3,176 research outputs found
On the use of Neumann's principle for the calculation of the polarizability tensor of nanostructures
The polarizability measures how the system responds to an applied electrical
field. Computationally, there are many different ways to evaluate this
tensorial quantity, some of which rely on the explicit use of the external
perturbation and require several individual calculations to obtain the full
tensor. In this work, we present some considerations about symmetry that allow
us to take full advantage of Neumann's principle and decrease the number of
calculations required by these methods. We illustrate the approach with two
examples, the use of the symmetries in real space and in spin space in the
calculation of the electrical or the spin response.Comment: 7 pages, 5 figures, accepted for publication in the Journal of
Nanoscience and Nanotechnolog
Condition monitoring and diagnostics of power transformers
This paper addresses the subject of condition monitoring and diagnostics of power transformers. The main
results of two reliability surveys, carried out under the auspices of CIGRE and IEEE in order to assemble objective data
on the performance of transformers in service, are presented, providing useful information on the main causes of
transformer failures, the most likely affected components and the related outages times. A survey of the most important
methods, actually in use, for condition monitoring and diagnostics of power transformers is also given, which stresses
the need for the development of new diagnostic methods, that can be applied without taking the transformers out of
service, and that can also provide a fault severity criteria, in particular for determining transformers windings integrity.
Preliminary results, concerning the on-going research activity on the development of a new approach for inter-turn
winding fault diagnosis in three-phase transformers, are also reported in the paper
Modelling and simulation of three-phase power transformers
This paper presents the development and implementation
of a digital simulation model of a three-phase,
three-leg, power transformer. The proposed model, implemented
in MATLAB environment, is based on the physical
concept of representing windings as mutually coupled coils,
and it is intended to study its adequacy to incorporate, at a
later stage, the influences of the occurrence of windings inter-
turn short-circuits. Both simulation and laboratory tests
results, obtained so far, for a three-phase, 6 kVA transformer,
demonstrate the adequacy of the model under normal operating
conditions
A permeance-based transformer model and its application to winding interturn arcing fault studies
This paper investigates the behavior of power transformers under the occurrence of permanent or intermittent winding insulation faults
Incipient turn-to-turn winding fault diagnosis of power transformers by the on-load exciting current Extended Park`s Vector Approach
This paper presents the application of the
on-load exciting current Extended Park's Vector
Approach for diagnosing incipient turn-to-turn
winding faults in operating power transformers.
Experimental and simulated test results demonstrate
the effectiveness of the proposed technique, which is
based on the spectral analysis of the AC component of
the on-load exciting current Park's Vector modulus
On-line diagnostics of transformer winding insulation failures, by Park's Vector Approach
This paper presents a non-invasive approach for diagnosing winding insulation failures in three-phase transformers, which is based on the on-line monitoring of the
primary and secondary current Park's Vector. Experimental and simulated results demonstrate the
effectiveness of the proposed technique, for detecting
winding inter-turn insulation faults in operating three-phase transformers
A coupled electromagnetic transformer model for the analysis of winding inter-turn short-circuits
This paper presents the development and implementation of a coupled electromagnetic transformer model
for the simulation of three-phase power and distribution
transformers affected by the presence of winding inter-turn
short-circuits. Generically, the coupled electromagnetic
transformer model is based on the simultaneous consideration of both magnetic and electric equivalent circuits. To incorporate the internal faults in this model a suitable equivalent
circuit of the faulty winding is proposed. Both simulation and
laboratory tests results are presented in the paper, which
demonstrate the adequacy of the proposed model
Power transformers' fault diagnostics by Parkās Vector Approach
The electricity market was, for a long time, dominated by national or regional monopolies.
During the last decades, this picture has changed drastically, particularly in Europe and
in the United States. The trend toward free competition and privatization with the related
demands on return on investments results in a cost consciousness among utilities. In this
context, monitoring and on-site diagnostics are seen as a possible way of optimizing existing
assets. The main driving forces are to reduce maintenance costs, to prevent forced
outages with the related consequential costs, and to work existing equipment harder and longer [1]. Transformers constitute the largest single component of the transmission and distribution equipment market
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