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Time domain reflectometry (TDR) has been used for localization of discontinuities in transmission lines in diverse applications. In this paper, its potential as an alternative to frequency response ...

Pantograph arcing in ac traction system generates transients, and causes asymmetries and distortion in supply voltage and current waveforms. These asymmetric voltage and current lead to a net dc component and harmonics that propagate within the traction power and signalling system and cause electromagnetic interference. This problem is enhanced during winter because of the layer of ice/snow on the overhead contact wire. The sliding contact becomes poor and a visible arc moves along with the pantograph. In this paper, it is shown how different parameters like traction current, line speed, power factor, and supply voltage influence the arcing, its characteristics, and the dc components. It is shown that the dc current component increases with increasing train speed and traction current, and reduces at a lower power factor. It is also discussed how the presence of an ice layer influences the arcing and the dc components. It is found that running the trains below the normal operating power factors is an effective choice to mitigate this problem. The findings presented in this paper could be beneficial to estimate the probable limit of the dc component at the planning stage so that proper precautions can be taken at the design stage itself.

Pantograph arcing is a common phenomenon in electrified railway systems. This is also a source of broadband-conducted and radiated electromagnetic interference (EMI) for vehicle as well as traction power and signaling systems. In this paper and the companion paper, experimental analyses of pantograph arcing phenomena are presented for dc and ac feeding systems, respectively. Influences of different parameters on dc traction system, such as supply-voltage polarity, relative motion between pantograph and overhead contact wire, namely, forward motion along the track (longitudinal), and lateral sliding motion of the pantograph (zigzag) are presented here. From the voltage and current waveforms of the test runs, it is shown that pantograph arcing is a polarity-dependent phenomenon. For the positive-fed overhead traction system, where pantograph is cathode, the supply interruptions due to zigzag motion are comparatively less compared to negatively fed traction systems. As a result, the transients, due to pantograph arcing, are more frequent in negatively fed traction systems. It is found that the arc root movement along the electrode surfaces (pantograph-contact wire) is governed by the relative motion and polarity of the electrodes. The analyses presented in this paper also form a foundation to understand the pantograph arcing process and the corresponding influential parameters with the ac supply presented in the companion paper. The findings presented in this paper could be beneficial for coming up with appropriate mitigation techniques from the EMI due to pantograph arcing in dc-fed traction systems.

In this article, we have seen how high-frequency system resonances, triggered by power electronic devices, may enhance both electrical and thermal stress on an insulation system. An important lesson from this study is that even an insulation system that is very well designed according to present standards may fail if it is subjected to stresses that were never considered in the design. The root cause of such failures is thus not within the failed part, and it can only be found by studying the entire system where the failed part is used. With the examples given here, we hope that the reader may more easily recognize situations which may warrant a deeper study and perhaps some extra precautions.

The dynamics of a globally neutral system of diffusing Coulomb charges in two dimensions, driven by an applied electric field, is studied in a wide temperature range around the Berezinskii-Kosterlitz-Thouless transition. I argue that the commonly accepted ``free particle drift'' mechanism of charge transport in this system is limited to relatively low particle densities. For higher densities, I propose a modified picture involving collective ``partner transfer'' between bound pairs. The new picture provides a natural explanation for recent experimental and numerical findings which deviate from standard theory. It also clarifies the origin of dynamical scaling in this context., 4 pages, RevTeX, 2 eps figures included; some typos corrected, final version to be published in Phys. Rev. Lett

Vacuum circuit breakers can cause repetitive transient overvoltages with steep fronts in some cases, depending on the combination of the system configurations and type of circuit breaker operations. Laboratory testing has been performed to improve the understanding of the high frequency system interaction between vacuum circuit breaker, transformer, and cables. A dry type medium voltage transformer has been energized and disconnected in a 20 kV medium voltage laboratory cable system using a vacuum circuit breaker. The transformer is a standard transformer, equipped with additional voltage measuring taps along the disks of one of the high-voltage windings, enabling the measurement of the voltages in the winding when exposed to transient overvoltages. The results show that normal no-load energizing and disconnection of the transformer do not cause any high overvoltages. Disconnection of the transformer with an inductive load or a trip on inrush current during energizing can cause transient overvoltages. Surge arresters, both phases to ground and phase to phase, are recommended for a transformer exposed to these types of circuit breaker operations. (4 pages)

