Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
EVALUATION OF ACOUSTIC EMISSION SIGNATURES OF ON-LOAD
TAP-CHANGERS
M.B. Trindade1* and H.J.A. Martins1
Electric Power Research Center, CEPEL,
Av. Horácio de Macedo 254, Cidade Universitária,
Ilha do Fundão, Rio de Janeiro, Brazil
*Email: mauro@cepel.br
1
Abstract: In this work, are presented results of the development of a methodology that, using the
acoustic emission technique, aims to verify the conditions of operation and diagnosis of on-load
tap-changers. Acoustic signatures obtained from on-site tests, in different models from four
manufacturers of tap-changers, are analyzed.
The developed method showed to be effective to the desired purpose, with capacity to enable the
acquisition of acoustic signatures, performing as an appropriate tool for the diagnosis of on load
tap-changers, allowing the discrimination between states of normal operation and with the
presence of defects.
1.
equipment. In a normal sequence operation briefly
occur the following events:
INTRODUCTION
On-load tap-changers (OLTC) are machines with
mechanical behavior following well-defined standards.
This characteristic is resulted from the need that the
events involved on tap-changer operations must be
processed with high precision and synchronism in
order to avoid risk to the integrity of power
transformers. Since these are mechanical switching
devices, on-load tap-changers are expensive and
vulnerable devices of power transformers. The largest
part of the defects is mainly related to mechanical
components and presents higher occurrence ratio for
contacts mechanism [1-2].
•
•
•
•
•
•
Each of these events is processed by mechanical
devices, which when operating act as acoustic emission
sources, generating signals that can be detected by the
measurement system.
OLTC can be designed as a single unit for single and
three-phase applications. Depending on the three-phase
rating, it might require three separate units, each
having its own insulated phases. Nowadays, there are
different OLTC models in operation, which vary
according to the manufacturer and application. In
general, these models can be fit in two basic types [3]:
•
•
On load tap changer, Figure 1 – generally, it
consists of a diverter switch with a transition
impedance and a tap selector which can be
with or without a change-over selector.
Selector switch – a device combining the
duties of a tap selector and a diverter switch.
The tap selection and the tap changing are
processed simultaneously with the use of the
same set of movable contacts.
Besides the type, other characteristics differentiate the
OLTC models:
•
•
•
Driving
mechanism
operation
(motor,
transmission shaft, gears);
Set in motion the change-over selector and
change the position of its contacts;
Spring energy accumulator loading and
releasing;
Set in motion the tap selector shaft;
Tap selector contacts transition between taps;
Diverter switch operation.
Figure 1: On load tap-changer
Insulation: oil (conventional technology) or
vacuum (vacuum switching technology);
Assembly in the transformer: it can be located
either inside the transformer tank (internal) or
outside in its own compartment (external);
Transition impedance: reactor or resistor.
Acoustic emission is a non-destructive and noninvasive technique useful in detecting internal active
defects in materials and equipment [4-5]. Due to their
convenience, acoustic emission tests are an important
tool of preventive inspection, and are an alternative to
other more expensive non-destructive test methods.
Previous researches have already indicated it as
adequate to support diagnosis of OLTC [6].
During the short period of a tap-changer switching,
automatically occur events whose precision and
synchronism depend on the right working of the
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Paper D-31
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
the difference in the signatures provoked by the change
in sensor position.
Acoustic signals, generated inside the equipment in
operation or when subjected to programmed strengths,
carry with them important information’s related to their
integrity. These signals, generally associated to
presence of defects or to typical performing
characteristics of the equipment and of its components,
can be picked up externally, using acoustic sensors,
and analyzed considering the variation in parameters
such as: amplitude, energy, counts, duration and
average frequency. Studies have revealed that, among
several relations between these parameters, the curves
of accumulated energy (AcE), during the tap change
time, are the acoustic signatures that best characterize
the OLTC behavior [6].
The environmental conditions during the tests were
considered satisfactory. However, in the case of tapchanger D, the presence of noise, originated inside the
power transformer, has interfered in the results. In none
of the results, the oil filter was activated.
The quantity of switchings, for each equipment, was
24, corresponding to a rising up to 3 positions above
the normal position operation, one fall up to 3 positions
down and other rising, returning to initial position.
Following, the cycle was repeated one more time.
Additionally, it were included results obtained from a
three-phase tap-changer, type selector switch, which
was subjected to monitoring before and after
maintenance, in the condition – de-energized, and in
which it was identified the presence of defects.
The energy of acoustic signals, informed by the
measuring system during the tests, refers to the
measured area under the rectified signal envelope
(MARSE), in amplitude versus time coordinates [5].
This parameter, for the form as it is obtained consists
in a dimensionless measure, being only indicated as
units of energy.
3.
