To detect the defects of transformer winding and confirm the causes, the data of two transformers of Beijing Electrical Power Company are analyzed. The cause is that there is quality problem in coil welding. Measures are put forward. The method of determining the defect is summarized.

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Analysis and Diagnosis of Typical Transformer DC
Resistance
Tu Mingtao, Yang Qinghua
HIMALAYAL - SHANGHAI - CHINA
Abstract: To detect the defects of transformer winding and confirm the causes,
the data of two transformers of Beijing Electrical Power Company are analyzed.
The cause is that there is quality problem in coil welding. Measures are put
forward. The method of determining the defect is summarized.
Key words: Transformer, DC resistance, defect, welding, detection, advice
Introduction
According to the Code for
Commissioning Test and Preventive
Test of Electrical Equipment, the
commissioning test should be carried
out prior to the running of power
transformer. The power transformer
is put into operation followed by
preventive test. The DC resistance of
power transformer is one of
important aspects, which reflect
physical property of transformer
winding. The abnormality of DC
resistance always indicates that the
coils are broken or poor partial
contact exist. The DC resistance tests
of one set of 250000kVA /220kV
transformer and one set of
50000kVA/110kV transformer are
analyzed in this paper.
1. Analysis of Defect in one set of
250000kVA/220kV Power
Transformer
1.1 250000kVA/220kV Power
Transformer Test
An preventive test is conducted on
Beijing Electric company’s #
3
transformer. It is found that the line
difference of low voltage coil exceeds
the limit and other electrical
indicators are normal. After one year
the DC resistance test is conducted
again and the line difference further
increases. The detailed data is listed in
Tab.1.
Tab.1 DC resistance of
250000kVA/220kV transformer low
voltage line
Rab&Rbc&Rca represents the DC
resistance of low voltage ab, bc and ca.
Ra, Rb and Rc represents the DC
resistance of low voltage phase-a, low
voltage phase-b and low voltage
phase-c.
From Tab.1, we can conclude that the

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difference of low voltage DC
resistance in phase increases,
exceeding the standard (＜2%). Rab and
Rca increase obviously compared with
the commissioning value, which
indicates that low voltage phase-a DC
resistance increase obviously. Based
on the formula (1)-(3), DC resistance
of low voltage phase can be calculated
by using the data shown in Tab.1. The
calculated data is shown in Tab.2.
Tab.2 DC resistance of #
3 transformer
low voltage phase
Ra = (Rca - Rp )-RabRbc/(Rca - Rp ) (1)
Rb =(Rab - Rp )-Rbc Rca/(Rab - Rp ) (2)
Rc =(Rbc - Rp )-RabRca/(Rbc - Rp ) (3)
In the formula, Rp= （Rab + Rbc + Rca）
/2.
The data of three tests are compared
in the Tab.2. It is assumed that the
phase-c DC resistance changes under
the influence of temperature, DC
resistances of both phase-a and b of
May 30, 2000 rise, up by 0.15m Ω for
phase-a and 0.08mΩ for phase-b.
From the test data of September 10,
2001, we can find that the DC
resistance of phase-a further goes up
by 0.28mΩ while that of phase-b does
not change basically. The comparison
indicates that the DC resistance
condition of phase-a further
deteriorates.
In order to figure out the problem of
low voltage DC resistance, the Beijing
Electrical Institute conducted the test
for confirming the causes and
position.
1.2 Transformer Defect Locating
After the removal of transformer oil,
all screw joints of low voltage coils are
fastened followed by the test. The test
data is shown in Tab.3.
Tab.3 DC resistance of joint low
voltage line after fastening
The comparison between Tab.1 and
Tab.3 indicates that there is no
obvious improvement in maximum
line difference pre-and-post fastening.
However, the difference between DC
resistance ab and bc decreases, which
shows that the DC resistance is
improved to a certain extent after
fastening all screws. The values of
each phase are shown in Tab.4.
Tab.4 DC resistance of joint low
voltage phase after fastening
The DC resistance of phase-b goes
down after the treatment while there
is no obvious change in that of
phase-a (not considering the effect of
temperature on the DC resistance).
The DC resistances of phase a and b
increase but the causes are different.
The joint screws loosen, causing DC
resistance of phase-b to go up while
the cause of phase a may be the defect
in the cold joints. To further figure out
the causes of phase-a defect, the low

