2. Routine & Type
tests of power
transformer as
per relevant
standard
TRANSFORMER TESTING DEPT
3. Measurement of Insulation Resistance
Measurement of Winding Resistance
Measurement of Turn Ratio
Measurement of Vector Group
Measurement of No Load Loss and no Load Current
Measurement of Load Loss and %Impedance
Di-electric Breakdown Test of Transformer Oil
Separate Source Voltage Withstand Test
Induced Over Voltage Withstand Test
Test of OLTC
TRANSFORMER ROUTINE TESTS
4. Temperature rise test
Lightning impulse voltage withstand test
Switching impulse voltage withstand test
Determination of sound level
Partial discharge measurement.
Tanδ measurement of winding & bushing.
TRANSFORMER TYPE & SPECIAL TESTS
5. The purpose of the measurement is to determine the leakage
current of the insulation resistance. This current is changing
with the moisture, impurity contents and temperature of the
insulation.
MEASUREMENT OF INSULATION RESISTANCE
The insulation resistance is measured by
means of an High Voltage Digital Insulation
Resistance Meter which apply a voltage
2500 V dc or 5000 V dc. Each winding is
measured separately by connecting the
voltage between the winding to be tested
and earth. While the other windings are
connected to the ground. The temperature
and humidity are recorded during the test. Digital Insulation
Resistance Meter
by Kyoritsu
6. MEASUREMENT OF INSULATION RESISTANCE
Insulation of shorted terminals are usually measured as
Insulation between HT terminals and ground
Insulation between LT terminals and ground
Insulation between HT terminals and LT terminals
Terminals not under test are shorted to ground
Connection for insulation resistance measurement
7. MEASUREMENT OF WINDING RESISTANCE
The Winding Resistance is the resistance of the HT/LT coils and it
causes the resistive losses in the form of heat.
Winding Resistance is measured with the basic ides of injecting
a DC current and measuring the voltage drop across the
winding.
Concept of winding resistance measurement
8. MEASUREMENT OF WINDING RESISTANCE
The resistance of HT/LT winding is
measured with a Digital Winding
Resistance Meter.
By the device, a DC current is injected
by the “Test Current” terminals and the
resistance of a coil is measured by
their Voltage Drops and this are being
read by the Channel I and II. The
current to be injected and the
measuring resistance range can be
varied by the adjusting knobs of the
device. The injected current and
resistance can be measured directly
from the digital meters “M1” and “M2”.
Winding resistance measuring
meter
9. Measurement of Winding Resistance:
Top Oil Temperature: 31 ºC
Average phase resistance in LT side: 13.37 m
ohm
Average phase resistance in HT side : 426.36 m
ohm
Sample data & calculation
10. Temperature Factor
Resistive Loss ambient at LT
Resistive Loss ambient at HT .
Total Resistive Loss ambient 28.287 kw
+33.409 kw = 61.696 kw
Sample data & calculation
11. The voltage ratio or the turn ratio of the transformer is the
ratio of voltage at no-load.
Purpose of the measurement is the verification of no-load
voltage ratios specified by the specification and detection
of any problem within the coils or tapping connections.
Measurements are carried out on all taps and on all
phases.
MEASUREMENT OF TURN RATIO
12. Turn ratio is measured by
Digital Turn Ratio Meter, in
which the vector group of
the transformer is an input
and the output is the three
ratios for a specific tap.
MEASUREMENT OF TURN RATIO
Connection for voltage ratio
measurement
Turn Ratio Meter
by Vanguard
13. a)For Delta Wye transformer,
Turn ratio : (HT Voltage* 3)/LT voltage
√
b)For Wye Delta transformer,
Turn ratio : HT Voltage/(LT voltage* 3)
√
c)For Delta Delta transformer,
Turn ratio : HT Voltage/LT voltage
d)For Wye Wye transformer,
Turn ratio : HT Voltage/LT voltage
MEASUREMENT OF TURN
RATIO
14. Vector Group of a transformer is a representation of the
coil interconnection and coil alignment on the core.
