Power Transformer Health Monitoring: A shift from
off-line to on-line detection
Ehnaish Aburaghiega
Glasgow Caledonian University
UK
Ehnaish.Aburaghiega@gcu.ac.uk
Dr. Mohamed Emad Farrag
Glasgow Caledonian University
UK
Mohamed.Farrag@gcu.ac.uk
Abstract- Transformers are an important part of the electrical
power system network, therefore, its fault detection is vital. Offline methods are commonly used for their fault detection. These
methods have associated costs derived from the necessity of
taking the transformer out of service. The application of on-line
methods reduces the expected costs and the possibility of
unpredicted failures. In the present study, off-line and on-line
methods are applied to the detection of short circuits in
transformers, demonstrating the possibility of moving from offline to on-line methods. Short circuits between sections in a
transformer winding, between winding and core and between
windings have been considered. PSPICE software is used to
simulate the transformer for both detection methods. A
comparison of the fault indication in both techniques proves the
possibility of moving from off-line to on-line method. Sweep
Frequency Response Analysis (SFRA) is considered an accurate
technique for off-line tests. The changes in the windings
inductance and capacitance affect the number of poles in the
system response, so the number of poles will indicate the number
of healthy sections. For on-line method, measurement of
primary/secondary voltage/current is used to determine the
measurable values which would result from a range of internal
short circuit faults. From the simulated results, it is found that
primary current can be used as the main indicator for the
primary winding short circuits; a combination of secondary
voltage and primary current is found to be useful for detecting
secondary winding faults. Secondary winding voltage can be the
main indicator for the cross windings short circuits.
Index Terms—Power Transformer monitoring, off-line and online method, SFRA, Section-to-section faults
I.
INTRODUCTION
Power transformer failures have a high financial impact in
the distribution and transmission companies, due to failure to
meet commercial contract and transformer replacement cost.
The majority of the transformers in service in the network
have been installed since 1970 [1]. Power transformer
reliability is dependent on the condition of its insulation
system. Most transformer failures are caused by the electrical,
thermal and mechanical stresses [2], [3] that appears in the
transformer under certain operating conditions. 70%-80% of
power transformer faults occur as result of internal short
circuit and, as indicated in research papers such as [4], [5],
[6], the very high current under short circuit conditions leads
to high mechanical force on the windings, These forces cause
changes in the dimensions through axial or radial deformation
[7]. Internal faults such as winding faults can lead to massive
damage in a short time [8] and are the most likely cause of
978-1-4673-9682-0/15/$31.00 ©2015 IEEE
Dr. Donald M Hepburn
Dr. Belen Garcia
Glasgow Caledonian University Universidad Carlos III de
UK
D.M.Hepburn@gcu.ac.uk
Madrid, Spain
bgarciad@ing.uc3m.es
disruption in transformer operation and power supply
interruption. The investigation of condition monitoring and
evaluation techniques of power transformers has become a
subject of interest for many researchers, e.g. [9]. Improving
understanding of the relationship between transformer fault
types and their indicators will help asset managers to
maintain equipment. In this study, an off-line and an on-line
method for detecting and identifying internal faults are
investigated. SFRA is applied as an off-line method to
investigate the indicators of short-circuit conditions that may
be expected to occur between sections and between winding
to core. The method correlates the frequency response of the
winding, which is affected by the transformer construction
and resulting coil parameters of resistance, inductance and
capacitance. It can provide an indication for winding and core
conditions by relating changes in frequency response to
changes in coil parameters. In addition, the possibility of
moving to an on-line method is considered through
determining the impact of short-circuit occurring between
sections in one winding, between windings and core and
between both windings on the measurement of voltages and
currents for the standard AC frequency.
