Freqwency Response Measurement of a Power Transformer.
Frequency Response Measurement of a
Power Transformer
W. G. Ariastina, I N. S. Kumara, C. G. Indra Partha, I G. Dyana Arjana,
I W. Arta Wijaya, A. A. N. Amrita, and I. A. D. Giriantari
Department of Electrical and Computer Engineering
Udayana University
Bali-80362, Indonesia
Abstract—This paper presents a field experience in condition
monitoring of a power transformer using SFRA technique. A
design based comparison approach of the SFRA test results is
discussed in the paper. In order to verify the latest condition of
the transformer, the SFRA test was complemented with DGA
and PD tests. The SFRA test results indicated that there is an
abnormality within the transformer windings. The PD test results
confirmed this interpretation, where very large ultrasonic
impulses were detected. A series of DGA tests have showed that
there is an early stage degradation of the insulating paper, which
may be associated with fault within the transformer windings.
The field experience showed that the monitoring approach has
been very useful in indicating the possible occurrence of failure
within the transformer structure.
Keywords—core and winding fault; power transformer;
condition monitoring; SFRA
I. INTRODUCTION
Power transformers play major roles in delivering the
electrical energy. Unpredictable major events such as
earthquake and external short circuit may affect the initial
position of transformer windings and core. Winding
deformation and displacement may be occurred accordingly,
which further may cause the torn of insulating paper and
pressboard. This condition obviously may affect the insulation
integrity of the transformer. Damage on the transformer
insulation may lead to an insulation failure, hence causing
interruption of power delivery. In order to assure the good
condition of a transformer, an immediate inspection of the
insulation condition after those major events should be carried
out. The inspection should also be carried out after relocation
and transportation of a transformer.
Sweep frequency response analysis (SFRA) has been used
widely for determination of transformer windings and core
condition. This technique utilises the electrical transfer
functions of the transformer windings over a wide range of
frequency. The low voltage with a variable frequency signal is
injected into one winding terminal and the response signal is
measured at the other terminal. The frequency response
measured from a particular winding then is compared to the
available reference response data. The frequency response
appearance of a particular transformer is greatly affected by the
relationship of the resistance, inductance and capacitance
within its winding structure. Because the frequency response is
unique for a particular transformer, thus a response signature of
an individual transformer can be developed for future
measurement reference [1,2].
Recently, advanced research in SFRA technology has
successfully implemented an online monitoring, a step forward
from a conventionally offline monitoring technique. The online
monitoring approach utilises appropriate network models for a
particular transformer. The impedance of the measuring
equipment thus must be included in the model, to achieve a
proper response. This technique has been implemented for
frequency response measurements in different size of
transformers [3,4].
To date, the SFRA condition monitoring techniques have
successfully assisted engineers to detect abnormalities in
transformers, including winding deformation and displacement,
shorted or open turns, faulty grounding, core movement, as
well as other internal structure and connection problems.
However, given the fact that the frequency response is also
very much affected by many factors within the transformer,
thus further investigations are still required to look at the
possible affecting parameters, such as: core residual
magnetism, oil and winding temperature, moisture level, and
the winding structure [5-7].
This paper presents a field experience in condition
monitoring of a power transformer using SFRA technique. The
analysis is carried out by comparing the frequency response
from one winding to another (i.e. design based comparison).
This approach was considered the best option due to
unavailability of SFRA reference data of the particular
transformer type. In addition to the SFRA test, complementary
tests of DGA and PD were also carried out.
II. SFRA MEASUREMENT
The electrical relationships between core, windings,
insulation materials and the tank structure of a transformer can
be modelled as a complex electrical network that consists of a
number of resistances, self and mutual inductances, as well as
shunt and series capacitances. Because this relationship is
unique for every single transformer structure, thus the transfer
function of a transformer winding can be considered as a
fingerprint for the individual transformer. Particular defect that
causing any changing in the structure of the transformer, will
change any quantity of the circuit resistances, inductances and
the capacitances. Consequently, the frequency response of the
transformer will alter accordingly. This fact has become a
development foundation of the SFRA monitoring technique.