The notation ‘Dyn11’ Vector Group indicates
MEASUREMENT OF VECTOR GROUP
15. MEASUREMENT OF VECTOR GROUP
Dyn11 Vector group defines that
Va-n is aligned with VA-B,
Vb-n is aligned with VB-C and
Vc-n is aligned with VC-A.
This means LT coil a-n is on the
same limb of the core as HT coil
A-B and so on.
16. MEASUREMENT OF VECTOR GROUP
Due to Vector summation, in Dyn11 grouped transformers, if
Terminal C and c is shorted and HT or LT side is supplied with
three phase voltage, the measured voltage VB-a will be higher
than VA-a = VA-b = VB-b .
In testing, terminal C and c is shorted, three phase voltage is
supplied to HT side and voltage between A and a, A and b, B and
a, B and b are measured and compared.
Connection for vector group
measurement
18. The no-load test or open circuit test determines the losses
the transformer carries out even when it is not supplying
any load.
The following losses occur at no-load :
◦ Iron losses in the transformer core and other metal
parts (main component of no-load loss).
◦ Dielectric losses in the insulation
◦ Load losses caused by the no-load current.
The no load test or the open circuit test is performed when
one of the winding is open (usually the H.V winding) while
the other winding is supplied with rated voltage at rated
frequency. Then the no-load losses ( P0) and the no-load
current ( I0) are measured.
MEASUREMENT OF NO LOAD LOSS
AND CURRENT
19. 1. Magnetization component Iµ
2. Loss component Iw
Magnetization component of no load current is
responsible to generate flux.
Loss component of no load current is responsible
for Hysteresis loss & eddy current loss.
Component of no load
current
20. MEASUREMENT OF NO LOAD LOSS
AND CURRENT
Connection for no load loss and current measurement
21. Short-circuit impedance voltage
is an important parameter
specially for the parallel
operation of the transformers,
where as load losses or the
short-circuit losses are important
from economical point of view.
The main components of full
load loss are:
◦ The resistive loss of the
windings and
◦ The stray loss
MEASUREMENT OF LOAD LOSS
AND IMPEDANCE
Digital Power Analyzer
23. MEASUREMENT OF LOAD LOSS
AND IMPEDANCE
Connection for load loss and impedance measurement
24. Let: A 40 MVA 132/11 KV Dyn1 transformer %z= 13 % is considered .
Here 13% of Impedance means that
If 132000 X 13/100 = 17160 V ph-ph is applied from 132 KV side
while 11 kv side short circuited then rated current will flow from
both side.( Tap position on have to be on principal) which are 175 A
& 2100 A respectively.
Again if 11000 X 13/100 = 1430 V is applied from 11 KV side while
132 KV side short circuited then rated current will flow from both
side.( Tap position on have to be on principal)
Here 171760 V and 1430 V are impedance voltage which are
commonly expressed as percentage to avoid confusion.
PERCENTAGE IMPEDANCE
25. For Load Loss at 75ºC:
For %Impedance at 75ºC:
MEASUREMENT OF LOAD LOSS
AND IMPEDANCE (FORMULAE)
26. Calculation of Load Loss at 75°C:
At Ambient Temperature = 30°C
At Standard Temperature = 75°C
Calculation of %Impedance at 75°C:
1
0
MEASUREMENT OF LOAD LOSS
AND IMPEDANCE (CALCULATION)
27. Transformer oil is used in a oil type transformer for
cooling and better insulation.
Breakdown Voltage is of an insulator is the voltage below
which, the material is an insulator but at and above this
voltage, the insulator looses its insulation property and
passes current through it.
Breakdown test of transformer oil ensures that the oil
used is in well condition and would work as insulation as
designed.
If the breakdown voltage is in unacceptable range, the
transformers risks failure at operation.
