RMSF Research Articles

${\rm CO}_{2}$ as an Arc Interruption Medium in Gas Circuit Breakers

<formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\rm CO}_{2}$</tex></formula> is a possible alternative to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm SF}_{6}$</tex> </formula> —which has a high global warming potential—as the interruption medium in gas circuit breakers. The performance of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm CO}_{2}$</tex></formula> is investigated in this paper by carrying out experiments in representative test devices and by performing computational fluid dynamic (CFD) simulations; some comparisons with <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm SF}_{6}$</tex></formula> and air are given. It is found that the thermal interruption performance of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm CO}_{2}$</tex></formula> is lower than that of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm SF}_{6}$</tex></formula> , but higher than that of air. The measured dielectric recovery after arcing is compared for <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm CO}_{2}$</tex> </formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm SF}_{6}$</tex></formula> ; a streamer model is used to calculate the breakdown voltage in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm CO}_{2}$</tex></formula> ; good agreement with measurement is found. The speed of sound and the adiabatic coefficient, important parameters that influence pressure buildup and gas flow, are compared for <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm SF}_{6}$</tex></formula> , <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm CO}_{2}$</tex></formula> , and air. CFD simulations of the pressure buildup in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm CO}_{2}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm SF}_{6}$</tex></formula> both illustrate the qualitative differences between the two and show good agreement with measurements.

Investigation of ${\rm SF}_{6}$ Arc Characteristics Under Shock Condition in a Supersonic Nozzle With Hollow Contact

Arc-shock interaction and its influence on steady and dynamic characteristics of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> arcs in a supersonic nozzle with hollow contact, which is representative of switching arcs in a gas blast high-voltage circuit breaker, are computationally investigated using the magneto-hydro-dynamic theory with fixed inlet stagnation pressures and at five different exit pressure to inlet stagnation pressure ratios (referred to as pressure ratios). The significant quantity in determining the thermal interruption capability of a circuit breaker, i.e., the critical rate of rise of recovery voltage, was predicted and compared with available test results. The deterioration of thermal interruption capability of a supersonic nozzle under shock conditions, which was observed in previous tests is verified and discussed by current arc model. It was found that the close coupling between the shock and the surrounding gas flow can greatly influence the aerodynamic and electrical behavior of a nozzle arc and hence the thermal recovery behavior of current interruption. Additionally, the velocity deceleration caused by the shocks and the enhanced turbulent cooling brought by the sucked gas and arc interaction both pay a significant role in the determination of thermal interruption capability.

HPM Formation in Air and ${\rm SF}_{6}$/Argon Mixture

Using fast-rising voltage pulses, corona discharges are created on the helix. Investigations are carried out in atmospheric air and SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> /argon mixture. The corona discharges produce electron beams on the helix. High-power microwaves are obtained in the structure that has two microwave sources. The helix as the slow-wave structure with in situ generated electron beams is one source of microwave generation. The experimental data are presented. The analysis of the data related to the helix is based on the backward-wave oscillator.

Application of Computational Fluid Dynamics to Reduce the New Product Development Cycle Time of the ${\rm SF}_{6}$ Gas Circuit Breaker

This paper describes thermal modeling, using computational fluid dynamics (CFD), to predict the temperature rise of a high-voltage SF6 gas circuit breaker during a heat-run test. In electrical equipment, maximum temperature rise in various components is limited by their material properties and, hence, the heat-run test forms part of type test. A methodology to predict the temperature rise at the design stage is developed using ANSYS-CFX and found to be beneficial to reduce the number of heat-run tests during the product development process. The developed methodology has been implemented on a 72.5-kV SF6 gas circuit breaker and validated with the experimental temperature rise results of the existing design at two current ratings: 1000 A and 2000 A. The methodology was further used to predict thermal performance of the breaker at a higher current rating of 3150 A. The prediction helped to understand temperature rise in various components and hot-spot locations. Various design modifications were then tested virtually to evaluate the temperature rise. Thereafter, the final modified design where the temperature rise of all the components was within the permissible limits and which also satisfied other constraints of manufacturing feasibility and cost was fabricated and physically tested. Good agreement is found between the CFD and experimental results.

