CHARACTERISTICS
WINDING
OF TRANSFORMER
FAULTS
PARAMETERS
BASED ON EXPERIMENTAL
Peter Palmer-Buckle
Student Member, IEEE
Karen L. Butler
Member, IEEE
DURING
INTERNAL
MEASUREMENTS
N. D. R. Sarma
Member, IEEE
Power System Automation Laborato~
Department of Electrical Engineering
Texas A&M Universi~
College Station, TX 77840-3128
This paper describes in detail, field experiments
performed on a single-phase, distribution transformer to
study the behavior of transformer terminal parameters during
internal winding faults. A custom-built transformer provided
with external taps was used for these tests. The taps were
used to stage various internal winding faults in the
transformer. Terminal values of voltages and currents were
monitored and the results presented. A comparison of these
results with simulation results is also presented.
Abstract:
Keywords: distribution transformers, windings, internal winding
faults, fault detection, fault diagnosis, incipient faults.
I.
INTRODUCTION
Internal winding faults comprise about seventy to eighty
percent of modern transformer breakdown [1] and this is
likely to increase since loading transformers to their optimum
capacity is becoming normal practice. These winding faults
are a result of the degradation of the transformer winding due
to aging, high voltages, etc., which tend to cause a
breakdown in the dielectric strength of the insulation. This
breakdown either causes adjacent windings to short (turn-totum) or a winding to be shorted to ground (turn-to-earth).
Several techniques have been developed for the detection and
diagnosis of these faults with a large proportion using the
gases dissolved in the transformer oil (dissolved gas analysis)
[2, 3] or determination of the degree of polymerization [4] of
paper insulation. Other parameters used are temperature,
thermal and electrical stress of insulation, insulation aging
and overloading incidents.
This research investigates the viability of utilizing
electrical parameters (mainly terminal voltages and currents)
of a transformer in an on-line transformer incipient fault
detection method. The magnetizing current in a normal
transformer is about ten percent of the full load current.
During internal winding faults, depending on the location of
the fault, the magnetizing current increases rapidly, The
distribution of transformer current subsequent to an internal
electrical fault therefore differs entirely fi-om the distribution
of normal load or no-load currents and is governed mainly by
the internal reactance of the windings [5].
This paper presents results of experiments performed on a
custom-built, 25kVA, 60Hz, 7200V1240VI120V, singlephase transformer connected to a 25kW load. To prevent
damage to the transformer during faults due to high
circulating currents, the experiments were performed at a low
voltage level. The results presented in this paper are therefore
at the reduced voltage level. Future work would be
performed at the rated values. The transformer was provided
with taps on both the primary and secondary windings which
were used to stage the following cases of faults:
1. Turn-to-earth and turn-to-turn faults on the primary
windings.
2. Turn-to-earth and turn-to-turn faults on the secondasy
windings.
A variac capable of supplying up to 140V was used to supply
power to the primary side of the transformer and digital
meters were used to monitor the terminal voltages and
currents.
The recorded values of voltages and currents were used to
validate computer models compatible with the Alternative
Transients Program (ATP) [6] to simulate internal winding
faults of single-phase, distribution transformers. The models
used were based on those developed for three-phase, power
transformers by Bastard et al. [7].
II. COMPUTER SIMULATION MODELS
In ATP, a single-phase, two-winding transformer shown as a
T-circuit in fig. 1 is represented by 2X2 matrices of [R] and
[L].
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‘R]’E
:1‘L]=[$l
+
1,
(1)
?1
where the Ri represents the winding resistance, Li, the
winding inductance and A4~,the mutual inductance between
windings i andj. Subscript ‘1’ refers to the primary winding
and ’2’ the secondary winding.
11
R1
L,
L2
+
R,
na, i=
v,
‘b, ‘b
—
-
nmi=
4’
J’
I
R(?O
o
[R] =
o
0
[L] =
OR2 I
La
~.b
Mac
M.2’
Mb.
Lb
Mb.
Mb2
Mca
Mcb
L.
Mc2
LM2.
