Unit 1-2-b

Testing of Transformer
Preliminary Tests:
Following tests are carried out in the works at different
stages, before the core and the coil assembly is placed in its
tank .
1.Core insulation: After the core is assembled, 2kV test is
done to ensure that the insulation between clamp plates,
and core bolts to find core is adequate.
2.Core loss test: Some turns are wound over the core and it
is energized at normal flux density. Core loss and
magnetizing current are noted and compared with design
values.

3. Check of Ratio, Polarity, Vector relationship and winding
resistance of Transformer Assembly .
a. Conducted to check that all connections to the bushings, tap
changers etc. have been made correctly during final
assembly.
b. Conducted to ensure the correctness of voltage ratio
between different windings on each tapping.
c. The tolerance for ratio is ±0.5% of the declared ratio or ±10%
of the percentage impedance voltage.
d. In order to get accurate ratio, a ratiometer is employed .
e. It also indicates the polarity of transformer windings.
f. With the turns ratio tester, the turns ratio is directly read on
the tester for each tap and for each phase of the winding.

g. The turns ratio can also be tested by applying a single
phase ac voltage (approximately 230V) on the HV side and
measuring the voltage on the low voltage side at all tap
positions.
h. For a three phase transformer a vector relationship test is
also to be done.
i. The dc resistance of each winding is measured by Kelvin’s
double bridge to check that there is no faulty joint.
4. Preliminary Load loss and Impedance voltage
measurements to be done.

Final Tests
The completely assembled transformer is tested in accordance with
the International standards.
Routine tests:
1. Measurement of winding resistance.
2. Measurement of voltage ratio and check of voltage vector
relationship.
3. Measurement of impedance voltage (principal tapping), short
circuit impedance and load loss.
4. Measurement of no load loss and current.
5. Measurement of insulation resistance.
6. Dielectric tests. : Separate source ac voltage and Induced over
voltage.
7. Tests on-load tap changers.

Final Tests
Type Tests
In a batch only one transformer will undergo these tests.
1. The temperature rise test
2. Lightning impulse test
3. Air Pressure test
4. Permissible flux density and over fluxing test
5. Noise level test

Final Tests
Special Tests: Based on agreement between the manufacture and
Purchaser.
1. Dielectric special tests.
2. Measurement of zero-sequence impedance of three phase
transformers.(The zero-sequence is needed for earth-fault protection
and earth-fault current calculations.)
3. Short-circuit test.
4. Measurement of acoustic noise level.
5. Measurement of harmonics of the no load current.
6. Measurement of power taken by the fans and oil pumps.
7. Measurement of capacitances between windings to earth and
between windings.
8. Measurement of transferred surge voltage on low voltage windings.
9. Measurement of insulation resistance to earth of the windings, or
measurement of dissipation factor of the insulation system
capacitances.

Final Tests
Note:
• Special Tests and type tests are to be performed in the
presence of the purchaser or his representative.
• Commissioning tests are performed at site before
commissioning.

1.Measurement of Winding Resistance
Importance of test:
1. Check transformer windings and terminal connections.
2. Use as reference for future measurements.
3. Calculate the load loss values at reference (e.g. 750C)
temperature.
4. For calculation of I2R losses in the winding, it is necessary to
measure dc resistance of each winding.
5. Winding resistances are measured between all connection
terminals of windings and at all tap positions.
6. The resistance of each winding, the terminals between which it
is measured and the temperature of the windings shall be
recorded.

7. The measuring current can be obtained either from
a battery or from a constant(stable) current source
8. The measuring current value should be high enough
to obtain a correct and precise measurement and
small enough not to change the winding
temperature.
9. The resistance measurement should be done after
the direct current circulating in the winding has
reached a steady state.

11. In some cases, it may take some minutes depending upon the
winding inductance.
12. Temperature of the winding must be stable and for this
reason, this test is usually carried out before load loss
measurement.
13. The average oil temperature is determined as the mean of
top and bottom oil temperatures and is taken as average
winding temperature.
14. Normally winding resistance values 1 ohm or above is
measured using Wheatstone bridge and winding resistance
values less than 1 ohm is measured using micro-ohm meter
or Kelvin Bridge.