The frequency response measurement of a transformer is
carried out by injecting a low voltage with a variable frequency
signal into one winding terminal and measuring the response
signal at the other terminal. If the injected voltage is denoted
by Vin and the response signal is denoted by Vout, thus the
attenuation of voltage magnitude (denoted by K) can be
expressed as [1,4]:
K = 20 log10 (Vout/Vin) dB
(1)
The frequency for SFRA measurement usually ranges from
20 Hz to 2 MHz, although frequencies up to 25 MHz have been
used by some researchers [2]. The response magnitude K is
commonly depicted in Bode diagram, where the horizontal and
vertical axes represent the input frequency and the response
magnitude, respectively [1].
The overall measuring frequency range can be divided into
different sections, at which the frequency response is sensitive
to defect on particular components of the winding structure. It
is common to divide the entire frequency range into 3 sections,
which are [8]:
• Design based; the SFRA signatures from three different
phases of a transformer are compared one to another.
Due to similarity in their structure, the trace of the
frequency responses from a transformer is comparably
similar between one winding to another. This approach
is usually sufficient for analysis in case reference data is
unavailable.
To achieve appropriate comparison results, factors affecting
measurement results such as core magnetism, tap setting, and
terminal connection, must be similar for one measurement to
another. The comparison of measured frequency response data
can be further analysed using statistical indices. The correlation
coefficient and standard deviation have been found to be very
useful for interpretation of the frequency response data. In this
work, magnitude difference and cross correlation of the
frequency responses are explored [2,4].
III. RESULTS AND DISCUSSION
The SFRA test was conducted on a 150/20 kV, 60 MVA
power transformer, as demonstrated in Fig. 1. The SFRA
measurement was carried out for individual winding of the
primary (HV) and the secondary (LV). During the SFRA
measurement, the primary and the secondary winding
connections were opened, while that of the tertiary winding
was short circuited.
• Low frequency range (up to 20 kHz). Within this range,
the frequency response is mainly determined by
inductance of the circuit.
• Mid frequency range (between 20 kHz to 400 kHz).
Within this frequency range, the response is determined
by combination of circuit inductance and capacitance,
thus may initiate multiple resonances.
• High frequency range (above 400 kHz). The circuit
capacitance dominates the winding response within this
frequency range.
Some investigators divide the measurement frequency
range into four narrower frequency bands. It is intended to look
at more details on the sensitivity of a particular frequency
range to an associated fault within core and winding [5].
The frequency response is analysed by comparing the
appearance of measured response to the available reference
response data. The comparison can be done based on the
following approaches [1,2]:
• Time based; the latest SFRA measurement results from
a particular transformer is compared to those of
previous results. Significant changing in the SFRA
signature from time to time may indicate a progressing
defect within the winding structure.
• Type based; the SFRA measurement results from a
particular transformer are compared to those from
another transformer of the same type. Although the
SFRA signature is unique for an individual transformer,
the frequency responses of the transformers of the same
type are usually somewhat similar.
Fig. 1. Power transformer monitoring
The applied frequency range of the SFRA test was 20 Hz –
2 MHz. The frequency trace was set in a logarithmic scale. No
previous data was available at the time of measurement, hence
design based comparison would be the best option for
comparison of the measured frequency responses.
The work reported here is focused on the frequency
response of the primary and secondary windings. Fig. 2 shows
the frequency response of all windings. The lower traces
describe responses of the primary windings, while upper traces
describe those of the secondary and tertiary windings. In
general, the frequency responses show a consistency at low
frequency range. At frequency above 20 kHz, however, large
discrepancies between the traces can be clearly observed. For
primary windings, large differences between phase frequency
responses can be noticed at 20 – 40 kHz (within mid frequency
range) and above 500 kHz (within high frequency range). This
situation can also be observed for frequency responses of
secondary windings at above 20 kHz.
Fig. 3. SFRA responses from phase U and V of HV windings
Low frequency range
Mid frequency range
High frequency
range
Fig. 2. Frequency response of the transformer
A comparison of frequency responses from phase W and U
of primary windings is demonstrated in Fig. 4. It is clear that
within the low frequency band (up to 20 kHz), the two traces of
the frequency response possess a high similarity. The two
windings are constructed at outer core hence similar frequency
response is expected. Within the mid and high frequency bands
however, comparable characteristics to those in Fig. 3 are
observed.
