Chapter 1
Introduction
Chapter 1
Introduction
1.1 General Introduction
The transformer is a crucial component in power distribution and transmission
networks. Power outages cause loss of revenue for power consumers and the power
industry itself, as well as loss of consumer confidence in the power provider.
Deregulation of the power distribution industry in recent years has lead to strategies
for optimizations of the power networks. As a consequence, the operating stress levels
in the networks are closer to the transformers withstand levels and the amounts of
spare capacity in the networks are decreasing. This has lead to increased quality
demands on the transformer manufacturers, and an incentive for established
transformer assessment routines in the power industry.
For the Indian market it is mostly a large amount of older equipment that causes
increasing interest in efficient assessment methods. Another factor is that new
transformers are more optimized and thus more sensitive, which also gives reason for
systematical assessment routines [16].
The loss of mechanical integrity in the form of winding deformation and core
displacement in power transformers can be attributed to the large electromechanical
forces due to fault currents, winding shrinkage causing the release of the clamping
pressure and during transformer transportation and relocation. These winding
deformation and core displacement if not detected early will typically manifest into a
dielectric or thermal fault. This type of fault is irreversible with the only remedy been
rewinding of the phase or a complete replacement of the transformer. It therefore
imperative to check the mechanical integrity of ageing transformers periodically and
particularly after a short circuit event to provide early warning of impending failure.
Hence an early warning detection technique of such a phenomena is essential.
Frequency response analysis is recognized, as been the most sensitive diagnostic tool
to detect even minor winding movement and core displacement. [33]
Once a transformer is damaged, even if only slightly, its ability to withstand further
short circuits is reduced. Utility personnel need to effectively identify such damage. A
visual inspection is costly and does not always produce the desired results or the
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correct conclusion. Since so little of the winding is visible, little damage can be seen,
other than displaced support blocks. Often a complete teardown is required to identify
the problem. An alternative method is to implement field-diagnostic techniques
capable of detecting damage. [46]
The traditional methods of electrical tests carried out on transformers such as winding
capacitance, excitation current and leakage reactance measurements have proven to be
not particularly sensitive to detect winding movement. Each of these methods has
drawbacks.
•
Winding
capacitance
measurements
can
detect
winding
movement
successfully only if reference data is available or if measurements can be made
on each phase. In almost all older transformers, reference data is unavailable
and on site per phase measurements are not possible.
•
The excitation current method is an excellent means of detecting turn-to-turn
failure as a result of winding movement. However, if a turn-to-turn failure is
absent, winding movement can remain undetected.
•
Per phase leakage reactance measurements generally shows little or no
correlation between the phases. The three phase equivalent measurement is a
broad test and can mask a variance in one of the phases. Further, the
discrepancies from the name plate value of 0.5 to 3% can be a reason for
concern. This makes accurate assessments of the mechanical integrity of the
transformer very difficult.
•
Other condition monitoring tools such as dissolved gas analysis (DGA) do not
aid in the detection of winding deformation and core displacement. [33]
There is a direct relationship between the geometric configuration and the distributed
electrical elements known as RLC networks of a winding and a core assembly. This
RLC network can be identified by its frequency dependent transfer function.
Frequency Response Analysis testing can be accomplished by the sweep frequency
method. Changes in the geometric configuration alter the impedance network, and in
turn alter the transfer function. Changes in the transfer function reveal a wide range of
failure modes. [46]
The SFRA is a powerful method for the detection and diagnosis of the defects in the
active part of power Transformers. It can deliver valuable information about the
mechanical as well as electrical condition of core, windings, internal connections and
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contacts. No other single test method for the condition assessment of power
transformers can deliver such a diversity of information. Therefore the SFRA is an
increasingly popular test. The value of fingerprint data is more and more recognized
by users all over the world. Comparing the time and frequency domain FRA test
methods is seems to be obvious the SFRA, measuring directly in frequency domain,
prevailed. Reproducibility is the key for a successful application of SFRA. [2]
Sweep Frequency Response Analysis (SFRA) testing has become a valuable tool for
verifying the geometric integrity of transformers. SFRA provides internal diagnostic
information using nonintrusive procedures. The SFRA test method has been proven to
provide accurate and repeatable measurements. [46]
1.2 Scope of Work
SFRA Testing produces traces or “fingerprints”, which provides information related
to the physical geometry of the test specimen. Interpretation of the data is often
subjective unless baseline data is available. Base line data can be a previous test or
data collected from a similar test specimen, such as a sister unit. Interpretation of the
first time results can be limited, and results are generally analyzed by comparing
phases and recognizing obvious faults, such as severe deformation, open circuits and
short circuits. [7]
A substantial amount of work has been focused on identifying faults, such as core and
winding movement. Case study history and experience have been the primary sources
for identifying such faults. Identifying known fault conditions provides an analysis
tool that is not completely dependent on present baseline data. On the other hand,
having a known expectation of how “healthy” SFRA results behave can also provide a
similar benefit. Since the majority of SFRA results do not indicate problems, it is easy
to collect large populations of data and identify common characteristics. [7]
Comparison with other diagnostic techniques show that the key advantages of FRA
are its proven sensitivity to a variety of winding faults and a lesser dependency on
previous reference measurements, but there is a need for an objective and systematic
interpretation methodology. [15]
In recent years a lot of work has been done to develop analysis methods where automatic,
or at least guided, analysis of FRA response curves is performed. So far there is no
method that is widely accepted in the industry or the academic community. [16]
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The approach that is most commonly used in practice is to write guidelines for SFRA
interpretation, based on experience and field examples.