R2 : winding resistance at temperature t2
R1 : winding resistance at temperature t1

Refer Manual :Transformer Winding Resistance
Measurement.docx

2. Measurement of voltage ratio & check of
voltage group
 These tests are conducted to check that all connections to the
bushings, tap changers etc. have been made correctly during final
assembly.
By using ratiometer this test can be conducted.
Ratio test will confirms the polarity of the windings.
Test is conducted on every transformer for position of every tap.
Measurement of turn ratio is based on, applying a phase voltage
to one of the windings using a bridge (equipment) and measuring
the ratio of the induced voltage at the bridge.
The measurements are repeated in all phases and at all tap
positions, sequentially.

2. Measurement of voltage ratio and check of
voltage group
In general, the measuring voltage is 220 V A.C. 50 Hz. However,
equipments which have other voltage levels can also be used.
The theoretical no-load turn ratio of the transformer is adjusted
on the equipment by an adjustable transformer, it is changed until
a balance occurs on the % error indicator.
The value read on this error indicator shows the deviation of the
transformer from real turn ratio as %.
Voltage ratio & vector group test:volt ratio test transformer.docx

3.Measurement of Impedance Voltage & Load loss
 Short-circuit voltage is an important criteria especially during
parallel operations of the transformers .
 The short-circuit loss is a data which is also used in the heat
test.
 Load loss = I2R losses in the winding + stray losses due to eddy
currents in conductors, clamps and tank
 Stray losses vary with frequency
 Hence in this test, supply is given to the transformer at rated
frequency .
 The test is carried out by short circuiting, usually the LV
winding and by supplying the impedance voltage to HV
winding.
 The measured power will also include small core-loss .
 Since the supply voltage during the test is a small fraction of
normal voltage, this loss can be ignored.

 For a high impedance transformer ( Z > 15% ) the core loss may
become appreciable and can be deducted after separately
measuring it at impedance voltage
 As per IS: 2026(Part I) -1977, the measurements can be made at
any current between 25% to 100%, but preferably not less than
50% of the rated current
 Load loss and impedance voltage can be corrected for rated MVA
as below

While measuring load loss and impedance at different tap
positions, readings should be taken quickly.
 and the interval between measurements at different taps should
be adequate to avoid significant errors due to momentary
temperature rise of windings.
 The difference in temperature between the top oil and bottom
oil should preferably be small to enable the average temperature
to be determined accurately .
 Three wattmeter method should be used instead of two
wattmeter method to avoid large value of wattmeter multiplier
constant .
•The power factor during load loss test can be less than 0.1 and
wattmeters suitable for such low power factors should be used.

Impedance voltage at rated current (principal tapping):
If the principal tapping
corresponds with the mean
tapping position or with one of the
two middle tapping positions:
1)two-winding transformers ± 10 percent of the declared impedance
voltage for that tapping
2) multi-winding transformers ± 10 percent of the declared impedance
voltage for one specified pair of
windings
± 15 percent of the declared impedance
for a second specified pair of
windings

4.Measurement of no load loss & current
Importance of the test
To assess the efficiency of transformer
Also as a check that high voltage tests have not caused any
damage to the winding insulation
For large transformers, no-load loss measurement is carried out
before and after completion of dielectric tests
Test is carried out at rated frequency feeding usually LV winding
I2R will be negligible
Power factor for medium power transformers - around 0.3
Power factor for large power transformers - around 0.1
Core losses = hysteresis losses + eddy current losses
 Hysteresis losses dependent on average value of voltage.
Eddy current losses dependent on rms value of voltage.
In this test, two voltmeters are used
•(1) bridge rectifier type to indicate average voltage
•(2) dynamometer type to indicate rms voltage

Supply is set such that specified value is indicated on the average
voltmeter.
Hysteresis component of no load loss is measured correctly .
No load losses can be measured from the L.V. side using an
adjustable three phase voltage source with neutral.
The voltage and frequency should be steady and at rated values
and as near as possible to 50 Hz and it should be measured.
This test can give a basic value near rated conditions if all
precautions are taken.
The L.V. side is energised at the rated tap at rated voltage and
power is measured by three watt meters or 3 phase, 4 wire single
wattmeter/energy meter