Fig. 2 demonstrates that within low frequency range, antiresonances for HV and LV windings occur around a frequency
of 500 Hz. There are two resonances occur within this
frequency range. At a higher frequency range however; a high
ripple occurs on the traces. Parameters of magnitude difference
and cross correlation are implemented for analysis. The
magnitude difference and the statistical cross correlation are
calculated by software of the measurement instrument. The
software compares two frequency response traces from two
different phases at a time.
Fig. 3 illustrates the comparison of frequency response
from phase U and V of HV windings. It can be seen that there
is a large discrepancy between the two traces at around 500 Hz
(low frequency range). This is indicated by a large deviation on
the magnitude difference and a reduced value of cross
correlation between the two. Any abnormality within the low
frequency band may be associated with the core deformation,
open or shorted turns, and residual magnetism [8]. Within the
low frequency band, the frequency response of the U winding
comprises two minima, while that of V winding comprises a
single minima (see Fig. 2 for details). This discrepancy is
associated to a difference in magnetic flux path within the core
of phase U and V windings. The winding of middle core
usually poses a slightly different response at around antiresonance frequency then that of windings of outer core [2].
Fig. 3 also shows that within the mid frequency range (20
kHz to 400 kHz), there is an increased in trace discrepancy at
around 20-40 kHz. The mid frequency range is sensitive to
main or tap winding deformation [8]. An increased in
discrepancy of the frequency response within this frequency
band may indicate abnormality in the windings. This
circumstance also occurs within the high frequency band
(above 400 kHz), which may signify an abnormality within the
main winding.
Fig. 4. SFRA responses from phase W and U of HV windings
Fig. 5 shows the comparison of frequency response from
phase u1 and v1 of secondary (LV) windings. It can be
observed that the traces of the two frequency responses are
quite similar, but the position of the u1 trace is much lower
than that of the v1. This is indicated by a large magnitude
difference over the measurement frequency range as depicted
in the middle graph of Fig. 5. Similar phenomenon related to
the anti-resonances at low frequency band as demonstrated in
Fig. 3, is also observed. Similar reason thus can be explained
for this fact. A low cross correlation between the two traces can
be observed at frequencies of 20 to 100 kHz (within mid
frequency range). This again may indicate an abnormality in
the LV winding. A comparatively similar interpretation to that
from Fig. 5 can also be observed from Fig. 6, where there is a
large difference in response magnitude over the measurement
frequency range. Note also that the cross correlation between
the two comparison is reasonably similar, except for that within
the low frequency range.
degradation of the insulating paper. Details on the PD and the
DGA test results are discussed in [9] and [10].
IV. CONCLUSIONS
The SFRA measurement on a power transformer has been
carried out. The test results indicated that there is an
abnormality within the transformer windings, most probably at
LV side. The SFRA test results also showed that there is
increasing losses in the LV winding. The increasing losses may
be caused by dielectric leakage current or failure in the winding
connection. In order to obtain a better interpretation of the
SFRA test results, a type based and the time based comparisons
have been planned for future monitoring.
Fig. 5. SFRA responses from phase u1 and v1 of LV windings
Previously published PD test results confirmed the
interpretation of the SFRA results. Large ultrasonic impulses
were detected during the test. Series of DGA tests have showed
that there is an early stage degradation of the insulating paper,
which may be associated with a winding fault within the
transformer.
ACKNOWLEDGMENT
The authors would like to express their high appreciation to
the manager and staffs of the PT PLN (Persero) Transmission
and Load Dispatch Centre of East Java and Bali for facilitating
the undergoing research collaboration on the condition
monitoring of high voltage apparatus.
REFERENCES
[1]
Fig. 6. SFRA responses from phase w1 and u1 of LV windings
The cross correlation of the frequency responses is
presented in Table 1. The analysis indicates that there is an
abnormality in the transformer windings, most possibly at LV
side. A comparative analysis of the frequency responses shows
that the trace from winding u1 poses a lower magnitude than
those from the other two LV windings, which indicates an
increase in losses. The increasing losses may be caused by
dielectric leakage current or winding connection fault [8].
