Design, Realization and Experimental Test of a Coaxial Exponential Transmission Line Adaptor for a Half Impulse Radiating Antenna

This paper describes the simulation, design, realization, and experimental test of a tapered transmission line (TL) for adapting a broad-band impulse generator to a radiating antenna, for a frequency range of 50 MHz-1 GHz. Two different taper geometries are considered and discussed in the analysis: exponential and logarithmic. Two analysis methods are also used: 1) analytical equations obtained by applying the TL theory and 2) full-wave numerical simulations in both frequency and time domains using Comsol. It is shown that, in general, an exponential taper performs better than a logarithmic one, particularly at high frequencies. Time-domain simulations reveal that, for fast transient subnanosecond pulses, both tapers can be used equivalently and the signal does not suffer from any significant distortion while traveling along the tapers. We also show that analytical equations obtained using the TL theory are in very good agreement with full-wave simulation results and can be used advantageously in the design of tapers. This paper also presents the mechanical design and the realization of an exponential taper used for the connection of a 50-Omega pulser to a half-impulse radiating antenna (HIRA) having an input impedance of 100-Omega. The realized taper is fully characterized in the frequency domain using a vector analyzer and in the time domain using a reflectometer and shown to be performing in agreement with the simulations.

Design, Realization, and Experimental Test of a Coaxial Exponential Transmission Line Adaptor for a Half-Impulse Radiating Antenna

Nicolas Mora Parra

Ieee-Inst Electrical Electronics Engineers Inc2013

Fast temperature cycling of the catalytic CO oxidation using microstructure reactors

Martin Luther

The possibility to increase the performance (productivity or selectivity) of a chemical reactor by using periodic variations of reaction parameters (e.g. reactant concentration or temperature) has been theoretically envisaged since the beginning of the 70th. The experimental validation of the predicted positive effects was successful in the case of concentration variation but failed for temperature variation. This was mainly due to the high thermal inertia of the conventional chemical reactors used for the measurements which prevented to create variations having a sufficiently high frequency. Microstructure reactors own, at the contrary to conventional reactors, a very low thermal inertia and allow to generate temperature oscillations with an amplitude of about ten to hundred Kelvin at a frequency in the order of magnitude of 10-1 to 4 Hz. These properties, coupled with the possibility to introduce a catalytic active material within the devices, seem to make them well suited for the study of the effects of fast periodic temperature variations of a catalytic reaction. The objective of this work was to demonstrate that non-stationary temperature conditions may increase the reaction rate of a heterogeneously catalyzed reaction up to values not predicted by the classical Arrhenius dependency towards the temperature. Two different types of microstructure reactors have been used. The reaction was taking place in the first one (FTC-type 2) in microstructured channels on whose walls a catalytic layer was deposed. In the second one (FTC-type 3), the reaction was taking place on a piece of sintered metal fibres (SMF) plate placed in a reaction chamber. The catalytic active material was deposed on the SMF plate filaments. Both devices where permanently heated by electrical resistors and periodically cooled down with water flowing through cooling channels incorporated in the devices. The test reaction chosen for the experimental measurements was the CO oxidation reaction, heterogeneously catalyzed by platinum supported on alumina (Pt/Al2O3). The reaction behaviour under stationary thermal conditions was consistent with the one predicted by the Arrhenius law. The dependency of the reaction rate with the temperature was exponential with an apparent activation energy of 104 kJ·mol-1, a negative partial reaction order for CO and a positive one for O2. The measurements effectuated under quasi-stationary thermal conditions (slow temperature ramps) have shown that a temperature change rate between 7 and 14 K·min-1 was not sufficient to observe any non-trivial effect of the temperature. The reaction behaviour was always predicted by the Arrhenius law. The surface coverage of the reactive species is, in this case, always able to follow the slow temperature changes and the reaction behaves as being always under steady-state conditions. The experiments realized under non-stationary thermal conditions using the FTC-type 2 reactor have also failed to demonstrate any non-trivial effects of the temperature oscillations. This device allows indeed to generate temperature oscillations with an amplitude of up to 120 K with a frequency of 0.1 Hz but unfortunately correlated with a very high thermal inhomogeneity. A temperature difference of up to 80 K was measured between a cold spot and a hot spot inside the device. Due to their very low local reaction rate the colder areas of the reactor have attenuated the product concentration (CO2) oscillations which should have been created by the temperature variations. This attenuation prevented the temperature oscillations to have any positive effect compared to the stationary thermal conditions. The FTC-type 3 reactor allowed temperature changes of lower amplitude but the thermal homogeneity was much better. The maximal temperature difference measured between two points within the reactor was only 15 K. Under temperature oscillation conditions, the measured instantaneous CO2 concentration was higher for any temperature within the oscillation range compared to the one recorded under stationary thermal conditions. The increase obtained in the mean CO2 concentration was ranging from 34% for a frequency of the oscillations of 0.035 Hz to 85% for a frequency of 0.052 Hz. The amplitude of the oscillations was kept constant at approximately 40 K and the mean temperature value at 437 K. The simulations effectuated using a theoretical model for the catalytic CO oxidation including a feed-back step in the form of the oxidation-reduction of the catalyst have shown that the experimentally observed increase may be qualitatively explained. Above a certain frequency, the temperature oscillations are fast enough compared to the characteristic time of the feedback step and are able to perturb the stationary established reactive species surface coverage. The formation of a transient surface coverage more favourable for the reaction than the surface coverage at high temperature allows during the transient period to reach an instantaneous surface reaction rate values higher than the one at high temperature. This reaction rate peak is then responsible for the increase of the mean reaction rate under non-stationary thermal conditions and, thus, for an increased yield.

