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Modified Hierarchical Clustering Algorithm for Partial Discharge Separation
Full text linkTo date, one of the main tools for evaluating the
reliability of an insulation system is the continuous monitoring
of those phenomena which, by interacting with the elements of
the system, can induce aging processes or failures. For power
grids, a signal that identifies possible aging or improper use
of the component is Partial Discharge (PD) activity. Generally,
the evaluation of the PD phenomenon is carried out through
a two-step procedure: measurement and data analysis. To
optimize the PD analysis process, increasingly sophisticated PD
separation/classification algorithms are needed. Especially for
the measurements carried out in HVDC systems for which the
absence of a phase reference makes more difficult to identify
the different types of discharge. The purpose of this article
is to investigate the possibility of optimizing the input data
to a hierarchical clustering algorithm in order to obtain a
subdivision of the dataset more faithful to the real behavior
of the phenomena. Specifically, the proposed approach is based
on the use of the cross-correlation matrix to carry out the
clustering operation. This matrix replaces the matrix of the
distances among the points distributed in the map used for
the representation of the data. Results show that with this
modification it is possible to separate phenomena that present
partially or completely overlapping patterns. Moreover, the
algorithm turns out to be automatic and does not require the
choice of references or thresholds to define the similarity among
pulses
Partial Discharge Separation by Using Pulses Cross-Correlation
No full textTo date, continuous monitoring of power grid
elements susceptible to aging processes is essential to increase
the reliability of these systems and avoid failures. In the field of
energy transmission, an index of the state of a cable insulation
system is partial discharge activity, which is influenced by the
presence of structural defects (e.g. voids within the material). In
High-Voltage-Direct-Current applications, given the dependence
of dielectric conductivity on temperature, this phenomenon is
also influenced by the load. Thus, partial discharge activity is
strongly related to the operating condition of the line and not
only by the aging of materials. Given the importance of partial
discharges monitoring, it is necessary to develop techniques
for analyzing them that allow the identification of different
types of discharges. The lack of a reference standard prompts
researchers to propose different approaches to achieve these
goals. This paper describes a comparison of two separation
algorithms both based on cross-correlation. Data used for
comparison have been obtained through a partial discharges
measurement, under DC voltage stress, on XLPE model cable.
The first algorithm evaluates the similarity among the pulses
and separates them into clusters following the acquisition order.
The second one, on the other hand, uses the correlation matrix
as input data, which must be calculated before running the
algorithm. The results show that both algorithms succeed in
identifying the same phenomena. The former with less accuracy
but employing less computation time than the latter. The latter,
on the other hand, provides higher accuracy but requires longer
computation time. The choice in using either algorithm can thus
be traced back to the desired accuracy/time ratio
Effect of Temperature on Main Partial Discharges Phenomena Under DC Voltage Stress
Full text linkThis work is aimed at characterizing the main
partial discharges phenomena under HVDC stress and with
the presence of thermal gradient. Specifically, the increase
in discharge activity without changes in applied voltage is
investigated. Even the supply and disconnection phases are
excluded from the analyzed acquisition range. In this way, any
variation in discharge activity can be attributed only to thermal
effects. A setup consisting of a pair of specimens connected in
series and supplied with a voltage of 20 kV DC has been used
for the tests. One specimen is defect-free and immersed in a
tank containing silicone oil. The other is designed to give rise
to certain discharge phenomenon. Three tests were carried out
using specimens for internal, surface, and corona discharges.
For each test, two acquisitions, lasting 30 minutes, have been
made. One with both specimens at room temperature and
another with the healthy specimen heated through a resistive
element in order to obtain a discrete thermal gradient. Thus,
a total of 6 acquisitions have been made and analyzed. The
results show that for all three specimens the discharge activity
undergoes a significant increase in the transition from the roomtemperature condition to the thermal gradient condition. The
increase in the amount of detected discharge is greater in tests
with corona and surface specimens. In general, an increase in
average discharge amplitude is also observed. The observed
changes can be attributed to a different distribution of the
applied voltage between the two specimens due to changes in
the conductivity of the defect-free specimen caused by heating.