TABLE I.
Structure
CROSS CORRELATION OF THE FREQUENCY RESPONSES
HV Windings
U-V
LV Windings
V-W
W-U
u1-v1
v1-w1
w1-u1
Core
0.5448
0.5861
0.9781
0.129
0.1493
0.9768
Winding
0.5651
0.8757
0.8258
0.4653
0.1949
0.478
In relation to the diagnosis status verification of the
transformer, the SFRA tests were complemented with DGA
and PD tests. The PD test results confirmed the interpretation
here, where very large ultrasonic impulses were detected. A
series of DGA tests have showed that there is an early stage
S. A. Ryder, “Diagnosing Transformer Faults Using Frequency
Response Analysis”, IEEE Elect. Insul. Mag., Vol. 19, No. 2, pp. 16-22,
Mar/Apr 2003.
[2] A. Kraetge, M. Krüger, J. L. Velásquez, “Experiences with the practical
application of Sweep Frequency Response Analysis (SFRA) on power
transformers”, Proc. of the 16th ISH, Johannesburg, Paper D-45, 2009.
[3] V. Behjat, A. Vahedi, A. Setayeshmehr, H. Borsi, and E. Gockenbach,
“Diagnosing Shorted Turns on the Windings of Power Transformers
Based Upon Online FRA Using Capacitive and Inductive Couplings”,
IEEE Trans. on Power Deliv., Vol. 26, No. 4, pp. 2123-2133, Oct 2011.
[4] M. Bagheri, M. S. Naderi and T. Blackburn, “Advanced Transformer
Winding Deformation Diagnosis Moving from Off-line to On-line”,
IEEE Trans. on DEI, Vol. 19, No. 6; pp. 1860-1870, Dec. 2012.
[5] E. Al Murawwi, R. Mardiana, C. Q. Su, “Effects of Terminal
Connections on Sweep Frequency Response Analysis of Transformers”,
IEEE Elect. Insul. Mag., Vol. 28, No. 3, pp. 8-13, May/Jun 2012.
[6] M. Bagheri, B. T. Phung, and T. Blackburn, “Transformer Core
Influence on Frequency Response Spectrum Oscillations”, I Proc. of the
18th ISH, Seoul, Paper OF2-03, pp. 1772-1777, Aug. 2013.
[7] M. Bagheri, B. T. Phung and T. Blackburn, “Influence of Temperature
and Moisture Content on Frequency Response Analysis of Transformer
Winding”, IEEE Trans. on DEI, Vol. 21, No. 3; June 2014.
[8] A. Abu-Siada, N. Hashemnia, S. Islam, and M. A. S. Masoum,
“Understanding Power Transformer Frequency Response Analysis
Signatures”, IEEE Elect. Insul. Mag., Vol. 29, No. 3, pp. 48-56,
May/Jun 2013.
[9] W. G. Ariastina, et al., “Application of an Integrated Diagnostic
Technique on Power Transformer for Reliable Electrical Energy
Supply”, Proc. of the 1st Int. Conf. on Sust. Tech. Developt., Bali, pp.
E46 – E53, Oct. 2010.
[10] W. G. Ariastina, I N. Setiawan, I. A. D. Giriantari, R. P. Sari and I. K.
Solin, ”An Experience of DGA Monitoring on Power Transformers”,
Proc. of the 18th ISH, Seoul, Paper PF-40, pp. 2136-2140, Aug. 2013.
Power Transformer
W. G. Ariastina, I N. S. Kumara, C. G. Indra Partha, I G. Dyana Arjana,
I W. Arta Wijaya, A. A. N. Amrita, and I. A. D. Giriantari
Department of Electrical and Computer Engineering
Udayana University
Bali-80362, Indonesia
Abstract—This paper presents a field experience in condition
monitoring of a power transformer using SFRA technique. A
design based comparison approach of the SFRA test results is
discussed in the paper. In order to verify the latest condition of
the transformer, the SFRA test was complemented with DGA
and PD tests. The SFRA test results indicated that there is an
abnormality within the transformer windings. The PD test results
confirmed this interpretation, where very large ultrasonic
impulses were detected. A series of DGA tests have showed that
there is an early stage degradation of the insulating paper, which
may be associated with fault within the transformer windings.