II. OFF-LINE FAULT DETECTION METHOD
SFRA is an advanced method for defining transformer
health conditions, by applying low and constant voltage with
varying frequencies to the transformer windings, the ratio of
the measured input and output signals gives the required state
of the transformer [10]. This method must integrate the
measurement and analyze the data while the device is not
connected to the electrical supply in order to provide winding
health conditions [11]. Typically the SFRA method consists
of measuring the transformer response in a range of
frequency from 0.1 Hz to 5 MHz (based on the transformer
construction), to find the resonant frequencies of the winding.
The identification of changes to these frequencies from a
known baseline measurement, or in comparison with
transformers of the same type, allows the detection of
changes in the geometry of the winding.
A.
Winding Model and faults
A transformer model has been implemented in PSPICE
based on the data for transformer following the example of
[12], the transformer is iron-core insulated winding. The
transformer is modeled as 5 sections of coil that are
connected in series (Fig. 1). Each section is represented by
lumped resistances, inductances for iron core and series
capacitances between sections and shunt capacitances
between sections and core. Winding insulation is made from
(paper) with thickness of 0.1 mm and air insulation was
considered to be between discs and core, so any two sections
that are adjacent to each others are insulated by solid paper.
Model parameters are given in Table I.
Fig. 1. Winding Model (5-Sections)
TABLE I
MODEL PARAMETERS
No
1
2
3
4
Component
Section Resistance
Section Inductance
Series capacitance
Shunt capacitance
Code
R
L
Cs
Cg
Value
0.0194 Ω
0.3655mH
0.914pF
0.011nF
C. SFRA response for Short Circuit Tests
 One section short circuited
In practical cases, a damaging in the insulation between any
two sections leads to the current by-passing parts of the
winding. In this investigation this type of fault is called a
short circuit between two sections.
To investigate this type of fault, the same SFRA
procedure that was applied to investigating a healthy winding
is applied to study section to section faults and a section to
ground fault. The following scenarios are investigated:
1) Short circuit between two adjacent sections.
2) Short circuit between sections and the core.
Figure 4 compares SFRA output for faults at different
locations: the upper figure is from a short circuit between
sections 1 and 2, the middle figures is a short circuit between
sections 2 and 3 and the lower figure a short circuit between
sections 3 and 4. As can be seen, all faults lead to the same
reduction in number of poles (from 5 to 4), independently of
fault location. This is due to the fact that the number of poles
is related to the number of RLC sections in the network,
equivalent to healthy sections in the winding.
B. Healthy winding
The model is initially run as a healthy winding connected to a
constant resistive load of 100 Ω and with a fixed supply of voltage
230 V to obtain the SFRA response of a healthy winding, which will
be later compared to those of faulty models. The output voltage is
measured on the load side. Any change of any section’s impedance
leads to changes in the winding impedance which theoretically
impacts on the measured voltage.
Figure 2 shows the SFRA output of the healthy transformer
between 0.1 Hz to 5MHz. As can be seen, five healthy sections
results in a response which has 5 peaks in frequency, i.e. 5-Poles.
Fig. 4. Comparison of SFRA response for one short circuited section in
different positions
Fig. 2. Peak voltages versus sweeping frequency
Due to the nature of R-L-C circuits, resonance will occur
at the defined frequencies. Figure 3 shows the non-linear
increase in frequency of each pole for the healthy model.
 Two and more sections short circuited
In the second set of tests, the number of sections short
circuited was increased, considering 2, 3 and 4 sections.
Table II compares the number of poles and their frequencies
for healthy winding and transformers with shorted sections.
As can be seen, as the number of shorted sections increases
the number of resonant peaks decreases. The number of poles
and their frequencies provide an indication of the fault type.