DI-ELECTRIC BREAKDOWN TEST
OF TRANSFORMER OIL
28. DI-ELECTRIC BREAKDOWN TEST
OF TRANSFORMER OIL
The test is performed with a digital Oil Test Set. Two electrodes
are placed with 2.5mm distance in the test oil. The voltage
between the electrodes are increased gradually until breakdown
occurs.
The breakdown value more than 32KV with 2.5mm distance is
acceptable for transformer oil.
Process of breakdown voltage of oil measurement
29. The purpose of the Separate Source Voltage Withstand
Test or High Voltage Test is testing the insulation between
the windings and the insulation between windings and
earth parts (tank press iron, etc.) under the temporary and
switching over-voltage conditions which may be occurred
during the service.
The applied voltage is defined by IEC60076-3. For a
33/11kV transformer, the 33kV side is tested with 70kV
and the 11kV side is tested with 28kV voltages.
The test is performed at rated frequency
The test duration is one minute.
SEPARATE SOURCE VOLTAGE
WITHSTAND TEST
30. SEPARATE SOURCE VOLTAGE
WITHSTAND TEST
In the test, one side of terminals are shorted
and supplied with the high voltage while the
other side terminals are shorted to ground.
The test is successful if no collapse of the
test voltage occurs.
Connection for separate source voltage withstand test
31. The separate source AC voltage test shall be
made with single-phase alternating voltage as
nearly as possible on sine-wave form.
The peak value of voltage shall be measured.
The peak value divided by 2 shall be equal to
√
the test value.
The test is successful if no collapse of the test
voltage occurs.
SEPARATE SOURCE VOLTAGE
WITHSTAND TEST
34. A The purpose of the test is testing the insulation between the phase
windings, turns, coils, tapping leads and terminals, for non-uniformly
insulated windings also the insulation between these parts and earth,
under the temporary and switching over-voltages conditions to which
the transformer may be subjected during its life time.
Normally, the excitation voltage is applied to the terminals of the low
voltage winding while the terminals of the other windings are left
open .
Since the test voltage is much higher than the rated voltage, the
frequency of the test voltage is chosen at least two times greater than
the rated frequency without causing the over saturation in the core.
Test duration is dependent on frequency.
If no collapse in test voltage and abnormal increase in the current
occurs during the test, then the test said to be satisfactory.
INDUCED OVER VOLTAGE
WITHSTAND TEST
36. With the tap-changer fully assembled on the transformer the
following sequence of operations shall be performed without
failure:
a)with the transformer un-energized, eight
complete cycles of operation (a cycle of
operation goes from one end of the tapping
range to the other, and back again).
b)with the transformer un-energized, and with the
auxiliary voltage reduced to 85 % of its rated value, one
complete cycle of operation.
Test of On Load Tap Changer
37. c)with the transformer energized at rated
voltage and frequency at no load, one
complete cycle of operation.
d)with one winding short-circuited and, as far as
practicable, rated current in the tapped winding,
10 tap-change operations across the range of
two steps on each side from where a coarse or
reversing changeover selector operates, or
otherwise from the middle tapping.
Test of On Load Tap Changer
38. The purpose of this test is to verify the insulation integrity for
transient voltages, caused by atmospheric phenomena
(lightning).
LIGHTNING IMPULSE WITHSTAND TEST
39. Lightning Impulse voltages are mainly divided
into :
1)Full Wave Impulse and
2)Chopped Wave Impulse.
After striking of Impulse voltage in the power
system if flashover occurs, system will
experience “Chopped Impulse wave”
Whereas, after striking Lightning Impulse in the
power system if flashover does not occur, system
will experience “Full Impulse Wave”.