Eddy current component will be either lower or higher than the true value,
depending upon the form factor of the supply voltage
Ratio between two components is known for any particular quality of core
steel
Where, P = no-load loss for sinusoidal voltage
Pm = the measured no-load loss
P1 = ratio of hysteresis losses to total iron losses
P2 = ratio of eddy current losses to total iron losses
K = ( rms voltage / ( 1.11*average voltage) )2
For normal flux densities and frequencies of 50Hz and 60Hz, P1 and P2 are each
0.5 for grain oriented steel
•P1 and P2 are 0.7 and 0.3 , respectively, for non-grain oriented steel

4.Measurement of no load loss
and current

5.Measurement of Insulation
Resistance
 Measured between two parts separated by
insulation.
Resistance between conducting part and earth.
Instrument used is Megger.
The ‘Megger’ consists of a D.C power source (hand
operated or electrically driven D.C generator or a
battery source with electronic circuit ) and a measuring
system.
Microprocessor based insulation testers are also now
available.

5.Measurement of Insulation
Resistance(IR)
 The insulation test reveals the condition of the
insulation inside the transformer.
The insulation resistance values are affected by
temperature, humidity and presence of dirt on
insulators and bushings.
oil temperature is also recorded
IR at 15th second and 60th second are noted for
determining Polarization Index

Factors influencing Insulation
Resistance (IR) value
1. Surface condition of the terminal bushing
2. Quality of oil
3. Quality of winding insulation
4. Temperature of oil
5. Duration of application and value of test voltage

IR values are monitored in
Transformers
1. Winding to ground
 HV to LV and earth connected together
 LV to HV and earth.
2. Winding to winding. Eg. HV to LV
3. All windings to ground .Eg. HV and LV to earth.
 By comparing the results obtained in insulation
resistance measurements with periodical measurements,
the insulation conditions can be evaluated.
 For comparison they have to be at the same
temperature

IR values are monitored in
Transformers

Steps for measuring the IR
1. Shut down the transformer and disconnect the jumpers
and lightning arrestors.
2. Discharge the winding capacitance.
3. Thoroughly clean all bushings
4. Short circuit the windings.
5. Guard the terminals to eliminate surface leakage over
terminal bushings.
6. Record the temperature.
7. Connect the test leads (avoid joints).
8. Apply the test voltage and note the reading.
The IR. value at 60 seconds after application of the test
voltage is referred to as the Insulation Resistance of the
transformer at the test temperature.

Steps for measuring the IR
IR values recorded over a period of time may be
plotted as a curve to study the history of the
insulation resistance
A curve showing a downward trend indicates a
loss of IR due to unfavourable conditions such as oil
deterioration, excessive moisture in paper,
deterioration/damage to terminal bushings etc.
 A very sharp drop is a cause for concern and
action shall be taken to ascertain the exact cause of
insulation failure and for corrective steps.

Polarization Index
 Tells true idea about insulation quality and extent of dryness.
As the insulation draws capacitive currents in the beginning
current is more & within 15 secs insulation resistance measured
by megger.
Repeat the same for 60 secs. Take the reading.
IoR60 / IoR15 = polarization index
Dependence on which temperature insulation resistance is
measured.
Polarization index should be greater than 1
Reason for low value of Insulation
1. Dust & dirt sticking on the surface of the insulation.
2. Absorption of moisture by insulation & insulating oil.