TABLE II
POLE RESONANT FREQUENCIES (IN MHZ) FOR HEALTHY & FAULTY WINDINGS
Fig. 3. SFRA frequencies-poles for healthy winding
Pole Number
Healthy winding
1
0.1*10-7
2
1.5
3
2.7
4
3.6
5
4.1
1 Section is shorted
0.1*10-7
1.8
3.2
4.0
-
2 Sections are shorted
0.1*10-7
2.4
3.8
-
-
3 Sections are shorted
0.1*10-7
3.2
-
-
-
4 Sections are shorted
-7
-
-
-
-
0.1*10
D. Inspection of resonant frequencies and voltage
As can be seen in Fig. 5, the change in number of poles and
shift in their frequencies is an indicator of the health of a
transformer winding. The greater the number of shorted
sections, the fewer the number of poles and greater the
resonant frequency shifts. Note that the first pole’s frequency
is constant for all cases.
defined constant load. The model parameters are shown in
Table IV and the load parameters can be seen in Table V.
Fig. 5. Pole resonant frequencies for healthy and un-healthy windings
E. Number of poles
Table III indicates the number of poles for the different
conditions that have been simulated. Note that the change in
SFRA response is the same for shorting the relevant section
to ground (core) as intra-turn shorting.
TABLE III
HEALTHY AND UN-HEALTHY WINDING CONDITIONS
Fig. 6. Model of two windings transformer connected to a load
TABLE IV
POWER TRANSFORMER MODEL PARAMETERS
Components
Primary section resistance RP
1.2 Ω
Secondary section resistance Rs
Primary section inductance LP
Condition
Number
of Poles
1
Healthy winding
5
2
One section is shorted or section 4 to ground
4
Secondary section inductance LS
Primary and secondary series capacitance C S 1 , C S 2
3
Two sections are shorted or section 3 to ground
3
4
5
Three sections are shorted or section 2 to ground
Four sections are shorted or section 1 to ground
2
1
Primary and secondary shunt capacitance C g 1 , Cg 2
Cases
A. Healthy Transformer Model
A modified version of the transformer model set out in [13]
and used in [14] is used. This model is a one to one power
transformer having two interleaved windings of five sections
that contain twenty turns. Each section is represented by a
lumped resistance, an inductance and a shunt and a set of
series capacitances. Primary and secondary windings are
linked by mutual inductance that is dependent on the
transformer construction. The model modification considers
iron core instead of air core which increases the magnetic link
between the windings and gives a truer indication of a power
transformer construction. Insulation materials considered are
paper of thickness of 0.1 mm wrapped around the conductor
and oil to provide insulation and cooling. The construction of
the healthy model and the connection to the load is shown in
Figure 6. The system is simulated using 230 V, 50 Hz supply
on the primary and the secondary side is connected to a
7.2H
7.2H
0.0133nF
3nF
5nF
Capacitance between windings C w
TABLE V
LOAD PARAMETERS
III. ON-LINE FAULT DETECTION METHOD
In this case, internal fault detection is investigated by using the
measurement of both voltages and current in both transformer
windings for 50 Hz frequency and connected to constant load. Faults
between sections in one winding, between sections and ground and
between two windings were studied. The impact of the faults on the
primary and secondary voltages and currents is considered, to detect
internal short circuit using measurable parameters.
Values
1.2 Ω
Total impedance
Total reactance
Resistance
Inductance
Capacitance
Power factor
11309.7 Ω
6784.3Ω
9048.9 Ω
27.9 H
1.59µF
0.8
The simulation was run for both healthy and faulty
transformers and the primary and secondary voltage and
current were recorded. The measured parameters for healthy
transformer can be seen in Table VI.
B. Faulty Transformer model
Referring to the equivalent circuit of the transformer
winding, healthy winding impedances are constant. In an
unhealthy winding the values will vary from these, as is the
case when the transformer suffers from a short circuit or
mechanical deformation causing changes in winding
impedance. Due to the changes induced by the faults, it is
expected that the primary/secondary voltages/currents would
deviate from the operating values under known load
conditions. Interpreting the measurable signatures is to be
used to identify the un-healthy conditions of the transformer.
Figure 7 indicates the current flow during short circuit
between sections in each winding and between windings.
