1,720,988 research outputs found
Impact of DC Transient Disturbances on Harmonic Performance of Voltage Transformers for AC Railway Applications
Full text linkThis paper analyzes the impact of typical DC transient events occurring in railway grids on the frequency performance of instrument transformers (ITs) installed onboard trains and in AC substations for power quality (PQ) applications. PQ monitoring in railway systems is an issue of great interest because it plays a key role in the improvement of energy efficiency. The measurement chain for the PQ measurements, at 15 kV at 16.7 Hz and 25 kV at 50/60 Hz, commonly includes ITs to scale the voltage to levels fitting the input of the measurement units. Nevertheless, the behavior of ITs in the presence of PQ phenomena represents an open issue from a normative point of view, even for those installed in conventional AC power supply systems. In this context, the paper presents a possible definition of DC transient disturbances test waveforms, a measurement procedure, and a setup to assess the impact of these disturbances on the harmonic performances of ITs for railway systems. Preliminary experimental tests carried out on two commercial ITs under wide ranges of variation for the amplitude and the time duration of DC disturbances show that, in some cases, the error introduced in harmonic measurements can exceed 100%
Energy Efficiency Improvement with Reversible Substations for Electrified Transportation Systems
No full textMore and more often overall energy efficiency of an electrified transportation system appears as a target of new constructions, sometimes with incomplete definitions of performance indexes to assess that target has been reached. A worked out example is based on the introduction of
reversible substations. Reversible substations are a method to improve energy efficiency, whose application in an existing system can be progressive and does not require large traffic intensity to be effective. Their effectiveness depends on some system parameters (nominal catenary voltage in particular). A simulation model fed with experimental data from a line section of Metro de Madrid is used to demonstrate the operation and optimization of reversible substations
Measurement Repeatability of a Supercapacitor Equivalent Circuit Parameters
No full textThe determination of the equivalent parameters of a supercapacitor requires the galvanostatic measurement of a charge and self-discharge cycle. If a more general determination is needed than that on the single cycle, it is necessary to take into account the repeatability of the charging and self-discharging cycles. A cycle for determining the parameters can also extend over a few hours for larger supercapacitors, also considering the time needed to identify the parameters. Therefore, having a large set of dozens of cycles including the parameters determination is definitely a time-consuming procedure. In this work, an efficient method for the repeatability assessment of the equivalent circuit parameters is proposed; this approach relies on a limited set of experimental curves and on a single parameter identification process
A New Approach to Measure the Energy On-Board Train during Braking
Full text linkMeasuring energy wasted during braking by trains is a nontrivial task because it is performed by power semiconductor devices that switch high current levels at a few hundred hertz. The chopped signals produced are difficult to analyze, not only for the high switching frequency but also for the high slew rate that produces even higher harmonic content. In this harsh scenario, it is challenging to guarantee an adequate level of accuracy and it could be very expensive for railway operators, who often completely renounce measuring these quantities of interest, estimating it from mechanical quantities. In a previous article, we have already faced this issue by developing an accurate technique usable offline. Here, two simplified methods with suitable accuracy levels for train energy management and easy to be implemented on train control unit are proposed. This article describes the phenomena of interest and related measurement issues. A numerical solution together with a deep analysis on the input quantities needed is presented. The methods are applied to experimental data from a measurement campaign, conducted on-board train in the northern part of Italy. Furthermore, the metrological performances of the methods are assessed by adopting as a reference the offline technique
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Setup for the Calibration of Current Measuring Systems under DC Signals Affected by Ripple
No full textIn recent years, the adoption of DC technology in power systems is attracting renewed interest from the technical-scientific side not only in traditional high voltage applications (HVDC) which allow more efficient transmission of large quantities of energy over large distances but also in low voltage DC networks (LVDC), which allow for more efficient and sustainable integration of renewable energy sources and battery storage systems. An obstacle to its widespread diffusion is the lack of traceable reference systems able to evaluate the performance of energy meters for billing purposes in operating conditions characterized by DC with AC components (up to hundreds of kHz). The present work proposes a methodology to calibrate current measuring systems experiencing DC with AC ripple providing the metrological traceability for this combined signal. The developed and tested archetype, here presented, proves the feasibility of the methodology
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Filter transients onboard DC rolling stock and exploitation for the estimate of the line impedance
Full text linkPower supply transients in dc railways related to filter charging may trigger network and filter oscillatory responses, as well as cause very fast voltage spikes. These phenomena are relevant not only for Power Quality and EMC, but also for their impact on the measured pantograph quantities e.g. for power and energy consumption estimate. The broadband excitation of the system gives the possibility of attempting the identification of the network impedance. The experimental results are discussed and compared to the output of a circuit and a distributed parameter simulator. Matching between simulated and experimental data is very good
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