– $\sigma$ Pareto Optimization of Si and SiC Multilevel Dual-Active-Bridge Topologies With Wide Input Voltage Range

D $v$ /D $t$ -Control Methods for the SiC JFET/Si MOSFET Cascode

EMI Filter Volume Minimization of a Three-Phase, Three-Level T-Type PWM Converter System

Protection of MV Converters in the Grid: The Case of MV/LV Solid-State Transformers

12-pulse rectifier for more electric aircraft applications