A short circuit occurring between nodes 2 and 3 is equivalent
to removing one section from the overall model and,
accordingly, the ratio of number of turns, the winding
impedances and mutual inductance will be different .
In theory shorting one section in the transformer primary
winding will have the same impact irrespective of its actual
location. Similarly, a short across one section of the
secondary will produce similar, but different, measurable
changes. Simulations were run for both healthy and damaged
transformers and measured values in both transformer sides
are taken to consider the relationships between transformer
health and measurable parameters. Table VI shows the
measured parameters for healthy and damaged transformers,
with the faults outlined in the following sections.
agreement with the work carried out in [15] and the
simulation conducted in [16].
No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
TABLE VI
MEASURED PARAMETERS FOR PRIMARY WINDING FAULTS
Vin
Iin
Vout
Case
(Volts)
(Amps)
(Volts)
Healthy Transformer
229.99
0.022
229.798
Section 1 to 2
229.99
9.599
229.809
Section 2 to 3
229.99
9.599
229.809
Section 3 to 4
229.99
9.599
229.809
Section 4 to 5
229.99
9.599
229.809
Section 4 to Ground
229.99
9.599
229.809
Section 1 to 3
229.99
25.572
229.803
Section 2 to 4
229.99
25.572
229.803
Section 3 to 5
229.99
25.572
229.803
Section 3 to Ground
229.99
25.572
229.803
Section 1 to 4
229.99
57.518
229.801
Section 2 to 5
229.99
57.518
229.801
Section 2 to Ground
229.99
57.518
229.801
Section 1 to 5
229.99
153.35
229.796
Section 1 to Ground
229.99
153.35
229.796
Iout
(Amps)
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
Fig. 8. Impact of shorted primary sections on primary current
Fig. 7. Short circuit current flow between sections and windings
C.
Windings Short Circuit Test
 Primary Winding Faults
The inter-turn short circuit faults that are expected to occur in this
winding and their locations have been investigated. Short circuit
between sections or between section and core occur because of
degradation of the insulation. As the model contains five sections,
the maximum number of shorted sections that can be considered is
four. In addition, possible short circuits from section to core have
also been considered. This makes a total of 14 possible faults in the
primary winding: the faults are listed, along with the simulation
output, in Table VI. To investigate whether the fault location has any
impact on the measurable voltages and currents, the primary and
secondary voltages and currents have been measured for the 14
cases and compared with the healthy values.
Table VI shows the different faults and their impact on the on the
measurable parameters. As can be observed the values of voltages
and currents are not affected by the location of the shorted sections,
but they are affected by the number of shorted sections. The greater
is the number of shorted sections the higher is the input current. This
can also be observed clearly in Fig. 8.
It should be noted that although the value of secondary
voltage varies with the number of shorted sections, in practice
the slight change would be difficult to detect. In consequence
the primary current is considered the fundamental variable for
detecting faults in the primary winding. This result is in
 Secondary Winding Faults
Simulation of short circuits in the secondary winding used
the same scenarios as were applied to the primary winding.
Table VII shows the 14 faults simulated and the impact of the
different faults on voltage and current values.
Again, it can be seen that, for a defined fault, location has no
effect on the measured values. However, as was discussed for
the primary winding faults, the greater the number of shorted
sections the greater the effect on the operational values.
In this case, the output voltage, the input current and the
output current are affected. Figure 9 indicates that the input
current increases and output voltage decreases as the number
of sections short circuited increases.