The field experience showed that the monitoring approach has
been very useful in indicating the possible occurrence of failure
within the transformer structure.
Keywords—core and winding fault; power transformer;
condition monitoring; SFRA
I. INTRODUCTION
Power transformers play major roles in delivering the
electrical energy. Unpredictable major events such as
earthquake and external short circuit may affect the initial
position of transformer windings and core. Winding
deformation and displacement may be occurred accordingly,
which further may cause the torn of insulating paper and
pressboard. This condition obviously may affect the insulation
integrity of the transformer. Damage on the transformer
insulation may lead to an insulation failure, hence causing
interruption of power delivery. In order to assure the good
condition of a transformer, an immediate inspection of the
insulation condition after those major events should be carried
out. The inspection should also be carried out after relocation
and transportation of a transformer.
Sweep frequency response analysis (SFRA) has been used
widely for determination of transformer windings and core
condition. This technique utilises the electrical transfer
functions of the transformer windings over a wide range of
frequency. The low voltage with a variable frequency signal is
injected into one winding terminal and the response signal is
measured at the other terminal. The frequency response
measured from a particular winding then is compared to the
available reference response data. The frequency response
appearance of a particular transformer is greatly affected by the
relationship of the resistance, inductance and capacitance
within its winding structure. Because the frequency response is
unique for a particular transformer, thus a response signature of
an individual transformer can be developed for future
measurement reference [1,2].
Recently, advanced research in SFRA technology has
successfully implemented an online monitoring, a step forward
from a conventionally offline monitoring technique. The online
monitoring approach utilises appropriate network models for a
particular transformer. The impedance of the measuring
equipment thus must be included in the model, to achieve a
proper response. This technique has been implemented for
frequency response measurements in different size of
transformers [3,4].
To date, the SFRA condition monitoring techniques have
successfully assisted engineers to detect abnormalities in
transformers, including winding deformation and displacement,
shorted or open turns, faulty grounding, core movement, as
well as other internal structure and connection problems.
However, given the fact that the frequency response is also
very much affected by many factors within the transformer,
thus further investigations are still required to look at the
possible affecting parameters, such as: core residual
magnetism, oil and winding temperature, moisture level, and
the winding structure [5-7].
This paper presents a field experience in condition
monitoring of a power transformer using SFRA technique. The
analysis is carried out by comparing the frequency response
from one winding to another (i.e. design based comparison).
This approach was considered the best option due to
unavailability of SFRA reference data of the particular
transformer type. In addition to the SFRA test, complementary
tests of DGA and PD were also carried out.
II. SFRA MEASUREMENT
The electrical relationships between core, windings,
insulation materials and the tank structure of a transformer can
be modelled as a complex electrical network that consists of a
number of resistances, self and mutual inductances, as well as
shunt and series capacitances. Because this relationship is
unique for every single transformer structure, thus the transfer
function of a transformer winding can be considered as a
fingerprint for the individual transformer. Particular defect that
causing any changing in the structure of the transformer, will
change any quantity of the circuit resistances, inductances and
the capacitances. Consequently, the frequency response of the
transformer will alter accordingly. This fact has become a
development foundation of the SFRA monitoring technique.
The frequency response measurement of a transformer is
carried out by injecting a low voltage with a variable frequency
signal into one winding terminal and measuring the response
signal at the other terminal. If the injected voltage is denoted
by Vin and the response signal is denoted by Vout, thus the
attenuation of voltage magnitude (denoted by K) can be
expressed as [1,4]:
K = 20 log10 (Vout/Vin) dB
(1)
The frequency for SFRA measurement usually ranges from
20 Hz to 2 MHz, although frequencies up to 25 MHz have been
used by some researchers [2]. The response magnitude K is
commonly depicted in Bode diagram, where the horizontal and
vertical axes represent the input frequency and the response
magnitude, respectively [1].