6.Dielectric Tests
1) Separate source voltage withstand test:
Purpose – to verify the power frequency withstand
strength of the winding under test to earth and other
windings
line terminals of windings under test are connected
together and appropriate test voltage is applied to them
Other windings and tank are connected together to
the earth

Rated withstand voltages for transformer windings with highest
voltage for equipment Um up to and including 420kV

6.Dielectric Tests
 Windings with graded insulation, which have
neutral intended for direct earthing, are tested at
38kV
 Supply voltage should be nearly sinusoidal
The peak value of the voltage is measured
 Digital peak voltmeter associated with capacitive
voltage divider is used
 Peak value divided by √2 shall be equal to the test
value
 Duration of application of test voltage is 60s

Procedure
 The test shall be commenced at a voltage not
greater than one-third of the specified test value and
shall be increased to this value as rapidly as is
consistent with measurements.
 At the end of the test, the voltage shall be reduced
rapidly to less than one-third of the test value before
switching off
The full test voltage shall be applied for 60s
The test shall be successful if no collapse of the
test voltage occurs

Example:
HV high voltage test : LV winding connected together
and earthed. HV winding connected together and
given 28 KV ( for 11KV transformer) for 1 minute.
LV high Voltage test : HV winding connected together
and earthed. LV winding connected together and
given 3 KV for 1 minute.
Equipment used : High Voltage tester ( 100KV & 3KV)

6.Dielectric Tests
(2) Induced over-voltage withstand test:
Purpose : to verify the power frequency withstand strength
along the winding under test, between its phases, and to
earth and other windings
Transformers with uniformly insulated windings, the test
voltage is twice the corresponding rated voltage
For test voltages up to 66kV, three-phase transformers are
generally supplied direct from a three-phase source
For higher values, each HV terminal in turn is raised to a
specified test voltage, by applying single-phase voltage to
each winding

6.Dielectric Tests
For higher values, each HV terminal in turn is raised to a
specified test voltage, by applying single-phase voltage to
each winding
Supply frequency higher than normal is used to avoid core
saturation
When the frequency is chosen is in the range of 150 to
240Hz, capacitive currents may cause overloading of the
generator set
Reactor is used
Frequency must be selected properly to avoid series
resonance
Test duration in seconds = (120*Rated frequency)/Test
frequency

6.Dielectric Tests
This test checks the inter turn insulation
For a 11KV/433V transformer,866 Volts are applied
at the 433V winding with the help of a Generator
for 1 minute. This induces 22KV on 11KV side. The
frequency of the 866V supply is also increased to
100HZ.
Equipment used : MOTOR GENERATOR SET

Type Tests
(i) Temperature rise test
Purpose: to confirm that under normal
conditions, the temperature rise of the windings
and the oil will not exceed the specified limit
Temperature rises are measured above the
temperature of the cooling air
In water cooled transformers, the temperature
rise is measured above the inlet water
temperature

Temperature rise for top oil
LV winding is short circuited
Voltage is applied to HV winding such that power input =
no load loss + load loss corrected to a reference
temperature of 75oC
Measurement by three wattmeter method
Losses are maintained constant until the top oil rise has
reached a steady state
Hourly readings of the top oil temperatures are taken
Thermometer placed in a pocket in the transformer top
cover
Temperature at inlet and outlet of the cooler bank is also
taken hourly and mean oil temperature is determined
Ambient temperature is measured

Duration of temperature rise test
Method (a) : Test should not be regarded as
complete until the temperature rise increment of top
oil is less than 3oC in 1 hour

Duration of temperature rise test
Method (b): It should be demonstrated that the top oil
temperature rise does not vary more than 1oC per hour
during four consecutive hourly readings.
The last reading is taken for the determination of the
final oil temperature rise.

Winding temperature rise
When top oil temperature rise is established, the
current is reduced its rated value
It is maintained for 1 hour to allow the winding to
attain its normal temperature
At the end of the test, supply is switched off
Cooling fans or water pumps should be stopped
Oil pumps should remain running
Short circuit connection is removed

Winding temperature rise
Hot resistance of winding is measured by KDB
Time elapse between switching off of power supply
and taking the first reading
Resistances are measured at intervals over a period
of about 15min
Graph of hot resistance versus time is plotted
From the extrapolated value, winding resistance at
the instant of shut-down can be measured

Impulse Testing
Lightning – common cause of flashover on overhead
transmission lines
(1) Lightning stroke makes direct contact with a
phase conductor and produces a voltage in excess of
impulse voltage level
(2) stroke makes contact with an earth wire or tower
-> back flashover
Terminal equipment experience lightning impulses
in service
Measure of effectiveness of insulation strength is
dielectric strength