No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
16
TABLE VIII
MEASURED PARAMETERS FOR SECONDARY WINDING FAULTS
Vin
Iin
Vout
Iout
Case
(Volts)
(Amps)
(Volts)
(Amps)
Healthy Transformer
229.99
0.022
229.798
0.02
Section 1 to 2
229.99
6.396
153.25
0.013
Section 2 to 3
229.99
6.396
153.25
0.013
Section 3 to 4
229.99
6.396
153.25
0.013
Section 4 to 5
229.99
6.396
153.25
0.013
Section 4 to Ground
229.99
6.396
153.25
0.013
Section 1 to 3
229.99
10.955
98.532
0.008
Section 2 to 4
229.99
10.955
98.532
0.008
Section 3 to 5
229.99
10.955
98.532
0.008
Section 3 to Ground
229.99
10.955
98.532
0.008
Section 1 to 4
229.99
14.376
57.487
0.005
Section 2 to 5
229.99
14.376
57.487
0.005
Section 2 to Ground
229.99
14.376
57.487
0.005
Section 1 to 5
229.99
17.038
25.555
0.002
Section 1 to Ground
229.99
17.038
25.555
0.002
TABLE IX
MEASUREMENT PARAMETERS FOR SHORT CIRCUIT BETWEEN DISSIMILAR
LOCATIONS IN THE WINDINGS
Fig. 9. Impact of shorted secondary sections on secondary voltage and
primary current
D. Inter-winding Short Circuit Test
The probability of short circuit between primary and secondary
windings may be low compared to faults within the primary and/or
secondary sections, however, for some constructions it is possible.
As such, this type of fault will change the transformer operation
from magnetic to direct electrical coupling between windings,
because current will travel directly between the two windings.
Therefore, this kind of short circuit is different to the cases discussed
previously. The faults conditions between windings are divided, and
investigated, as follows: Similar sections short circuit
A short circuit taking place between any two sections having
similar locations in different windings, ( such as, section 1 primary
winding and section 1 in secondary winding, etc ), does not affect
the ratio of turns in the transformer. It can be seen that this type of
fault has no effect on the input and output currents and has minimal
effect on the output voltage. The small change in output voltage is
graphed in Figure 10: this change in voltage would be difficult to
measure in practice.
No
Fault
1
2
3
4
5
6
1P-2S
2P-3S
3P-4S
1S-2P
2S-3P
3S-4P
Vin
(Volts)
229.99
229.99
229.99
229.99
229.99
229.99
Iin
(Amps)
1.393
1.94
3.216
0.923
1.288
2.139
Vout
(Volts)
279.2
275.86
268.144
186.152
183.937
178.804
Iout
(Amps)
0.024
0.024
0.024
0.016
0.016
0.016
Figures 11 and 12 show the changes in input current and output
voltage respectively, indicating how a short circuit between
windings and locations affects the parameters.
Fig. 11. Primary current for short circuit between dissimilar primary and
secondary windings
Fig. 12. Secondary voltage for short circuit between dissimilar primary and
secondary windings

Fig. 10. Secondary voltage for short circuit between similar sections in
primary and secondary windings
 One section cross-over short circuit
Short circuit between dissimilar sections in primary and secondary
windings are considered, e.g. section 1 primary winding and section
2 secondary winding (1 P - 2 S) are connected by a current path, etc.
Cross-over faults between two un-like section locations will affect
the transformer turn ratio, due to the direct connection between the
two affected sections. The simulated values, shown in Table IX,
suggest that this type of fault does affect the measurable parameters
and that the changes in input and output might be used to identify
the fault. For the short circuits between higher voltage sections in
primary coil to lower sections in secondary coil (shown in yellow in
Table X), the input current and output voltage become higher, with a
slight increase in output current compared to the healthy state. For
the short circuits between higher voltage sections in secondary coil
to lower sections in primary coil (shown in orange in Table X), the
input current and output voltage are again affected in comparison to
the healthy state.
Short Circuit between dissimilar winding sections, 2 and 3
sections.
The simulation carried out for a short circuit which traversed two
and three sections in primary-secondary arrangement. Table X and
Table XI show the short circuit faults across two and three sections,
and the simulation results, respectively. The values in both Table X
and Table XI indicate that input current and output voltage are the
most significant indicators of a fault. The magnitudes of voltage and
current are significantly different from those in previous fault
conditions and could be helpful in identifying the fault.