The overall measuring frequency range can be divided into
different sections, at which the frequency response is sensitive
to defect on particular components of the winding structure. It
is common to divide the entire frequency range into 3 sections,
which are [8]:
• Design based; the SFRA signatures from three different
phases of a transformer are compared one to another.
Due to similarity in their structure, the trace of the
frequency responses from a transformer is comparably
similar between one winding to another. This approach
is usually sufficient for analysis in case reference data is
unavailable.
To achieve appropriate comparison results, factors affecting
measurement results such as core magnetism, tap setting, and
terminal connection, must be similar for one measurement to
another. The comparison of measured frequency response data
can be further analysed using statistical indices. The correlation
coefficient and standard deviation have been found to be very
useful for interpretation of the frequency response data. In this
work, magnitude difference and cross correlation of the
frequency responses are explored [2,4].
III. RESULTS AND DISCUSSION
The SFRA test was conducted on a 150/20 kV, 60 MVA
power transformer, as demonstrated in Fig. 1. The SFRA
measurement was carried out for individual winding of the
primary (HV) and the secondary (LV). During the SFRA
measurement, the primary and the secondary winding
connections were opened, while that of the tertiary winding
was short circuited.
• Low frequency range (up to 20 kHz). Within this range,
the frequency response is mainly determined by
inductance of the circuit.
• Mid frequency range (between 20 kHz to 400 kHz).
Within this frequency range, the response is determined
by combination of circuit inductance and capacitance,
thus may initiate multiple resonances.
• High frequency range (above 400 kHz). The circuit
capacitance dominates the winding response within this
frequency range.
Some investigators divide the measurement frequency
range into four narrower frequency bands. It is intended to look
at more details on the sensitivity of a particular frequency
range to an associated fault within core and winding [5].
The frequency response is analysed by comparing the
appearance of measured response to the available reference
response data. The comparison can be done based on the
following approaches [1,2]:
• Time based; the latest SFRA measurement results from
a particular transformer is compared to those of
previous results. Significant changing in the SFRA
signature from time to time may indicate a progressing
defect within the winding structure.
• Type based; the SFRA measurement results from a
particular transformer are compared to those from
another transformer of the same type. Although the
SFRA signature is unique for an individual transformer,
the frequency responses of the transformers of the same
type are usually somewhat similar.
Fig. 1. Power transformer monitoring
The applied frequency range of the SFRA test was 20 Hz –
2 MHz. The frequency trace was set in a logarithmic scale. No
previous data was available at the time of measurement, hence
design based comparison would be the best option for
comparison of the measured frequency responses.
The work reported here is focused on the frequency
response of the primary and secondary windings. Fig. 2 shows
the frequency response of all windings. The lower traces
describe responses of the primary windings, while upper traces
describe those of the secondary and tertiary windings. In
general, the frequency responses show a consistency at low
frequency range. At frequency above 20 kHz, however, large
discrepancies between the traces can be clearly observed. For
primary windings, large differences between phase frequency
responses can be noticed at 20 – 40 kHz (within mid frequency
range) and above 500 kHz (within high frequency range). This
situation can also be observed for frequency responses of
secondary windings at above 20 kHz.
Fig. 3. SFRA responses from phase U and V of HV windings
Low frequency range
Mid frequency range
High frequency
range
Fig. 2. Frequency response of the transformer
A comparison of frequency responses from phase W and U
of primary windings is demonstrated in Fig. 4. It is clear that
within the low frequency band (up to 20 kHz), the two traces of
the frequency response possess a high similarity. The two
windings are constructed at outer core hence similar frequency
response is expected. Within the mid and high frequency bands
however, comparable characteristics to those in Fig. 3 are
observed.
Fig. 2 demonstrates that within low frequency range, antiresonances for HV and LV windings occur around a frequency
of 500 Hz. There are two resonances occur within this
frequency range. At a higher frequency range however; a high
ripple occurs on the traces. Parameters of magnitude difference
and cross correlation are implemented for analysis. The
magnitude difference and the statistical cross correlation are
calculated by software of the measurement instrument. The
software compares two frequency response traces from two
different phases at a time.