Breakdown Characteristics of Liquefied ${\rm SF}_{6}$ and ${\rm CF}_{4}$ Gases in Liquid Nitrogen for High Voltage Bushings in a Cryogenic Environment

High voltage cryogenic insulation issues need to be addressed in order to promote the commercialization of high temperature superconducting (HTS) equipment. One of the critical components for superconducting devices is the bushing whose role is to safely supply high current to the device. Due to a steep temperature gradient, commercial bushings which have been insulated with SF6 gas could not be directly applied to cryogenic equipment due to liquefaction of SF6 in the cryogenic environment; therefore, alternative suitable structure and insulation methods should be developed. As a fundamental step in the development of the optimum bushings for HTS devices, the breakdown characteristics of liquid nitrogen mixed with liquefied insulating gases such as N2, SF6 and CF4 have been investigated. In particular, we noted the insulation characteristics of CF4 gas whose liquefication temperature is much higher than SF6 gas. Thus, in order to investigate the possibility of substituting CF4 gas for SF6 gas for the bushings of HTS electrical equipment, impulse tests, AC withstanding voltage tests, and partial discharge (PD) tests have been performed. As a result of these tests, it was shown that mixtures of liquefied insulating gases have a much higher breakdown voltage compared to pure liquid nitrogen. Especially in a cryogenic environment, the usage of SF6 gas should be evaluated due to freezing effects. On the other hand, CF4 gas has shown excellent insulation properties even in a cryogenic environment and could be utilized as an insulation gas for high voltage bushings of HTS electrical equipment.

Erratum: “Highly accurate determination of the electron affinity of SF6 and analysis of structure and photodetachment spectrum of ${\rm SF}_6^-$ SF 6−” [J. Chem. Phys. 134, 054303 (2011)

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Highly accurate determination of the electron affinity of ${\rm SF}_{6}$ SF 6 and analysis of structure and photodetachment spectrum of ${\rm SF}^{-}_{6}$ SF 6−

The title system is thoroughly investigated by high-level electronic structure techniques and nuclear quantum dynamics calculations. Equilibrium geometries and harmonic frequencies are determined by coupled-cluster singles doubles [CCSD(T)] calculations with large AO basis sets. A C(4v) distorted geometry is found for the anion in contrast to previous assumptions. This is explained by the bonding situation in the electronic ground state and possible vibronic interactions with higher electronic states. The computed adiabatic electron affinity of 0.73 eV is considerably lower than the currently recommended value. Analysis of the electronic states of the anion shows that the σ* ground state at equilibrium position corresponds to a highly excited state at the neutral's geometry where the ground state is either a very weakly bound or scattering state. If the electron is captured by this latter state, a nonadiabatic transition to the σ* state followed by internal vibrational redistribution could explain the formation of a stable anion. The C(4v) distortion of the equilibrium geometry is essential for the explanation of recently measured photodetachment spectra. Since the distortion leads to six equivalent minima with very low barriers, an anharmonic potential energy surface (PES) of the four relevant vibrational modes is constructed and fitted to CCSD(T) computed energies. The remaining 11 modes are treated as harmonic oscillators. The vibrational dynamics of the anion is studied by diagonalization of the Hamiltonian in the basis of the neutral's eigenstates. The computed photoelectron spectra are in good agreement with recent experiments and demonstrate the quality of the PES and that C(4v) distortion is responsible for the observed irregularities. However, thermal effects play a significant role for the shape of the spectra because many low-lying initial states are populated.