TABLE X
MEASUREMENT PARAMETERS FOR SHORT CIRCUIT BETWEEN WINDINGS
ACROSS TWO SECTIONS
No
Fault
1
2
3
4
1P-3S
2P-4S
1S–3P
2S-4P
Vin
Iin
(Volts)
(Amps)
229.99
1.393
229.99
1.94
229.99
0.923
229.99
1.288
TABLE XI
Vout
(Volts)
279.2
275.86
186.152
183.937
Iout
(Amps)
0.024
0.024
0.016
0.016
MEASUREMENT PARAMETERS FOR SHORT CIRCUIT BETWEEN WINDINGS
ACROSS OVER THREE SECTIONS
No
Fault
1
2
1P-4S
4P-1S
Vin
(Volts)
229.99
229.99
Iin
(Amps)
34.541
8.627
Vout
(Volts)
367.751
91.979
Iout
(Amps)
0.032
0.00807
E. Summary of applying On-line method
Table XII summarizes the major and minor variables that could be
used to detect the type of short circuit that exists in a transformer.
TABLE XII
HEALTHY AND UNHEALTHY CONDITIONS OF POWER TRANSFORMER USING
ON-LINE METHOD
Case
High priority indicators Low priority indicators
Healthy transformer
Primary and secondary
voltage and current
Primary winding
Primary current
Secondary current
faults
Secondary winding
Secondary voltage and
Secondary current
faults
primary current
Short circuit between
Secondary Voltage
Primary and secondary
windings
current
IV. COMPARISON OF OFF-LINE AND ON-LINE METHODS
The possibility of moving from an off-line to an on-line method
has been proved. The results obtained for defined faults when the
on-line and the off-line methods are applied are summarized in
Table XIII. As can be seen, both methods are able to detect the
studied faults with same indication.
current are considered to be strong contenders to detect the
secondary winding faults and (iii) a short circuit between two
windings can be determined from the output voltage and the
input and output currents. It has been shown fault type is
more easily determined than the fault location as, for most
faults, location does not affect the measurable parameters
considered. However, in the case of a short circuit between
two windings, the measured values would give information
on fault location.
Measuring on-line voltages and currents can be used to
indicate faults that may occur within a winding and between
windings of a power transformer during normal operation.
This method also provides an ability to define fault severity
from external measurements. Comparison of both methods
proves the possibility of shifting from off-line to on-line
method for power transformer condition monitoring and fault
diagnosis.
VI. REFERENCE
[1]
[2]
TABLE XIII
COMPARISON BETWEEN OFF-LINE METHOD AND ON-LINE METHOD FOR ONE
TYPE OF FAULT DETECTION
No
Case
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
H-Transformer
Section 1 to 2
Section 2 to 3
Section 3 to 4
Section 4 to 5
Section 4 to G
Section 1 to 3
Section 2 to 4
Section 3 to 5
Section 3 to G
Section 1 to 4
Section 2 to 5
Section 2 to G
Section 1 to 5
Section 1 to G
Off-line faults
detection using
SFRA
Number of Poles
5
4
4
4
4
4
3
3
3
3
2
2
2
1
1
On-line faults detection
using measurement
parameters
Primary current A
0.022
9.599
9.599
9.599
9.599
9.599
25.572
25.572
25.572
25.572
57.518
57.518
57.518
153.352
153.352
V. CONCLUSION
In this paper, healthy and unhealthy conditions of a power
transformer are investigated using off-line and on-line
methods. SFRA is used for range of 0.1 Hz up to 5MHz that
was found suitable for the selected transformer parameters.
Each healthy section will give rise to one pole in the
frequency spectra, which means that the number of healthy
sections is indicated by number of poles. Shorted section
leads to a reduction of the number of poles according to the
sections that are out of circuit. It is not possible to identify
which sections are short circuited but it is possible to tell the
number of sections short circuited from the number of poles
present. For on-line method, the simulations indicate that: (i)
the primary current can be used as the main indication for
primary winding short circuits; (ii) output voltage and input
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
B. J. Small and A. Abu-Siada, “A new method for analysing
transformer condition using frequency response analysis,” in IEEE
Power and Energy Society General Meeting, 2011, pp. 1–5.