We study the two-dimensional Ginzburg-Landau model of a neutral superfluid in the vicinity of the vortex unbinding transition. The model is mapped onto an effective interacting vortex gas by a systematic perturbative elimination of all fluctuating degrees of freedom (amplitude {\em and} phase of the order parameter field) except the vortex positions. In the Coulomb gas descriptions derived previously in the literature, thermal amplitude fluctuations were neglected altogether. We argue that, if one includes the latter, the vortices still form a two- dimensional Coulomb gas, but the vortex fugacity can be substantially raised. Under the assumption that Minnhagen's generic phase diagram of the two- dimensional Coulomb gas is correct, our results then point to a first order transition rather than a Kosterlitz-Thouless transition, provided the Ginzburg-Landau correlation length is large enough in units of a microscopic cutoff length for fluctuations. The experimental relevance of these results is briefly discussed. [Submitted to J. Stat. Phys.], 36 pages, LaTeX, 6 figures upon request, UATP2-DB1-93

We investigate superconducting pair correlations in the attractive Hubbard model on a finite square lattice. Our aim is to understand the pronounced size dependence which they display in the weak and intermediate coupling regimes. These size effects originate from the electronic shell structure of finite systems and severely complicate a reliable extrapolation of numerical simulation data from small systems to the thermodynamic limit. To analyze the size effects in detail, we use the BCS approximation, as well as a particle number conserving modification of it and compare the results with those of quantum Monte Carlo simulations. As an application, we explore the possibility of reducing the shell effects in simulation data by changing the shape of the system and the imposed boundary conditions and by making use of the size dependence of corresponding BCS data.

For the modeling of infrared spectra it is a common approach to use a dielectric function that treats the vibrational modes as damped harmonic oscillators. This model was found to be rather crude for some applications to amorphous solids. A dielectric function model yielding a Gaussian shape of the absorption lines and satisfying Kramers–Kronig relations is suggested. The model function is constructed by a convolution of a Gaussian function with the dielectric function of the damped harmonic oscillator model. An analytical solution of this integral is given. It is demonstrated that this model describes the spectra of thermally grown ultrathin (1.3 nm) silicon oxide films, plasma‐deposited silicon films, plasma‐deposited silicon nitride films, and amorphous aluminum oxide films very well. The physical motivation of the dielectric function model suggested is the randomness of the vibrational frequencies in an amorphous structure.

One of the most common and yet unavoidable problems in electrified railways are because of pantograph arcing. The intensity of arcing increases during winter because of the ice layer on the overhead contact wire. In AC traction system, pantograph arcing distorts the sinusoidal waveforms of the supply voltage and current. It also generates transients at the current zero crossings (CZC). All these result in a net DC component, harmonics and radiated emission. In this paper we will present an experimental investigation to understand pantograph arcing and influence of power factor on conducted and radiated emission. We found that pantograph arcing generates both conducted and radiated emission in a wide band, starting from DC. Both the DC component and higher order conducted and radiated emission increases with line speed. Running the train at lower power factor can reduce the DC component and emerging as the most preferred solution against problems due to ice in cold weather.

PurposeFor efficient magnetic field calculations in electrical machines, the hysteresis and losses in laminated electrical steel must be modeled in a simple and reliable way. The purpose of this paper is to investigate and discuss the potential of a simple complex‐permeability model.Design/methodology/approachA frequency dependent complex‐permeability model as well as a more detailed model (describing hysteresis, classical eddy current effects, and excess losses separately) are compared to single‐sheet measurements on laminated electrical steel. It is discussed under which circumstances the simple complex‐μ model is an adequate substitute for the more detailed model.FindingsA satisfactory agreement of the simple complex‐μ model was found with both detailed model and measurements, improving with increasing frequencies. This is true not only for the effective permeability function, but holds also for the detailed H‐B characteristics (hysteresis).Originality/valueIt is demonstrated that the complex‐μ model is a reliable and convenient starting point for the estimation of flux distribution and losses in complicated magnetic core geometries.