This behavior is also found in variations in the electric field
profile in HVDC applications due to the presence of heat sources
that generate thermal gradients. For example, load currents for
cables
The Interaction between Electric Field and Partial Discharges Simultaneously Detected in a HVDC cable under operating conditions
Full text linkThe reliability assessment of HVDC systems greatly depends on the insulating materials aging
level and, among these, the cable insulation layer plays a fundamental role. With this in mind, in this work,
the correlation between the electric field distribution and the triggering and evolution of PD in a DC cable
containing an internal air void defect and subjected to normal operating conditions has been investigated. The
aim is to demonstrate that the PD activity under DC depends on the electric field distribution which, in turn,
is related to the conductivity gradient that varies with load. In a previous paper, the field distribution in a
cable specimen was simulated. Here, instead, the field profiles have been experimentally obtained starting
from the measured space charges detected simultaneously with the PD activity. For the charge detection and
for the PD monitoring, an innovative PEA cell and a PD acquisition system have been used, respectively.
Measurement results highlight that the PD behavior depends on both the electric field distribution and the
time constant tau. Specifically, during the first 10 minutes of the beginning of the test, the field in the outer
cable radius passes from 12 to 17 kV/mm and it is maintained around this last value until 30 minutes. PD are
triggered after 1 minutes from the start of the test, corresponding to a PDIEF of 12.6 kV/mm calculated in
the healthy cable section. The detected PD activity shows a PDRR with maximum value equal to 110
pulse/min at the beginning of the test. Whereas, after 15 minutes, the electric field variation dE/dt is
approximately zero and the PDRR, that is maintained only by dE/dt results around 10 pulse/minute
A Detailed Review of Partial Discharge Detection Methods for SiC Power Modules Under Square-Wave Voltage Excitation
Full text linkSilicon carbide (SiC) power modules are increasingly being used in high-voltage and high-frequency applications due to their superior electrical and thermal qualities. However, the issue of the partial discharge (PD) phenomenon raises serious reliability difficulties resulting in insulation failure, performance degradation, and potential device collapse. This paper provides a thorough assessment of the current PD detection strategies in SiC power modules. The issues provided by SiC devices’ distinct operational features, such as high switching frequencies and higher voltage stresses, which hinder PD detection and mitigation, are widely investigated. This review compares the effectiveness, benefits, and limitations of various detection methods, emphasizing the need for better strategies to ensure long-term reliability and performance. This study gives an in-depth overview of the numerous forms of PD phenomena that occur in power modules, including internal and surface discharges, as well as how they appear under various detection systems. It examines the performance of several methods for power module technologies such as SiC. To address these PD issues, this article proposes ways to improve reliability and detection accuracy
The Role of Defect Aspect Ratio in the Partial Discharge Phenomenology under DC Voltage and Temperature Gradient
No full textThis research is focused on the influence of defect
aspect ratio on partial discharge (PD) activity within solid
dielectrics subjected to direct current (DC) stress and thermal
gradients. Employing finite element method (FEM) simulations
by Comsol Multiphysics software, the PD inception conditions
under both transient and steady-state voltage conditions was
evaluated. The focus centered on investigating the effect of
charge injection at the dielectric-cavity interface and the
subsequent migration of these charges through the gas medium
within the defect. This migration was studied through a laminar
flow model considering ionized air movement within the defect,
driven by the electric field. Treating the charge motion within
the cavity akin to an electric current, an equivalent gas
conductivity characteristic of the cavity was derived. The
findings show that the shape of the defect profoundly influences
the variability of equivalent gas conductivity within the defect
during both voltage ramp-up and stable phases
A Review on Different Deconvolution Techniques on Frequency Domain for a PEA Cell for HVDC Cables
No full textThe processing of space charge signals, obtained
through the Pulsed Electro-Acoustic (PEA) method, is crucial
for understanding the space charge accumulation mechanisms
in dielectric material and the impact these have on the operation
of an HVDC cable. In this paper, the authors give a brief
description of the entire processing procedure required to avoid
making mistakes in the interpretation of the obtained profiles.
Specifically, we examine different deconvolution techniques in
the frequency domain for analyzing such signals. Deconvolution
techniques were implemented in MATLAB environment and
used to process a PEA profile obtained from a measurement on
a full-size HVDC cable with an applied DC voltage of 100 kV
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
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
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