In addition to the work on modeling and quantifiable interpretation, and as an
extension of the guide line approach, there are ideas to organize database containing
collected measurement results. Since the SFRA interpretation is based on experience,
such data bases are thought to be of great importance when interpreting SFRA
response.
The practical simulations are the main part of the thesis work and have the following
objectives.
•
To establish a guide line for the interpretation of SFRA responses based on the
experience from various data collected from the field and case studies. Then,
this knowledge can be utilized to analyze the experimental results and classify
the fault accordingly.
•
To find out if in-housed developed transformer can be used to simulate power
transformer damages, in a way that can be useful for demonstration of SFRA
testing.
•
To distinguish which fault conditions that is possible and useful to simulate
with in-housed developed transformer.
•
The evaluation of the SFRA response against guidelines and other experience
have to be performed as a part of the work in progress, while conclusions
regarding usefulness of each simulation should be drawn in the analysis phase.
It has been recognized that there are several approaches for using objective automated
techniques such as, cross correlation coefficients, pole-zero modeling, etc. to compare
FRA results. Limits have not yet been set on an acceptable variation for difference
between two traces on the same transformer at a given frequency; likewise, limits
have not been set on acceptable differences in correlation coefficient in a given
frequency range and it is found that further research work is required regarding to any
such automated interpretation procedures. Therefore literature work has been carried
out related to a survey of some promising approaches for SFRA interpretation, with
reference to relevant papers.
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1.3 Thesis Outline
A brief description of the work is mentioned below. In addition to the outline of the
work in this chapter, the other chapter includes:
Chapter 2: Literature Review
This section defines transformer failure in the traditional sense as well as in the sense
of condition monitoring scheme. An overview of the general causes of transformer
failure is given with specific focus on the failures related to winding movement. The
principal causes of winding deformation especially those arising from high current
conditions have also been discussed.
Chapter 3: SFRA Basics
This Section defines the basic of Sweep Frequency Response Analysis (SFRA) as a
tool that can give an indication of core or winding movement in transformers.
Changes in frequency response as measured by SFRA techniques may indicate a
physical change inside the transformer, the cause of which then needs to be identified
and investigated. An overview of SFRA History and SFRA To-day is given. The
purpose of FRA measurements and Measurement of different winding types has been
discussed.
Chapter 4: SFRA Testing
SFRA has been a key tool in the decision to scrap or reenergize a transformer. To get
value from an SFRA test it is necessary to make sure that the measurements are
credible, which requires good test technique. This section defines the SFRA test
procedure and Type of measurement. An overview of the test circuit and how the
circuit connection to be done has been discussed. The test types and Measurement &
Check points are also discussed.
Chapter 5: Interpretation of SFRA
There is a learning curve associated with interpretation of SFRA traces. The traces
need to be interpreted with experience, with reference to baseline results where
possible, with reference to manufacturer specific variations and with reference to
phase comparisons. This section defines Interpretation of SFRA responses based on
the Transformer circuit modeling to accurately represent the behavior of a transformer
across the wide range of frequency. An overview of Basics of FRA interpretation,
typical FRA responses and frequency range for interpretation has been given. The
general guide lines for accurate fault detection in SFRA and interpretation
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methodology have been discussed. Some examples of FRA interpretation have also
been discussed.
Chapter 6: Experimental Work
This section defines some design details of 10 KVA power transformer which is
specially designed & developed in-house for studying and analyzing SFRA traces.
Authors have performed various practically simulated faults (14 Nos.) on this 10
KVA power transformer. The results of these practically simulated faults are
presented and discussed. The evaluations of the SFRA responses against guide lines
and experience have to be performed and conclusions regarding usefulness of each
simulation have been drawn.
Chapter 7: Conclusion
This section concludes the main findings and significant contributions of the thesis
and provides a few suggestions for further research work in this area.
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