Hochdynamische Leistungsregelung eines dreiphasigen Hochleistungs-netzstromrichters zur Speisung von Gleichspannungszwischenkreisen

Übersicht In dieser Arbeit wird ein neues Regelungsverfahren für U-Netzstromrichter sehr hoher Leistung beschrieben. Die Einspeisungsschaltung besteht aus zwei GTO-Dreipunktstromrichtern, die über Dreiwicklungstransformatoren mit in Stern bzw. Dreieck geschalteten Sekundärwicklungen ans Netz geschaltet sind. Für den Betrieb als Netzstromrichter wird eine Steuerstrategie entwickelt, wobei eine möglichst geringe Verzerrung der netzseitigen Spannung unter Einhaltung der geringen zulässigen Schaltfrequenz der Ventile das Ziel ist. Die Regelung für netzfreundlichen hochdynamischen Energieaustausch zwischen Netz und Zwischenkreis ist auf der Basis der sogenannten Stromrichterflußbahn konzipiert.

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Highly dynamic control for a three-phase high power AC/DC converter

Contents This paper describes the implementation of a new control approach for a high power three-phase AC/DC converter. The converter consists of two three-level inverters connected to the line via a three-winding transformer with the secondaries connected in star and delta. The control scheme is developed to minimise the network distortion voltage while keeping the small permitted switching frequency of the valves. The control for highly dynamic exchange of energy between the network and the dc link voltage is proposed on the basis of the converter voltage space vector time integral.

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Diese Arbeit wurde von der Deutschen Forschungsgemeinschaft gefördert.     

Dielectric breakdown of a residual SF6 plasma at 3000 Kunder diatomic equilibrium

A residual plasma at 3000 K and under diatomic equilibrium is left inside an axially blown gas blast SF₆ circuit breaker after the thermal extinction of an arc. The breakdown of such a residual plasma depends on the electron generation and loss mechanisms. Altogether six ionization and chemical reactions relevant to the time scale of breakdown have been taken into account. It has been found that the critical field strength for breakdown is proportional to the pressure and is equal to 2.0 V/(m.Pa). This field strength is in agreement with the experimental results     

Advances in selective wet oxidation of AlGaAs alloys

We review the chemistry, microstructure, and processing of buried oxides converted from AlGaAs layers using wet oxidation. Hydrogen is shown to have a central role in the oxidation reaction as the oxidizing agent and to reduce the intermediate predict As₂O₃ to As. The stable oxide is amorphous (Al_xGa_1-x) ₂O₃ which has no defects along the oxide/semiconductor interfaces but can exhibit strain at the oxide terminus due to volume shrinkage. The influence of gas flow, gas composition, temperature, Al-content, and layer thickness on the oxidation rate are characterized to establish a reproducible process. Linear oxidation rates with Arrhenius activation energies which strongly depend upon AlAs mole fraction are found. The latter produces strong oxidation selectivity between AlGaAs layers with slightly differing Al-content. Oxidation selectivity to thickness is also shown for layer thickness <60 nm. Differences between the properties of buried oxides converted from AlGaAs and AlAs layers and the impact on selectively oxidized vertical cavity laser lifetime are reported     

Application of fuzzy logic in computer-aided VLSI design

Application of fuzzy logic structures in CAD of digital electronics substantially improves quality of design solutions by providing designers with flexibility in formulating goals and selecting tradeoffs. In addition, the following aspects of a design process are positively impacted by application of fuzzy logic: utilization of domain knowledge, interpretation of uncertainties in design data, and adaptation of design algorithms. We successfully applied fuzzy logic structures in conjunction with constructive and iterative algorithms for selecting of design solutions for different stages of the design process. We also introduced fuzzy logic software development tool to be used in CAD applications     

FEM for evaluation of weight functions for SIF, COD and higher-order coefficients with application to a typical wedge splitting specimen

In the evaluation of accurate weight functions for the coefficients of first few terms of the linear elastic crack tip fields and the crack opening displacement (COD) using the finite element method (FEM), singularities at the crack tip and the loading point need to be properly considered. The crack tip singularity can be well captured by a hybrid crack element (HCE), which directly predicts accurate coefficients of first few terms of the linear elastic crack tip fields. A penalty function technique is introduced to handle the point load. With the use of these methods numerical results of a typical wedge splitting (WS) specimen subjected to wedge forces at arbitrary locations on the crack faces are obtained. With the help of appropriate interpolation techniques, these results can be used as weight functions. The range of validity of the so-called Paris equation, which is widely used in the evaluation of the COD from the stress intensity factors (SIFs), is established.     

Comparison of corrosion behavior of zirconium bombarded with self-ion and molybdenum ion

In order to study the effects of zirconium and molybdenum ion bombardment on the aqueous corrosion behavior of zirconium, one group of specimens was implanted with zirconium ions with ions surface densities ranging from 1 × 10¹⁵ to 2 × 10¹⁷ ions/cm² at about 170 °C, using a metal vapor vacuum arc (MEVVA) source operated at an extraction voltage of 50 kV. The other group of specimens was bombarded with molybdenum ion with ions surface densities ranging from 1 × 10¹⁶ to 5 × 10¹⁷ ions/cm² at about 160 °C, using a MEVVA source operated at an extraction voltage of 40 kV. The valence states and depth distribution of elements in the surface of the samples were analyzed by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), respectively. Polarization curves measurement was employed to evaluate the aqueous corrosion resistance of the zirconium samples in a 1N H₂SO₄ solution. It was found that the aqueous corrosion resistance of zirconium implanted with 5 × 10¹⁶ Zr ions/cm² is the best in first group samples. For molybdenum ion implantation, the aqueous corrosion resistance of samples declined with raising ions surface densities. The natural corrosion potentials of zirconium samples bombarded with self-ions are more negative than that of the as-received zirconium. While, as for molybdenum ion implantation, the results are opposite. Finally, the mechanisms of the corrosion behavior of the zirconium samples implanted with zirconium and molybdenum ions are discussed.     

Effect of synthesis temperature on density and microstructure of MgB2 superconductor at ambient pressure

The experimental thermodynamic of MgB₂ synthesis process and phase compositions have been investigated by diffraction thermal analysis (DTA) technology and X-ray diffraction. The fabrication of MgB₂ bulks and superconducting properties at the temperatures range from 600 to 800°C were reported. And microstructure of MgB₂ bulks were observed by scanning electron microscope (SEM). A method was developed to determine the porosity of MgB₂ and the highest density can be obtained in MgB₂ prepared at 650°C at ambient pressure. It is found that the vapor pressure of Mg increases remarkably at high temperature, leading to the high porosity in MgB₂ samples. MgB₂ bulk with good superconducting property and fine microstructure was synthesized at 750°C.