New joint frame synchronisation and carrier frequency offset estimation method for OFDM systems

AbstractWe propose a new joint frame synchronisation and carrier frequency offset estimation scheme for burst transmission mode OFDM systems. This scheme uses a central‐symmetric and comb‐like (CSCL) training sequence, which eases the power detection at the receiver without increasing the total training sequence power. Fine frame synchronisation as well as carrier frequency offset acquisition with a maximum acquisition range of $\pm {{N} \over {4\times {\rm SF}}}$ times the sub‐carrier spacing can also be performed based on the proposed CSCL training sequence, where N is the discrete Fourier transform (DFT) length and SF is an integer‐valued spreading factor used to generate CSCL. The post‐acquisition residual carrier frequency offset can be further estimated and corrected via a fine adjustment algorithm. In order to reduce performance loss due to the high peak‐to‐average power ratio (PAPR) of the CSCL training sequence, a time‐domain constant‐envelope (CE) training sequence is also proposed. The superior estimation accuracy of the proposed algorithm over that of the Moose algorithm and the SS (Shi and Serpedin) algorithm is proved by computer simulation. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Heat Conduction Problems in ${\rm SF}_{6}$ Gas Cooled-Insulated Power Transformers Solved by the Finite-Element Method

In this paper, the finite-element method is used to perform heat transfer analysis to obtain the steady state and the transient temperature distribution of gas cooled-insulated power transformers. All significant parameters that influence transformer operation have been included. Also, the analysis results, which are obtained from the thermal analysis, could be reviewed at any specified location within the transformer as a function of time. The predicted temperature values are compared with experimental results reported in the literature.

Transport between multiple users in complex networks

We study the transport properties of model networks such as scale-free and Erd\H{o}s-R\'{e}nyi networks as well as a real network. We consider the conductance $G$ between two arbitrarily chosen nodes where each link has the same unit resistance. Our theoretical analysis for scale-free networks predicts a broad range of values of $G$, with a power-law tail distribution $\Phi_{\rm SF}(G)\sim G^{-g_G}$, where $g_G=2\lambda -1$, and $\lambda$ is the decay exponent for the scale-free network degree distribution. We confirm our predictions by large scale simulations. The power-law tail in $\Phi_{\rm SF}(G)$ leads to large values of $G$, thereby significantly improving the transport in scale-free networks, compared to Erd\H{o}s-R\'{e}nyi networks where the tail of the conductivity distribution decays exponentially. We develop a simple physical picture of the transport to account for the results. We study another model for transport, the \emph{max-flow} model, where conductance is defined as the number of link-independent paths between the two nodes, and find that a similar picture holds. The effects of distance on the value of conductance are considered for both models, and some differences emerge. We then extend our study to the case of multiple sources, where the transport is define between two \emph{groups} of nodes. We find a fundamental difference between the two forms of flow when considering the quality of the transport with respect to the number of sources, and find an optimal number of sources, or users, for the max-flow case. A qualitative (and partially quantitative) explanation is also given.

Star Formation Rate from Dust Infrared Emission

We examine what types of galaxies the conversion formula from dust infrared (IR) luminosity into the star formation rate (SFR) derived by Kennicutt (1998) is applicable to. The ratio of the observed IR luminosity, $L_{\rm IR}$, to the intrinsic bolometric luminosity of the newly ($\la$ 10 Myr) formed stars, $L_{\rm SF}$, of a galaxy can be determined by a mean dust opacity in the interstellar medium and the activity of the current star formation. We find that these parameters area being $0.5 \le L_{\rm IR}/L_{\rm SF} \le 2.0$ is very large, and many nearby normal and active star-forming galaxies really fall in this area. It results from offsetting two effects of a small dust opacity and a large cirrus contribution of normal galaxies relative to starburst galaxies on the conversion of the stellar emission into the dust IR emission. In conclusion, the SFR determined from the IR luminosity under the assumption of $L_{\rm IR}=L_{\rm SF}$ like Kennicutt (1998) is reliable within a factor of 2 for all galaxies except for dust rich but quiescent galaxies and extremely dust poor galaxies.