M2b
M2c
L2
III. FIELD TESTS
A.
EXPERIMENTAL SETUP
The experimental setup (shown in figure 4) comprises
mainly the transformer, primary and secondary panel boards
and a variable resistive load bank.
‘R]=l’NIL]=E::~l
‘2)
Figure (3) shows the case of a turn-to-tun fault on the
primary. This divides the winding into three sub-coils and as
by 4X4 matrices
(3b)
In both cases, the values in italics for the resistance matrices
are computed using proportionality while those of the
inductance matrices are computed using the rules of
proportionality, leakage and consistency as outlined in [7].
“R2” and “L2” are given by BCTRAN.
Fig. 2. SingIe-phase transformer with a turn-to-earth fault on primary.
in (3a) and (3 b), it is represented
—
(3a)
OORCO
—
BCTRAN, a supporting routine of ATP is used to derive a
linear [R] and [L] representation for the single-phase, twowinding transformer using open-circuit and short-circuit test
data at rated frequency.
To model internal faults, the [R] and [L] matrices are
revised where some of their elements are from the BCTRAN
output and the other elements computed using mathematical
equations modeling the faulted transformer as proposed by
Bastard et al [7].
A turn-to-earth fault on the primary (originally having N1
turns with Nz turns on the secondary) divides the winding
into two sub-coils as shown in figure (2).
In this case the faulted transformer is represented by 3X3
matrices of [R] and [L] shown in (2).
[R] and [L].
V2
0
OR bi)o
V*
Fig. 1. T-Circuit Representation of a Single-Phase, Two-Wkding
Trrmsfonner.
shown
+
Fig. 3. Single-phase transformer with a turn-to-turn fault on primary.
12
L.
.
L
NJ
T
+
v
12
—~
of
Fig. 4. Experimental setup for the field tests
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The transformer, shown in fig. 5, is custom-built based on the
following specifications:
Both panel boards (fig. 6) are made of G- 10 (epoxy), 0.5”
thick. Copper strips bolted to the panel board at 1.0”
separation are used as bus bars on the primary to facilitate
ease in connections while on the secondary side, mechanical
lugs are used due to the thickness of tihe cables. Instrument
transformers on both the primary and the secondary sides
reduce terminal voltages and currents to measurable levels.
B.
Fig. 5. Single-phase, two-winding transformer with taps
●
.
●
.
.
.
.
Power rating
Rated primary voltage
Rated seconda~ voltage
Rated primary current
Rated secondary current
Primary turns
Secondary turns
25kVA
7200V
1201240V
3.472A
208.4I1O4.4A
780
13/26
The transformer is provided with taps on both the primary
and the secondary sides. There are 12 taps on the primary and
10 taps total on the secondary. These taps are external to the
transformer as shown in figure (5). For ease of simulation of
internal winding faults, these taps are connected to panel
boards through cables. The cables used on the primary side
are 15kV class capable of withstanding not more than
10Arms of current and the secondary side cables are of the
600V class. They can withstand up to 400Arms.
REDUCED
VOLTAGE TEST PROCEDURE
The simulation revealed very high circulating currents in
the shorted windings. Currently we are investigating ways of
reducing these currents to levels that would not damage the
windings of the transformer. The results presented in this
paper therefore represent values obtained when the supplied
voltage was far less than the rated on the primary.
The primary was supplied using a single-phase variac
capable of supplying up to 140V, 22A. The variac was
connected to the primary panel board (fig. 7) and a resistive
load of 2.3040hms connected to the secondary side. Digital
meters were used to monitor primary and secondary voltages
and currents as well as circulating currents during faults. To
stage an internal fault, a tap is either connected to the ground
(turn-to-earth fault) or to another tap (turn-to-turn fault).
The voltage on the variac is varied sllowly till a voltage of
about 10OV is reached. The primary voltage was limited to
100 V due to high circulating currents flowing through the
shorted windings. This could damage the transformer. The
maximum allowable current that could flow through the
windings was calculated based on (4), supplied by the
transformer manufacturer. The terminal voltages and currents
as well as the circulating current were recorded.
n
“
“ =
Load
IL--+JJ
,
—
Fig. 7. Setup for reduced voltage tests.