Impulse Testing
Power frequency tests alone are not adequate to
check dielectric strength
Distribution of impulse voltage stress along the
winding is non-linear
Necessary to ensure adequate dielectric strength of
transformers for impulse conditions
Switching impulses occur during switching operations
in the system
Magnitude and waveform of impulses differ from case
to case
Maximum voltage can be about 3.5 times the service
voltage
Switching impulse tests are worthwhile for
transformers with greatly reduced insulation level

Lightning Impulses
System disturbances from lightning can be represented
by three basic wave shapes
(1) Full wave 1.2/50 wave
(2) chopped wave
(3) front of waves

Lightning Impulses
These three waves – different in duration and in rate
of rise and decay of voltage
 Full wave - relatively long duration, cause major
oscillations to develop in the windings, produce stresses
in turn-to-turn and section-to-section insulation, stresses
across large portions of winding, between windings to
ground
Chopped wave – higher turn-to-turn and section-to-
section stresses due to rapid change in voltage
Front of wave – high turn-to-turn and section-to-
section voltages

Switching Impulse
 Front time – several hundred microseconds
Periodically damped or oscillating one
IEC-60060 - 250/2500
Test at negative polarity
SI – do not cause non-uniform voltage distribution
along the winding
They affect the insulation to ground, between
windings and phases and the external insulation of the
bushings
Open circuited LV windings are subjected to impulse
voltage stresses
Separate test on LV winding is not essential

Generation of Impulses & Test circuit


Lightning Impulse Test Circuit
Basic Impulse Generator circuit
Voltage measuring circuit
Chopping circuit, where applicable
Standards - 1.2μs/50μs
Tolerances - ±3% on peak value, ±30% on front time,
±20% on time to half value

Lightning Impulse Test Circuit

Test with transferred voltages

Testing of HV windings of large transformers,
Difficult to obtain wave front due to large capacitance of
transformer, inherent self-inductance of the generator and
the leads connecting the generator and transformer
On LV windings, difficult to obtain specified wave tail due
to extremely low impedance of these windings
 Effective impedance of transformer controls the wave tail
duration
Effective impedance can be varied by different terminal
impedances at the non-impulsed winding terminals
If these are left isolated, effective impedance will be
maximum

They cannot be left isolated because high voltages
are induced in these windings
Voltage on terminals that are not being tested is
limited to 75% of BIL
Non-impulsed terminals of windings are earthed
directly or through resistors( max 400Ω)

Full wave test sequence
It consists of application of a reduced full-wave and three full
waves
The impulse test-sequence is applied to each of the line
terminals of the tested winding in succession.
In the case of a three-phase transformer, the other line
terminals of the winding shall be earthed directly or through a
low impedance, such as a current measuring shunt.
If the winding has a neutral terminal, the neutral shall be
earthed directly or through a low impedance, such as a current
measuring shunt.
The tank shall be earthed.
In the case of a separate-winding transformer, terminals of
windings not under test are likewise earthed directly or through
impedances so that under all circumstances the voltage
appearing on them is limited to less than 75 percent of their
rated withstand voltage.

Test with impulse chopped on the tail
Greater stresses on windings due to fast rate of
collapse of voltage
Time to chopping between 2 and 6μs
Triggered type of chopping provides better
consistency
Failure during chopped wave test are reflected on
application of full waves, immediately after the
chopped wave

Test with impulse chopped on the tail

Sequence of tests with impulse
chopped on the tail
1.One reduced full impulse
2.One 100% full impulse
3.One reduced chopped impulse
4.Two 100% chopped impulses
5.Two 100% full impulses
In principle, the detection of faults during a
chopped impulse test depends essentially on a
comparison of the oscillographic records of 100
percent and reduced chopped impulses.