Pressure Studies of the Unconventional Superconductors CeTIn 5 (T: Co, Ir)

In the quest for new superconductor compounds which adopt the superconducting state at increasingly higher transition temperatures T c , a non-phonon mediated coupling between the charge carriers seems to play a key role. In order to enhance our understanding of such unconventional coupling mechanisms, we studied a new family of heavy fermion (HF) superconductors CeTIn 5 (T: transition metal) whose properties point toward the realization of unconventional superconductivity (SC): the specific heat, thermal conductivity and nuclear spin-lattice relaxation rate of CeIrIn 5 and CeCoIn 5 decrease as a power law of temperature instead of exponentially for T < T c . We report on measurements of the heat capacity of CeIrIn 5 and CeCoIn 5 at hydrostatic pressures p h 1.6 GPa. In both compounds, T c increases with increasing pressure, while the mass of the quasi-particles m eff decreases, as indicated by the ratio C / T | T c . As a working hypothesis based on theories of a nearly antiferromagnetic Fermi-liquid (NAFFL), this may be interpreted as the stabilization of the superconducting state by an increase of the characteristic spin fluctuation temperature T_{\\rm SF}\\ (T_{\\rm SF}\\propto k_{\\rm F}^{2}/m_{\\rm eff}).

Non-fermi-liquid behavior in a disordered Kondo-alloy model

We study a mean-field model of a Kondo alloy using numerical techniques and analytic approximations. In this model, randomly distributed magnetic impurities interact with a band of conduction electrons and have a residual RKKY coupling of strength $J$. This system has a quantum critical point at $J=J_{c} \sim T_{K}^0$, the Kondo scale of the problem. The $T$ dependence of the spin susceptibility near the quantum critical point is singular with $\chi(0)-\chi(T) \propto T^{\gamma}$ and non-integer $\gamma$. At $J_{c}$, $\gamma = 3/4$. For $J\lesssim J_{c}$ there are two crossovers with decreasing $T$, first to $\gamma=3/2$ and then to $\gamma=2$, the Fermi-liquid value. The dissipative part of the time-dependent susceptibility $\chi''(\omega)\propto \omega$ as $\omega \to 0$ except at the quantum critical point where we find $\chi''(\omega) \propto \sqrt{\omega}$. The characteristic spin-fluctuation energy vanishes at the quantum critical point with $\omega_{\rm sf} \sim (1-J/J_{c})$ for $J\lesssim J_{c}$, and $\omega_{\rm sf} \propto T^{3/2}$ at the critical coupling.

Kinetics and thermochemistry of electron attachment to SF6

AbstractThermochemical analysis of the electron capture process of SF6 leads to a rate constant for the reverse process \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm SF}_6^ - \mathop \to \limits^2 {\rm SF}_6 + e^ -,k_2 = 1.5 \times 10^{13 - 31.4/\theta } {\rm s}^{{\rm - 1}} $\end{document}, where θ = 2.303RT, in kcal/mol. The electron affinity of 32±3 kcal/mol is deduced from the observed bimolecularity of the capture process down to 0.1 torr Ar bath gas and estimated entropies of SF6 and SF. The capture process is discussed from the view point of the formation of a metastable SF electron (SF6·e) Langevin complex which appears to have a lifetime of about 2 × 10−13 s. Curve crossing from the SF6·e complex to vibrationally excited (SF)* appears to have a normal rate and A factor. This is interpreted to indicate near‐resonant coupling between the orbiting electron and the vibronic motions of SF6, together with similarity in structure of SF6 and SF. It is shown that the apparent slowness of thermal electron ejection from SF is a result of an unfavorable equilibrium constant rather than a slow rate.

Modification of Recovery from Potentially Lethal X-Ray Damage in Plateau Phase Chinese Hamster Cells

The survival of density inhibited Chinese hamster cells assayed immediately after 1500 rads ( ${\rm SF}_{0}$) was compared to that obtained from cells held for 6 hr at 37°C in their high density condition ( SF6). The ratio of SF6 to ${\rm SF}_{0}$, designated the surviving fraction ratio (SFR) was determined for various post irradiation conditions. If the density inhibited cells were overlaid with full medium following irradiation, the SFR was approximately 4 while in Hanks' BSS the SFR was 15-20. The recovery from X-ray damage was inhibited by lowered temperature: the SFRs were 10 and 2 at 25° and 4°C, respectively, compared to 16 at 37°C. This inhibition was almost completely reversible following transfer of the cells from the lower temperatures to 37°C and further incubation. The SFR under full medium (postirradiation) conditions was not significantly modified by the presence of metabolic inhibitors at concentrations nontoxic to at least 70% of the cell population. When the irradiated cells were kept i...