The A ‘,s are ammeters, V’s are voltmeters, AC is a 120V,
60Hz mains, Vu is single-phase, 60Hz, 120V input, 140V
maximum output, variac, HI and H2 represent terminals of
the primary winding and XI and X.2 are the terminals of the
secondary winding.
Fig. 6. Primary and secondmy panei boards showing connectors.
AT= K .12. T~in,. (Q/#)
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(4)
Table 2. Terminal values of turn-to-turn tkadtson secondary.
where AT is the temperature rise in degrees Celsius,
K = 0.343 is a constant,
I is current in amps,
T is time in minutes and
Q/# is the ohms per pound of conductor.
E
Short
Normal
The maximum AT for the transformer was limited to 115°C.
Knowing the ohm per pound of the conductor used (which is
copper for the primary and aluminum for the secondary in
this case) and the time, the maximum current was calculated.
IV. RESULTS
A.
S16-17f
Tables 1 and 2 give recorded values of voltages and
currents for turn-to-turn faults on the primary and secondary
respectively. Both tables show an increase in primary current
as the number of turns shorted increase. The primary voltage
was kept almost constant at 100V. The secondary voltage in
Table 1 remained fairly constant as expected. Since the load
was purely resistive and maintained at 2.304 ohms, the
current flowing through the load was in direct proportion to
the secondary voltage. In Table 2, the secondary voltage
decreased as the number of shorted turns increased. This is
because as more turns are shorted, few effectively remain in
circuit.
Tables 3 and 4 give results of turn-to-earth faults. Like in
Tables 1 and 2, the primary current increases with increasing
shorted turns in both cases. The secondary voltages in this
case decrease with increasing shorted turns for turn-to-earth
fault on both windings.
All voltage and current values are rms.
99.8
3.165
0.14
1.37
s19-2of
2
99.8
3.166
0.12
1.28
s2-4f
3
100.2
2.987
0.41
1.32
S8-10f
3
100.2
3.021
0.39
1.37
slo-13f
4
100.0
2.682
0.68
1.21
S16-20f
4
100.1
2.390
1.11
1.04
5
99.9
2.333
0.92
0.97
S16-19f
5
100.4
2.665
0.82
1.34
S8-13f
6
100.0
2.076
1.20
0.91
S2-8f
7
100.0
1.933
1.30
0.86
s4-lof
7
100.1
1.931
1.32
0.91
s13-2of
8
100.3
1.622
1.54
0.65
s2-lof
9
100.0
1.450
1.55
0.58
s4-13f
10
100.4
1.190
1.75
0.59
s2-13f
12
100.4
0.970
1.88
0.36
E
s13-17f
FIELD TESTS
2
Table 3. Terminal values of turn-to-earth faults on primary.
m=
Short
Number Primary
Secondruy Pfimrrry
Table 1, Terminal values of turn-to-turn faults on primary.
Fault on primary
Table 4. Terminal values of turn-to-earth faults on secondary.
Secondary
Short I Number I Primary I Secondwy I Primary
Current
Turns [ Voltage I Voltage I Current
1.49
100.OI
3.3091
0.03 I
Normal I
01
P9-lof
21
99.8
3.326
0.38
1.46
P4-5f
100.2
3.316
0.51
1.46
P5-6f
28
25
100.C
3.320
0.53
1.53
P1-2f
41
100.C
3.234
0.78
1.49
P4-6f
5(
100.3
3.334
1.02
1.54
P6-7f
57
100.3
3.337
1.03
1.53
P5-7f
8$
100.1
3.336
1.61
1.53
P4-7f
112
100.4
3.348
2.29
1.53
P4-8f
22L
100.2
3.467
5.34
1.61
E
kRiiF=$=F=d
Short
B.