Types of Lightning Impulse
40. International standard for lightning impulse
voltage testing of transformer
stipulates a 1.2/50 µs wave shape with
following permissible tolerance
Front time T1 = 1.2 µs ± 30 %
Time to half value T2= 50 µs ± 20 %
Overshoot < 5%
Condition of Full Wave Lightning
Impulse wave shape
43. If an impulse voltage is applied to a piece of
insulation and if a flash over or puncture occurs
causing sudden collapse of the impulse
voltage, it is called a chopped impulse
voltage,
Chopped voltage wave is generated when a
protective spark gap at the input connection of
a transformer starts to operate with a relatively
low time delay (4-6 microseconds)
Chopped wave Lightning Impulse
44. Front time T1 = 1.2 µs ± 30 %
Chopped time Tc = 2-6 µs
Condition of Chopped Wave
Lightning Impulse wave shape
47. The switching impulse test is applied to confirm
the withstand of the transformer’s
insulation against excessive voltages occurring
during switching. During switching impulse
voltage test, the insulation between windings
and between winding and earth is checked.
Purpose of the Test
48. The polarity of the voltage is negative and the
voltage waveform should normally be T1/ Td/
Tz 100/200/1000 μS according to IEC 60076-3.
Front : T1 100 µS = 1,67 T
≥
90% value : Td 200 µS
≥
Time for cutting the axis : Tz 1000 µS
≥
Switching Impulse Voltage
Waveform :
49. The polarity of the voltage is negative
according to IEC 60076-3.
Due to over-saturation of the core during
switching impulse test, a few low amplitude,
reverse polarity (e.g. positive) impulses are
applied after each test impulse in order to reset
the transformer core to it’s starting condition
(Demagnetized). By this way, the next impulse
voltage waveform is applied.
Polarity of Switching impulse
51. Transformers are subject to a
temperature rise test during
Factory Acceptance Tests to verify
the calculated temperature rise.
The test is performed in two
steps. The "no-load" test results
in temperature rise due to the
hysteresis losses in the magnetic
core. The "short-circuit" test
results in temperature rise due to
the thermal losses on the
windings.
TEMPERATURE RISE TEST
52. In this test, we check whether the temperature-
rising limit of the transformer winding and oil
as per specification or not. In this test of the
transformer, we have to check oil temperature
rise as well as winding temperature rise limits
of a transformer.
Purposes of Temp. Rise test
54. Power supplied during the test is as follow:
PTest= Po+ PL
Here,
Po= No load loss
PL= Full load loss at reference temp.
The calculation method of winding temperature is,
Δ ur= (km+ T1)-(km+ T2)
Here, T1= room temperature at which cold resistance is measured
T2= ambient temperature at which voltage is switched off
R1= cold resistance
R2= hot resistance
km= 235 for Copper
= 225 for Aluminium
POWER SUPPLY DURING
TEMPERATURE RISE TEST
55. SAMPLE GRAPH OF OIL TEMPERATURE RISE
2:00
AM
2:30
AM
3:00
AM
3:30
AM
4:00
AM
4:30
AM
5:00
AM
5:30
AM
6:00
AM
6:30
AM
7:00
AM
7:30
AM
8:00
AM
8:30
AM
9:00
AM
9:30
AM
10:00
AM
10:30
AM
11:00
AM
25
35
45
55
65
75
Oil Temperature Rise Test for 225/300 MVA,
230/132/33 kV Power Transformer
Time in Hour
Oil
Temperature
(°C)
56. -100 0 100 200 300 400 500 600
124.5
125
125.5
126
126.5
127
127.5
128
127.5
127.13
126.8
126.56
126.4
126.26
126.13
126.01
125.89
125.77
f(x) = 1.74242424242418E-05 x² 0.0187055555555551 x + 130.821808080808
−
Common Winding Resistance (mΩ) Vs. Time (seconds) after shut
down
Time in seconds after shut down
LT
Winding
Resistance
(mΩ
SAMPLE GRAPH OF WINDING
TEMPERATURE RISE
57. As per IEC 60076-2
Temperature rise test of transformer should be
continued until the top oil temperature has
reached an approximate steady value that
means testing would be continued until the
temperature increment of the top oil becomes
less than 1o
C in one hour & will be continued 3
hours. This steady value of top oil is
determined as final temperature rise of
transformer insulating oil.
STEADY STATE CONDITION OF
TEMPERATURE RISE TEST