RESULTS
3.1.
2.
PROCEDURE
Figure 2 shows a picture framed with typical signatures
obtained from switchings performed in different groups
of tested OLTC. In this figure, are showed two curves
for each switching obtained from two cycles of
switchings performed in sequence.
The results showed in this report were obtained in tests
performed in 14 one-phase OLTC, from different
manufacturers and models which characteristics are
showed in Table 1. The OLTC were coded according
to the manufacturer (A, B, C and D), the model (1 and
2) and the connected sensor (S1, S2 and S3). Each
group of OLTC, of the same model and manufacturer
was installed in the same power transformer bank.
In these graphs, the marked dots correspond to the
events involved in the switchings. The interconnection
of these dots, utilized during curves construction, only
attends for better visualization of the accumulated
energy values evolution, since the events are discrete.
The adopted origin for the time measurements was the
instant of the occurrence of the event of major energy.
For this reason, some period of time appear indicated
with negative values.
Table 1: OLTC characteristics
Manufacturer /
model - sensor
A1-S1, S2, S3
Diverter
switch
C
Insulation Assembling
Oil
A2-S1, S2, S3
VI
B-S1, S2, S3
VI
C-S1, S2, S3
C
Oil /
Vacuum
Oil /
Vacuum
Oil
D-S1, S2
C
Oil
Internal
Transition
impedance
Resistor
External
Reactor
External
Reactor
Internal
Resistor
Internal
Resistor
Acoustic signatures
The main stages of the switching process can be
identified in curves of Figure 2: starting of operation of
driving mechanism, tap selection and operation of the
diverter switch. The absence of signals between -2000
and 0 ms, observed in Figure 2(b), is explained by the
fact that switchings present in this figure are related
only to inversion in the direction of the switching,
without moving the contacts of the tap selector. This
ascertaining comes to confirm the indication of this
interval as that in which the tap selection is processed.
NOTE: Conventional diverter switch (C) and Vacuum interrupter (VI)
The tests consisted in monitoring acoustic signals
emitted during switchings performed on-site, with the
OLTC in normal operation. The monitoring were
simultaneously made in each group of transformers. In
general, for each tap-changer, it was used only one
fixed sensor outside the tank wall. The sensor positions
varied according the constructive characteristics of the
set tap-changer / transformer, in that way both
selection signals and tap change signals were detected
with suitable levels of intensity.
The main observations about the curves showed in
Figure 2 are:
•
•
•
In one of the tests, for OLTC B-S3, it was used one
more sensor, S4, positioned at the same height of the
sensor S3, but in an adjacent side, in the OLTC
compartment. This assembly had as objective to verify
Pg. 2
The repeatability of the tests verified by the
overlapping of the curves, by the repeating of
the dots and by the occurrence of time
coincidence;
The difference between a full switching, Figure
2(a), and a switching without tap selection,
Figure 2(b);
The difference between the curves generated
by the signals detected by sensors in different
positions, Figure 2(d). Despite the gap between
the energy values of the signals, the shape of
Paper D-31
ISBN 978-0-620-44584-9
•
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
•
both curves and the repetition of the dots at the
same period of detection are maintained;
The difference between a simple switching
Figure 2(e), and a switching with intermediate
positions, Figure 2(f);
The effects of noises inside the transformer,
during the monitoring, Figure 2(g). The
interference is more evident when the curves
are compared with the curves in Figure 2(h),
generated by a similar OLTC, without noises.
(a) Tap-changer A1
(b) Tap-changer A1
(c) Tap-changer A2
(d) Tap-changer B
(e) Tap-changer C
(f) Tap-changer C
(g) Tap-changer D
(h) Tap-changer D
Figure 2: AcE (MARSE) versus time (ms)
Typical signatures of tested OLTC
In Figure 3, it is possible to notice similar behaviors in
the curves of average of the total accumulated energy
for switchings in the same group of OLTC. The ripples
in some of those curves are generated according to the
proximity of the sensor to the taps that are being
switched, due to the circular disposition of the fixed
contacts of the tap selector. In the specific case of the
OLTC C-S1, C-S2 and C-S3, the peaks of total
accumulated energy correspond to the switchings that
involve intermediate positions and change-over
operation.
Values of the average of the total accumulated energy
(AcE) for each OLTC in each switching, obtained
through the average of the sums of the signals energy
in the repetitions, are presented in Figure 3, together
with the respective variation coefficient, VC%.
In this work, the VC%, calculated by the following
formula, was used as a parameter to evaluate the
repeatability of results:
VC % = ( s / X ) * 100
where s is the standard deviation and X is the average
of values, for each group of results obtained under the
same conditions.
In general, the VC% calculated were lower than 5%.