Fig. 3 illustrates the comparison of frequency response
from phase U and V of HV windings. It can be seen that there
is a large discrepancy between the two traces at around 500 Hz
(low frequency range). This is indicated by a large deviation on
the magnitude difference and a reduced value of cross
correlation between the two. Any abnormality within the low
frequency band may be associated with the core deformation,
open or shorted turns, and residual magnetism [8]. Within the
low frequency band, the frequency response of the U winding
comprises two minima, while that of V winding comprises a
single minima (see Fig. 2 for details). This discrepancy is
associated to a difference in magnetic flux path within the core
of phase U and V windings. The winding of middle core
usually poses a slightly different response at around antiresonance frequency then that of windings of outer core [2].
Fig. 3 also shows that within the mid frequency range (20
kHz to 400 kHz), there is an increased in trace discrepancy at
around 20-40 kHz. The mid frequency range is sensitive to
main or tap winding deformation [8]. An increased in
discrepancy of the frequency response within this frequency
band may indicate abnormality in the windings. This
circumstance also occurs within the high frequency band
(above 400 kHz), which may signify an abnormality within the
main winding.
Fig. 4. SFRA responses from phase W and U of HV windings
Fig. 5 shows the comparison of frequency response from
phase u1 and v1 of secondary (LV) windings. It can be
observed that the traces of the two frequency responses are
quite similar, but the position of the u1 trace is much lower
than that of the v1. This is indicated by a large magnitude
difference over the measurement frequency range as depicted
in the middle graph of Fig. 5. Similar phenomenon related to
the anti-resonances at low frequency band as demonstrated in
Fig. 3, is also observed. Similar reason thus can be explained
for this fact. A low cross correlation between the two traces can
be observed at frequencies of 20 to 100 kHz (within mid
frequency range). This again may indicate an abnormality in
the LV winding. A comparatively similar interpretation to that
from Fig. 5 can also be observed from Fig. 6, where there is a
large difference in response magnitude over the measurement
frequency range. Note also that the cross correlation between
the two comparison is reasonably similar, except for that within
the low frequency range.
degradation of the insulating paper. Details on the PD and the
DGA test results are discussed in [9] and [10].
IV. CONCLUSIONS
The SFRA measurement on a power transformer has been
carried out. The test results indicated that there is an
abnormality within the transformer windings, most probably at
LV side. The SFRA test results also showed that there is
increasing losses in the LV winding. The increasing losses may
be caused by dielectric leakage current or failure in the winding
connection. In order to obtain a better interpretation of the
SFRA test results, a type based and the time based comparisons
have been planned for future monitoring.
Fig. 5. SFRA responses from phase u1 and v1 of LV windings
Previously published PD test results confirmed the
interpretation of the SFRA results. Large ultrasonic impulses
were detected during the test. Series of DGA tests have showed
that there is an early stage degradation of the insulating paper,
which may be associated with a winding fault within the
transformer.
ACKNOWLEDGMENT
The authors would like to express their high appreciation to
the manager and staffs of the PT PLN (Persero) Transmission
and Load Dispatch Centre of East Java and Bali for facilitating
the undergoing research collaboration on the condition
monitoring of high voltage apparatus.
REFERENCES
[1]
Fig. 6. SFRA responses from phase w1 and u1 of LV windings
The cross correlation of the frequency responses is
presented in Table 1. The analysis indicates that there is an
abnormality in the transformer windings, most possibly at LV
side. A comparative analysis of the frequency responses shows
that the trace from winding u1 poses a lower magnitude than
those from the other two LV windings, which indicates an
increase in losses. The increasing losses may be caused by
dielectric leakage current or winding connection fault [8].
TABLE I.
Structure
CROSS CORRELATION OF THE FREQUENCY RESPONSES
HV Windings
U-V
LV Windings
V-W
W-U
u1-v1
v1-w1
w1-u1
Core
0.5448
0.5861
0.9781
0.129
0.1493
0.9768
Winding
0.5651
0.8757
0.8258
0.4653
0.1949
0.478
In relation to the diagnosis status verification of the
transformer, the SFRA tests were complemented with DGA
and PD tests. The PD test results confirmed the interpretation
here, where very large ultrasonic impulses were detected. A
series of DGA tests have showed that there is an early stage
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