Using the BCS approach and quantum Monte Carlo simulations, we investigate the size dependence of the pairing correlations in the weak and intermediate coupling regimes. Pronounced size effects appear as long as the average spacing of the single-particle energy levels is larger than or comparable to the characteristic energy scale (gap). The irregularity of the size dependence is traced to variations in the occupation of the highest nonempty energy level with system size. We also propose means to reduce the size dependence and its irregularity.

The most common and yet unavoidable EMC problems with electrified railways are due to pantograph arcing. This distorts the waveform of the supply voltage and current, can generates transients during the zero crossings of the current and can cause interference with the traction power and signalling system. Pantograph arcing is a complex phenomenon and depends on speed of the train, current, presence of inductance etc. In a sliding contact like pantograph and contact wire, the arc root moves across both electrodes because of the relative motion between them. In this paper, we will present an experimental analyses of the arc root movement and influence of different parameters on it using a laboratory setup.

The dynamical behavior of nonlinear, dilute scalar product models is investigated using generating functional techniques. In the first part of the paper (Sects. 2–4), bond averages are performed by a cumulant expansion of the interaction term. The coefficients of this expansion are calculated by diagrammatic means. Thereby special attention is paid to the role of correlations between bonds and dynamical initial conditions. The resulting averaged problem only depends on a finite number of real parameters; they thus label equivalence classes of models which behave identically at all times (including transients) on the dynamical average. In particular, the previous suggestion of Sompolinsky that nonlinearity and dilution of the bonds act just like additional stochastic bond contributions if the stored patterns are ±1-variables is confirmed for the exact dynamics at all times. However, if the ±-symmetry of the pattern distribution is removed, additional memory effects with respect to the system activity are found. In the second part (Sects. 5–6), the dynamical mean field theory of the averaged problem is derived as usual by functional saddle point integration. The resulting self-consistent singlenode problem is discussed qualitatively; as a new feature, it contains quenched disorder, introduced by the abovementioned memory effects. Finally, some remarks on the solution of particular cases are made.

Pantograph arcing generates a net DC voltage which create interference with the track circuit signaling, saturate transformer core, generate even harmonics, create problems on control of the switching of power electronics and drives. Experiments were conducted by Bombardier Transportation and ABB as a part of the ICE project to understand the physical processes that generates this DC component, the coupling mechanisms and the distribution paths, the identification of DC vulnerable components and their response to this DC. The amplitude of the DC voltage shows a wide variation with respect to applied voltage, train speed, type of electrical load, gap between the contact wire and the pantograph and current. In this paper, the authors analyzed those experimental data to understand the typical nature of sliding arc between the pantograph and the contact wire and possible mechanisms involved with this.

We study the dynamics of a classical, two-component plasma in two dimensions, in the vicinity of the Kosterlitz-Thouless (KT) transition where the system passes from a dielectric low-temperature phase (consisting of bound pairs) to a conducting phase. We use two ``complementary'' analytical approaches and compare to simulations. The conventional, ``intuitive'' approach is built on the KT picture of independently relaxing, bound pairs. A more formal approach, working with Mori projected dynamic correlation functions, avoids to assume the pair picture from the start. We discuss successes and failures of both approaches, and suggest a way to combine the advantages of both., 4 pages, LaTeX, 4 eps figures included; to appear in the proceedings of the ``1999 International Conference on Strongly Coupled Coulomb Systems'', 4-11 Sept 1999 in Saint-Malo (France); some typos corrected

Approximate dynamic mean field equations for a generalized Hopfield model are derived, which allow to calculate transient properties of this model. These equations are exact for short times and yield the replica symmetric solution as a stationary solution. They allow reliable computation of retrieval trajectories and basins of attraction of retrieval states, as demonstrated by comparison with simulations. The equations are derived for networks with arbitrary mean activity and results are given for the standard model and for low activities.