EPFL2008

Analog microelectronic emulation for dynamic power system computation

Ira Nagel

Power system dynamic simulators can be classified according to multiple criteria, including speed, precision, cost and modularity (topology, characteristics and model). Existing simulators are based on time-consuming numeric algorithms, which provide very precise results. But the evolution of the power grid constantly changes the requirements for simulators. In fact, power consumption is steadily increasing; therefore, the power system is always operating closer to its limits. Moreover, focus is put on decentralized and stochastic green energy sources, leading to a much more complex and less predictable power system. In order to guarantee security of supply under these conditions, real-time control and online security assessment are of the utmost importance. The main requirement for power system simulators in this context thus becomes the simulation time. The simulator has to be able to reproduce power system phenomena much faster than their real-time duration. An effective way to accelerate computation time of power system stability simulators is based on analog emulation of the power system grid. The idea is to avoid the heavy, time-consuming numerical matrix calculations of the grid by using an instantaneous analog Kirchhoff grid, with which computation becomes intrinsically parallel and the simulation time independent of the power system topology size. An overview of the power system computation history and the evolution of microelectronics highlights that the renaissance of dedicated analog computation is justified. Modern VLSI technologies can overcome the drawbacks which caused the disappearance of analog computation units in the 1960s. This work addresses therefore the development of a power system emulation approach from its theoretical principles to the behavioral design and the microelectronic implementation of a first demonstrator. The approach used in this research is called AC emulation approach and is based on a one-to-one mapping of components of the real power system (generator, load and transmission line) by emulating their behavior on a CMOS microelectronic integrated circuit (ASIC). The signals propagating on the emulated grid are the shrunk and downscaled current and voltage waves of the real power system. The uniqueness of this emulation approach is that frequency dependence of the signals is preserved. Therefore, the range of phenomena that can be emulated with an AC emulator depends only on the implemented models. Within the framework of this thesis, we restrict our developments to transient stability analysis, as our main focus is put on emulation speed. v We provide behavioral AC emulation models for the three main power system components. Thereby, special attention is paid to the generator model, which is shown to introduce a systematic error. This error is analyzed and reduced by model adaptation. Behavioral simulation results validate the developed models. Moreover, we suggest custom programmable analog building blocks for the implementation of the proposed behavioral models. During their design, application specific requirements, as well as imperfections, calibration, mismatch and process-variation aspects, are taken into account. In particular, the design of the tunable floating inductance used in all three AC emulation models is discussed in detail. In fact, major design challenges have to be addressed in order to achieve an inductance suitable for our application. Finally, a first AC emulation demonstrator is presented. A benchmark using a fixed two- machine topology has been implemented using a 0.35μm 3.3V CMOS technology. The characteristics of the emulated components (i.e. generators and transmission lines) are reprogrammable, allowing short circuits to be emulated at different distances from the generator. The emulated phenomena are shown to be 10′000 times faster than real time, therefore proving the high-speed capabilities of AC emulation.