S. Ab Ghani, Y. M. Thayoob, Y. Ghazali, M. Khiar “Evaluation of
transformer core and winding conditions from SFRA measurement
results using statistical techniques for distribution transformers,” in
IEEE Int, PEOCO, no. 6-June 2012, pp. 448–453.
S. Rotby, A. El-Hag, M. Salama, and R. Bartnikas, “Partial discharge
detection and localization inside the winding of power transformer,” in
IEEE An Rep Conf on Ele Ins and Diel Phenomena, 2012, pp. 84–87.
K. Ramesh and M.Sushama, “Inter-Turn Fault Detection in Power
Transformer Using Fuzzy Logic,” in IEEE (ICSEMR 2014).
O.Gouda, A. El Dein, and I. Moukhtar “Turn-to-earth fault modelling
of power transformer based on symmetrical components,” IET Gener.
Transm. Distrib., ISSN 1751-8687. vol. 7, no. 7, 2013, pp. 709–716.
H. Afkar and A. Vahedi, “Detecting and locating tum to tum Fault on
layer winding of distribution transformer,” in The 5th IEEE
Conference on Thermal Power Plants (lPGC2014), Shahid Beheshti
University, Tehran, Iran.10-11 Jone,2014 . pp. 109-116.
X. Liu and M. Chen, “An on-line monitoring method for winding
deformation of power transformers,” IEEE 8th Conf. Ind. Electron.
Application, Jun. 2013, pp. 255–260.
N. Joshi, Y. Sood, R. Jarial, and R. Thapliyal, “Transformer Internal
Winding Faults Diagnosis Methods: A Review,” MIT Int. J. Electr.
Instrum. Eng. ISSN 2230-7656 Publ, vol. 2, no. 2, 2012. pp. 77–81,
R. Schwarz and M. Muhr, “Diagnostic methods for transformers,” Int.
Conf. Cond. Monit. Diagnosis. China, Apr 21-24, 2008, pp. 974–977.
A. Pandya, B. Parekh “Sweep Frequency Response Analysis (SFRA)
an Approach To Detect Hidden Transformer Fault,” Int Journal of Adv
Res in El,Electr and Inst Eng. vol. 2,5 May,2013 no. 5, pp. 1893–1896.
J. Kumar, U. Prasadr “Expert System for Sweep Frequency Response
Analysis of Transformer Using MATLAB,” Int Journal of Scie and
Res Pub, vol. 2, no. 12, 2012. pp. 1–13.
T. Mariprasath and V. Kirubakaran, “Power Transformer Faults
Identification using SFRA,” Int. Journal. Sci. Eng. Res. ISSN 22295518, vol. 5, no. 5 May, 2014. pp. 81–87.
L. Satish and S. K. Sahoo, “An effort to understand what factors affect
the transfer function of a two-winding transformer,” IEEE Trans.
Power Deliv, vol. 20, no. 2 April, 2005. pp. 1430–1440.
E. Aburaghiega, M. E. Farrag, and D. M. Hepburn “Investigation and
Understanding the Conditions of Power Transformer Internal Faults
using On-line Technique” in (ICREPQ’15), no. 13. Mar 25-27, 2015
G. Díaz, A. Barbon, J. aleixandre and J. Coto “Currnet sequence
analysis of transformer under Turn-to-Turn faults,” 14th PSCC,
Sevilla, June 24–28, 2002, session 03, Paper 6, page 6.
L. M. R. Oliveira and A. J. M. Cardoso, “Power transformers behavior
under the occurrence of inrush currents and turn-to-turn winding
insulation faults,” in IEEE 19th ICEM, Rome, 2010.