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voltage Δ connection is open. The joint
at the position represented by the
letter y is unfastened shown in Fig.1
and the DC resistances of phase a and
b are measured. Ra = 4.129mΩ, Rb
=3.799mΩ.
Fig.1 Diagram of low voltage coil
structure
The DC resistance of phase-a (Fig.1
1-6 ) is tested. The six joints are
waggled in order. To facilitate the
comparison, the similar test is also
carried out for phase-b and its test
results are shown in Tab.5.
Tab.5 Low voltage phase DC
resistance of #
3 transformer
The Tab.5 data indicates that the DC
resistance of phase-a decreases when
waggling the No.1 and 3 joints. One
day later, the DC resistance restores to
the original value; there is no obvious
change in the DC resistance of phase-b
despite of waggling all joints. The
above-mentioned condition shows
that there is quality defect in phase-a
joints.
1.3 Treatment
It is confirmed that there is quality
defect in the low voltage joints of
transformer. The press plier is used to
weld the joints on site. The test data
after welding is basically the same as
Fig.5. The defect in phase-a is not
corrected. Hence, the current jolt
should be replaced by the new one.
The test data after treatment is shown
in Tab.6
Tab.6 DC resistance of #
3 transformer
low voltage line
The comparison between the results
of this test and that of commissioning
test indicates that the law of low
voltage DC resistance is different. For
further analyzing the causes of
difference, the line DC resistance is
converted into single-phase DC
resistance. The data is specified in
Tab.7.
Tab.7 DC resistance of No.3
transformer low voltage phase after
replacing the joint
The DC resistance values of all phases
in the Tab.4 and 7 are compared. The
impact of temperature on the DC
resistance is excluded. It is found that
there is no change in the DC
resistances of phase b and c while the

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phase-a DC resistance decreases.
There is serious defect in the phase-a
coil before installation.
2. Analysis of Defect in One Set of
50000kVA/110kV Transformer
2.1 50000kVA/110kV Transformer
Test
Based on the above Tab.8, it is found
that DC resistance value of each tap is
12-14mΩ more than that of phase-B
and C, which shows that there is
something wrong with main part of
phase A.
Tab.8 DC resistance of #
2 transformer
coil
To figure out the causes, the No.9
position of phase A are tested
repeatedly. The DC resistance value of
one time is 424.4mΩ and the earlier
one is 428.7mΩ. It is speculated that
the decline is caused by the
temperature change. But the change
in the tests of phase B and C is not
obvious, which indicates that the DC
value of phase A is not stable. The
workers knock the shield and outlet
wires outside the transformer coil on
site. As a result, the DC resistance of
phase A changes obviously when the
outlet wires are knocked. At last, the
value stabilizes at 405.9mΩ. Nothing
abnormal is detected at the welding
points. The cause lies at the end close
to the outlet wire within the coil.
2.2 50000kVA/110kV transformer
return-to-factory inspection
First of all, the test is performed on
the transformer coil and test results
are shown in Tab.9. Through the
analysis, it is concluded that the DC
resistance of phase A (RdA) increases
as the testing current decreases.
However, the DC resistances of phase
B and C do not change when the
testing current varies. Hence, it is
confirmed that the cause is that there
is weld defect within the phase A coil.
The phase-A coil is composed of upper
half a packet and lower half a packet
in parallel, which is made of two
strands in parallel. The upper and
lower half a packet are unfolded. Its
DC resistance is measured and the test
data is shown in Tab.10.
No.9 DC resistance of #
2 transformer
coil
Item RdA/m
Ω
RdB/
mΩ
RdC/
mΩ
Imba
lance
rate/
%
Meas
urin
g
curr
ent/
A
20 358.0 357.
0
355.3 0.76
10 360.2 357.
1
355.3 1.37
5 363.2 356.
8
355.1 2.26
No.10 DC resistance of #
2 transformer
phase A coil
From the Tab.10, we can see that there
is obvious difference between the DC

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resistance of lower half a packet #
1
coil and that of #
2 coil. In conclusion,
there is a problem with welding
quality of lower half a packet #
1 coil.
After the problem is identified,
phase-A coil is pulled out and the DC
resistance of phase-A lower half a
packet is measured. The DC resistance
of #
1 coil is not stable, a wood
hammer is used to knock the first
welding point to measure the open
circuit. It is found that the welding
point snaps fully.
To further analyze the causes of
snapped welds, factory test data and
semi-finished product test are
analyzed. The data is shown in Tab.11.
No.11 DC resistance of #
2
semi-finished transformer coil
In the semi-finished product test of
phase-A coil, the DC resistance values
of phase-A upper and lower half a
packet are abnormal but sufficient
attention is not paid to; in the product
test, the parallel of upper and lower
half a packet decreases the difference
among phase A, B and C, covering the
defect. Hence, the defect in the
phase-A weld exists during the
manufacturing process of coil.
3. Conclusions
a) The analysis of phase DC resistance
is more direct than line DC resistance.
b) After measuring value of DC
resistance stabilizes, the problem part
can be knocked or waggled and
measuring values are observed. If the
obvious change is found, there is a
defect in that part.
c) When there is defect in the coil
contact, the DC resistance value can
change according to the test current.
d) The sufficient attention should be
paid to the small change of DC
resistance in the future work.