الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
Transformers are major elements of any power systems. Normally they
must be properly protected by differential relays. This protection system
should be precise and reliable via implementation of strong algorithms
that able to differentiate between faulted and unfaulted conditions to fully
guarantee power continuity.
The used protection type should minimize the time of disconnection for
faults within the transformer and to reduce the risk of any failure to
simplify eventual repair and to increase the system reliability. The
operation of the transformer under any abnormal condition such as faults
or overloads compromises the life of the transformer, which means
adequate protection should be provided for quicker isolation of the
transformer under such conditions.
a false operating current in differential relays may exist due to mismatch
between the CTs ratios and the power transformer ratio, Variable ratio of
the power transformer caused by a tap changer, Phase shift between the
power transformer primary and secondary currents for delta–wye
connections and Magnetizing inrush currents created by transformer
transients where this false current may cause the relay mis- operation.
The relay percentage-restraint characteristic typically solves the first two
problems. A proper connection of the CTs solves the phase-shift
problem. A very complex problem is that of discriminating internal fault
currents from the false differential currents caused by magnetizing inrush
current which will be more investigated in the proposed work study.
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Most of the early methods of differential protection of transformers are
based on harmonic content of differential current. These methods are
based on the ratio of the second harmonic to the fundamental component
of the differential current. As for the inrush current condition this ratio is
greater than that in the internal fault conditions. But the second harmonic
can also be generated during internal faults on the transformers or due to
saturation of CTs or due to long transmission lines so this situation may
cause greater ratio of the second harmonic as in inrush current or more.
The wavelet transform is a relatively new and powerful tool in the
analysis in the transient phenomena because its ability to extract
information from the transient signals simultaneously in time and
frequency domain, rather than the conventional Fourier Transform which
can only give the information in the frequency domain.
This thesis presents an approach for the differential protection of power
transformers. This uses three different techniques (current wave shape,
time, wavelet transform (WT)) based discrimination techniques, in
combination with adaptive network-based fuzzy inference system
(ANFIS) to discriminate internal faults from inrush currents.
The performance of the simulated model is demonstrated by simulation of
different faults and switching conditions on a power transformer using
Matlab/Simulink software Package.
The wavelet transform technique is first applied to decompose the
differential current signals through the CTs secondary side into a series of
wavelet components, each of which is a time-domain signal that covers a
specific frequency band. Thus, more distinctive signal features that
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represent internal faults and inrush currents are extracted. As a result, by
quantifying the extracted features, a decision for distinguishing an
internal fault from an inrush current in different power transformer
systems can be accurately made by ANFIS in terms of the differences in
the quantified features. The extensive simulation results presented
show that the proposed technique needs very simple input signals ,
but can accurately discriminate between an internal fault and an inrush
current in different power transformer systems.