Number Primary
COMPARISON
RESULTS
Secondmy Primary
BETWEEN FIELD AND SIMULATION
Figures 8, 9 and 10 show a comparison between field and
simulation results of secondary voltage, primary current and
secondary
current
respectively.
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for
a turn-to-turn
fault
on the
primary
V. DISCUSSIONS AND CONCLUSION
taps shorted
IEiField ■ Simulation \
Fig. 8. Comparison between field and simulation results: secondmy
voltage, turn-to-turn fault on primary.
5.0
g
4.0
:
x
~2
?
3.0
2.0
1.0
0.0
taps shorted
H Field H Simulation
Fig. 9. Comparison between field and simulation results: primary
eurren~ turn-to-turn fault on primary.
2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
Results of field experiments carried out on a custom-built
transformer fitted with taps to simulate internal winding
faults are presented. Comparisons between field and
simulation results are also given. The terminal voltages and
currents behaved as the simulation. The slight discrepancies
could be due to the following factors:
●
During the field experiments, it was not easy to get the
exact primary voltage as used in the simulation.
●
Leakage factors were assumed to be negligible in some
cases while in others, they were assumed to be small.
This was for ease in calculation of the inductance
parameters using the method proposed in [7].
.
The cables used in the field for (connection to the tap
positions on the panel board were modeled as pure
resistors.
The primary current as stated earlier increased with
increasing number of shorted turns. In the case of faults on
the primary winding, as the shorted turns increase, the
effective number of turns across the primary decrease.
However, the primary voltage is maintained constant at
100V, This causes the magnetizing current and hence the
primary current to increase rapidly [5]. For faults on the
secondary side, the increase in primary current is not rapid.
The increased primary current is due to the high circulating
current in the shorted windings. This current flows in
opposition to the normal flow of current in the winding. The
effective flux which is dependent on the primary voltage is
reduced. More current must therefore be drawn from the
primary to bring the flux to the value proportional to the
primary voltage.
As expected, the secondary voltages were not significantly
affected when the faults were on the primary winding.
However, for faults on the secondary winding, the voltage
decreased with increasing shorted turns, This is due to the
decrease in effective number of turns across which the load is
connected, The secondary current as stated earlier is in direct
proportion to the secondary voltage in adlcases and hence has
the characteristics of the secondary voltage.
These experiments were performed to validate and if
necessary fine-tune the models used in the simulations. From
the results of the comparison plots, it can be concluded that
the models agree well with the actual transformer under
internal winding fault conditions. Future work would
investigate the dependence of the voltages and currents on
the fault position.
VI. ACKNOWLEDGMENT
taps shorted
❑
Field
.
The authors greatly acknowledge the National Science
Foundation through grant ECS-9522208 for their support of
this work.
Simulation
Fig. 10 Comparison of field and simulation results: secondary current, turnto-tum fault on primwy
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VI. REFERENCES
StigrmL A. C. Franklin, The .J&PTrarr<ormer
Book, New York Toronto, John Wiley and Sons, 1973.
[2]
“IEEE Guide for the Interpretation of Gases Generated in OilImmersed Transformers”, LE.E.EStandardC57.104-1991, New York
IEEE Press, 1992.
[3] Stebbins, D. Randy., et al., “Dissolved Gas Analysis of Transformers
Oil”, PanelSession, 1997 Winter Meeting, PES/IEEE, February 1997,
[1]
!&n,
D. M., “Practical Life-Assessment Techniques for Aged
Transformer Insulation”, IEEE Proceedings - A, Vol. 140, No.5
September, 1993, pp. 404-408.
[5] E. Billig, “Electrical and Mechanical Effects of Internal Faults in
Transformers”, The British Electrical and Allied Industries Research
Association, Technical Report Q/T103, London, 1944.
[6] Alternative Transients Program (ATP) Rule Book, Canadian/American
EMTP Users Group, 1987-1995.
[7] P. Bastard et al., “A Transformer Model for Winding Fault Studies”,
IEEE Transactions on Power Delivery, Vol. 9, No.2 April 1994, pp.
[4]
690-699.
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