The main exceptions were the OLTC D-S1 and D-S2,
Pg. 3
Paper D-31
ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
whose VC% reached up to 20%. The first case
happened due to the noises generated inside the
transformer and that varied during the repetitions,
affecting the results of each switching. For the OLTC
D-S2, the values suggest a more meticulous
investigation.
In Figure 4, the values of the variation coefficients are
presented. They were calculated for every switching
and every set of OLTC in a same group.
(a) Tap-changer A1
(b) Tap-changer A2
(c) Tap-changer B
Figure 4: VC%, for each switching and group of
OLTC
The VC% values varied a lot, reaching up to 15%,
which may be considered high. This behavior indicates
that the quantitative results generated by an OLTC will
not necessarily be repeated in another OLTC of the
same model and manufacturer.
(d) Tap-changer C
3.2.
Identifying defects
During the maintenance of a three-phase, selector
switch type on-load tap changer, one of the fixation
screws, placed at upper coupling gear of the selector
switch shaft, has been found broken and the others
loose, affecting the contact movement. The
accumulated energy curves versus time, acquired after
and before the repair of this tap-changer, can be
observed in Figure 5.
(e) Tap-changer D
The difference of the curves behavior on the two
conditions is very obvious and was verified in all tapchanger operations performed. It is possible to observe
a significant variation of the accumulated energy
values from the moment of the releasing of the spring
energy accumulator until the end of the movable arcing
contacts transition. These results indicate the presence
of a defect that appears at the same form in all tapchanger operations, compatible with the defect
identified during a later inspection. The variation of
accumulated energy was of 33% at the end of the
switching process.
Figure 3: Average of total AcE and VC%, for each
switching and each OLTC
Pg. 4
Paper D-31
ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
the results and should be taken under consideration
during monitoring.
(a) Tap change signatures
Quantitative comparisons between acoustic signatures
should always involve the same equipment, the same
switching, the same switching direction and the sensor
should be in the same position. The signatures of two
tap-changers of the same model can have the same
behavior and present the same switching events and
periods, but distinct intensities.
For the same switchings, the same OLTC and an
environment free of noise, the variation coefficients
presented in great part values lower than 5%, which
indicates good repeatability of the results when
compared to the 33% variation in the total accumulated
energy, caused by the presence of defects in the
selector switching type OLTC.
(b) After spring releasing
With the presented results, is possible to conclude that
the methodology under development, using the
acoustic emission technique, is a proper tool to assist in
the evaluation and diagnosis of on-load tap-changers.
5.
Figure 5: AcE (MARSE) versus time (ms), before and
after maintenance
4.
ACKNOWLEDGMENTS
The authors thank Luiz E. D. Santos and Roberto C. de
Menezes for the contribution during the execution of
this work.
REMARKS AND CONCLUSIONS
6.
The test results show that acoustic emission technique
is able to identify the many events involved in the
switching process, enabling to evaluate their intensity
and the periods when they occur.
REFERENCES
[1] P. Kang, D. Birtwhistle, J. Daley, D. McCulloch,
Non-invasive On-line Condition Monitoring of On
Load Tap Changers, Proceedings of IEEE Power
Engineering Society Winter Meeting, Singapore
2000.
[2] CIGRE SC 12 WG 12.05. An International Survey
on Failures in Large Power Transformers in
Service, ELECTRA, nº88, 1983, pp 21–42.
[3] International Electrotechnical Commission. Onload tap-changers, International Standard – IEC
214.
[4] American Society for Nondestructive Testing.
Acoustic Emission Testing, Nondestructive
Testing Handbook, Vol. 5, 2ª Ed.
[5] A. A. Pollock, Acoustic Emission Inspection,
Metals Handbook, 9ª Ed., Vol. 17, American
Society for Metals.
[6] M. B. Trindade, H. J. A. Martins, A. F. Cadilhe, J.
A. C. Moreira, On-Load Tap-Changer Diagnosis
Based on Acoustic Emission Technique, XIVth
International Symposium on High Voltage
Engineering – ISH/2005, Tsinghua University,
Beijing, China, August 25-29, 2005.
The repeatability of the results allowed the
establishment of acoustic signatures for on-load tap
changers of different models and manufacturers.
The methodology developed by CEPEL for obtaining
acoustic signatures is non-invasive and easy to apply. It
can be used even with the equipment energized and
operating. The amount of tap-changers that can be
simultaneously monitored is only limited by the
number of channels available in the measurement
system. The test duration is extremely short, being only
the time required for the switchings.
The comparison between acoustic signatures obtained
before and after the maintenance of a selector switch
type three-phase OLTC showed that the defects can be
clearly identified by changes introduced into the
accumulated energy curves.
Noises caused by the operation of the transformer or its
components, such as oil filters, besides the noises
produced by environmental factors can interfere with
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