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The balance between ice and water controls the habitability of an important fraction of the globe and influences the majority of the world's population. The freezing of water to form ice is one of the most common phase transformations in the natural environment. However, a complete understanding of its microscopics and their influ-ence on macroscopic phenomena still eludes us. As this millennium comes to a close, we are beginning to understand how the microscopic interfacial structure of ice con-trols pattern formation during ice-crystal growth, the evolution of the polycrystalline fabrics of the great ice sheets, the dynamics of ground freezing, ozone destruction, and the mechanism of charge transfer that drives thunderstorm electrification. This paper describes our evolving understanding, its implications for the basic principles of melting and freezing, and their environmental consequences.
Four classes of models of the lightning return stroke are
reviewed. These four classes are: (1) the gas dynamic models; (2) the
electromagnetic models; (3) the distributed-circuit models; and (4) the
“engineering” models. Validation of the reviewed models is
discussed. For the gas dynamic models, validation is based on
observations of the optical power and spectral output from natural
lightning. The electromagnetic, distributed-circuit, and
“engineering” models are most conveniently validated using
measured electric and magnetic fields from natural and triggered
lightning. Based on the entirety of the validation results and on
mathematical simplicity, we rank the “engineering” models in
the following descending order: MTLL, DU, MTLE, BG, and TL. When only
the initial peak values of the channel-base current and remote electric
or magnetic field are concerned, the TL model is preferred. Additionally
discussed are several issues in lightning return-stroke modeling that
either have been ignored to keep the modeling straightforward or have
not been recognized, such as the treatment of the upper, in-cloud
portion of the lightning channel, the boundary conditions at the ground,
including the presence of a vertically extended strike object, the
return-stroke speed at early times, the initial bi-directional extension
of the return stroke channel, and the relation between leader and return
stroke models. Various aspects of the calculation of lightning electric
and magnetic fields in which return stroke models are used to specify
the source are considered, including equations for fields and
channel-base current, as well as a discussion of channel tortuosity and
branches
A solution is presented for the electromagnetic fields radiated by an arbitrarily oriented current filament over a conducting ground plane in the case where the current propagates along the filament at the speed of light, and this solution is interpreted in terms of radiation from lightning return strokes. The solution is exact in the fullest sense; no mathematical approximations are made, and the governing differential equations and boundary conditions are satisfied. The solution has the additional attribute of being specified in closed form in terms of elementary functions. This solution is discussed from the point of view of deducing lightning current wave forms from measurements of the electromagnetic fields and understanding the effects of channel tortuosity on the radiated fields. In addition, it is compared with two approximate solutions, the traditional moment approximation and the Fraunhofer approximation, and a set of criteria describing their applicability are presented and interpreted.
A survey of nonlinear system identification algorithms and related topics is presented by extracting significant results from the literature and presenting these in an organised and systematic way. Algorithms based on the functional expansions of Weiner and Volterra, the identification of block oriented and bilinear systems, the selection of input signals, structure detection, parameter estimation and recent results from catastrophe theory are included. The limitations, relationships and applicability of the methods are discussed throughout.
Faraday observed in 1850 "that a particle of water which could retain the liquid state whilst touching ice on only one side, could not retain the liquid if it were touched by ice on both" (M. Faraday, Royal Institution Discourse, June 7, 1850; Experimental Researches in Chemistry and Physics (Taylor and Francis, New York, 1991)). Thus began the concept of interfacial melting, and the presence of a liquid water film on the surface of ice at temperatures of 0°C and below. Over the past few decades, there have been a number of measurements of interfacial melting. In some studies, the thickness of the thin film, variously called the quasi-liquid layer (QLL), liquid-like layer, surface melting layer, or premelting layer, has been determined. The results of these measurements demonstrate a striking variation depending on the experimental method and the nature of the ice samples. For example, at 0.1°C, the thickness values range over two orders of magnitude from around 1 to 100 nm. Although the disagreement can be partially explained by the differences in ice samples, the experimental techniques employed in measurements of the QLL thickness are based on different physical principals, and involve a web of assumptions for their deconvolution. We describe here the technique of infrared attenuated total reflection (ATR) spectroscopy that has been directed to the study of interfacial melting of ice for the first time.
PACS No.: 83.50Lh
Ultra-high-speed photographic techniques were used in order to observe and analyze the extremely rapid processes of corona streamer discharge inception and development on an ice surface during the first nanoseconds of occurrence. The investigations were performed on a simplified physical model under a positive lightning impulse voltage. The physical model consists of two metallic hemispherical capped rods of 12 mm diameter half submerged in ice. The distance between the electrodes was 11 mm. A number of discharge parameters such as inception field, critical volume and propagation velocity were investigated, discussed and compared with those parameters obtained in an equivalent air gap.
The objective of this investigation is to understand the inception of ice surface corona streamers. A validated onset criterion that is useful for practical electrode geometries is presented. High-speed streak photography techniques were used to observe the streamers propagating over an ice surface. The effects of several experimental parameters such as freezing water conductivity and HV electrode radius on the streamer inception parameters of an ice surface have been examined. Empirical models have been proposed, based on elements derived from the inception parameters to account for corona streamer propagation velocity and inception voltage/field on an ice surface. Since the propagation velocity is a power function of the electric field, its value has been expressed in terms of the inception voltage.
The shapes and density of ice accretions on an H.V. conductor is closely related to the electrical corona activities on ice asperities. We will present some effects of positive and negative corona discharges on a tip of conical ice as a function of corona current, temperature and atmospheric conditions. Firstly, the loss in the volume of the icicle and corona wind velocity were measured. Secondly, the influence of corona activities on the ice accretion was observed. The initial charge of water droplets impinging on the tip was also examined. It will be shown that the presence of corona activities does not promote ice accumulation and that the ice structure is independent of corona discharge.
Ageing of insulation materials and systems may well be detected by
quantitative measurements of their dielectric response. Such
measurements can be performed in the frequency or time domain. In this
contribution, a novel type of equipment based on relaxation current
measurements is introduced which is designed for on-site tests of high
voltage power apparatus. The introduction outlines the reasons for the
development. Then, the technique of the instrument is briefly explained.
Finally, some examples of on-site measurements on power transformers are
presented and post-processing and interpretation of the results is
demonstrated
Due to their approximation capabilities and inherent adaptivity
features, neural networks have been employed in modelling of complex
nonlinear systems. This paper presents an approach to effectively
identify a synchronous generator when only the inputs and outputs are
accessible for measurement. Such an identifier (or model) can be used in
an indirect adaptive control or internal model control schemes. A
one-hidden-layer feedforward neural network is proposed to identify the
synchronous generator. Training is done in an on-line mode to allow the
identifier to track the variable-parameter nonlinear plant, giving an
adaptive attribute to the identifier. Simulation results are presented
to complement the theoretical discussion
The results of dielectric response measurements in the time and
frequency domain on homogeneous impregnated pressboard and on series
arrangements of pressboard sheets and oil ducts are presented. A simple
equivalent circuit is proposed to model oil-paper insulation systems for
single impregnated pressboard sheets as well as for multi-layer
arrangements. This equivalent circuit which is derived from relaxation
current measurements is then used to calculate recovery voltages as well
as the frequency-dependent permittivity. Finally it is shown that the
same procedure can be applied to quantify the dielectric response
properties on high voltage power transformers
Equations to calculate the inducing scalar and vector potentials
produced by inclined return strokes are presented. Equations are also
given for calculating the induced voltages of overhead lines where
horizontal components of inducing vector potential exist. The adequacy
of the calculation method is demonstrated by field experiments. Using
these equations, induced voltages on overhead lines are calculated for a
variety of directions of return stroke
This paper presents a novel alternative to estimate armature
circuit parameters of large utility generators using real time operating
data. The proposed approach uses the Hartley series for fitting
operating data (voltage and currents measurements). The essence of the
method is the use of linear state estimation to identify the
coefficients of the Hartley series. The approach is tested for noise
corruption likely to be found in measurements. The method is found to be
suitable for the processing of digital fault recorder data to identify
synchronous machine parameters
Experiments on the lightning-induced voltage waveforms on an
overhead wire influenced by an inclined return-stroke channel are
carried out outdoors with a reduced-scale model. The measured voltages
are compared with those calculated by solving the Telegrapher's equation
in combination with numerical calculation of electric fields associated
with return-stroke currents, and the validity of the calculations is
verified. The effect of the ground conductivity on the lightning-induced
voltage waveform is studied based on numerical calculations and is found
to be dependent on the position of the return-stroke channel relative to
the overhead wire. When a return stroke hits the ground close to the end
of the overhead wire, the influence of the ground conductivity on the
induced voltage waveform is significant irrespective of the inclination
or the direction of the lightning channel. The degree of the influence
is dependent on the inclination and the direction of the channel when a
return stroke hits the ground on the side of the overhead wire
Compactly supported orthogonal wavelets have certain properties
that are useful for system identification and learning control. Drawing
on the rich theory of wavelets, we propose a system identification
scheme based on orthogonal wavelets. Better accuracy of estimation can
be obtained by adding more terms to the wavelet based identifier, and
these terms do not alter the coefficients of the existing terms. These
terms can be selectively added depending upon the region of interest,
for example we may require more terms in regions where the identified
functions vary rapidly. We illustrate the concepts by applying the
procedure to determine a speed-controller for an electric vehicle
„Online TE-Messung an Transformatoren: Möglichkeiten und Grenzen
- E Grossmann
Grossmann, E.: „Online TE-Messung an Transformatoren: Möglichkeiten und Grenzen.“ Micafil Symposium, March 20th-21st 2002, Stuttgart, Germany
„Betriebswirtschaftliche Gesichts-punkte im Asset-Management im Zeitalter der Fusion
- M Stach
Stach, M.: „Betriebswirtschaftliche Gesichts-punkte im Asset-Management im Zeitalter der Fusion.“ Micafil Symposium, March 20th-21st 2002, Stuttgart, Germany Depolarisation information Current) about the
„Die Aplikation und Integration eines Monitoringsystems für Stufenschalter in moderne Überwachungskonzepte
- C Viereck
Viereck, C.: „Die Aplikation und Integration eines Monitoringsystems für Stufenschalter in moderne Überwachungskonzepte.“ Hafely Symposium, 1998