Measurement and Recording of Impulses
Three types of dividers – resistive divider, capacitive
divider and compensated divider
Last two types also serve as load capacitor
Chopping can be obtained with a rod gap, triggered
sphere gap or with a multiple chopping gap

Oscillographic recording Lightning
Impulse Test
Applied voltage wave and one other parameter
are recorded
Oscillograms taken at reduced and full test levels
should be recorded to give equal amplitude ( by
using attenuators)

Fault detection
Insulation failure will change the impedance of the
transformer
Causes a variation in the impulse current and in the
voltage
Reduced full wave oscillograms are taken as reference
They match with subsequent full-wave oscillograms when
transformer is healthy
For fault detection , the following can be measured and
used separately
(1) the neutral current
(2) capacitively transferred current
(3) the tank current
Also, transferred voltages to non-tested windings can be
used

Interpretation of Oscillograms
Current Oscillograms
Most sensitive means of failure detection
Major changes in the current oscillogram indicate
probable breakdown within the winding and earth
Localized high frequency oscillations spread over 2
or 3μs are a possible indication of severe discharges
or a possible breakdown in the insulation between
turns or coils
Current oscillograms are recorded on two sweeps

Interpretation of Oscillograms

Transformer accessories, fitments
and safety devices
Accessories
Proper functioning
Provide protection under fault condition

(1) Oil level indicator:
Oil level in the conservator is maintained above a
pre-determined minimum level
Magnetic oil level gauge which also incorporates a
mercury switch

(2) Pressure relief valve:
Fitted on the tank to act as an exit for gases
formed of oil
Major fault inside the transformer causes
sudden vaporization of the oil
Leads to rapid build up gaseous pressure
Pressure to be relieved within few milliseconds
Otherwise transformer tank gets ruptured
Oil spilling
Damage and fire hazard

Pressure relief valves are usually installed
behind the pressure gauge on sealed tank units.
They are used in conjunction with pressurized
nitrogen systems and can be mounted in the gas
bottle cabinet or on the tank wall.
The bleeder valve is set to bleed-off any
pressures that exceed a, pre-set level (usually
around 8-10 psi).
This valve is an integral part of the pressurized
gas system, and its failure can result in a rupture
of the tank.

The operation of these devices can be checked
by manually increasing the tank pressure to the
preset level
It is important not to exceed the maximum
tank pressure.
If the valve does not bleed off the excess
pressure, it should be replaced

(3) Buchholz relay:
Gas operated relay
Fitted in the pipe between tank and conservator
Minor fault
–Damage to core bolt insulation
–Local overheating
Slow generation of gas in the oil
Alarm for incipient faults
Decomposition of oil due to fault contains more
than 70% of hydrogen gas
Hydrogen gas rises upward and tries to go into
conservator
Gas gets collected in the upper portion of the relay

(3) Buchholz relay:
Oil level in the relay drops
Float in the relay tilts with reduced oil level
Mercury switch attached to the float is closed
Switch closes the alarm circuit
Transformer is disconnected as early as possible
Gas sample is taken for test
Test indicates type of fault and insulation level
Relay operation is slow (0.1 sec and average time is
0.2sec)

(3) Buchholz relay:
In case of short circuits inside transformer, pressure
in the tank increases
Oil rushes towards conservator
The baffle(plate) gets pressed by the rushing oil and
closes another switch
This in turn closes the trip circuit of circuit breaker
Relay for earthquake zones

(4) Sudden pressure relay(rate of
rise of pressure relay):
 Relay responds to sudden rise of pressure due to
internal arcing

(5) Conservator:
Large cylinder connected by pipe to the transformer
Oil is filled up to some level in the conservator
Remaining portion is filled with air
Expansion and contraction of transformer tank oil is
accommodated by air cushion in the conservator
Direct contact with air is avoided
Conservators are of two types
(1) air cushion connected with external
atmosphere through breather
-air entering conservator is dried by
silica gel in the breather
(2) air cushion contained rubber bag installed
within the conservator

(6) Breather:
One end of the breather is connected to air
cushion in conservator
Other end is towards external air
It is filled with dry silica gel, which is blue in color
During expansion or contraction of oil, air is
breathed in/out through breather
Silica gel absorbs moisture and admits only
clean/fresh air
When silica gel absorbs moisture it becomes pink
It is dried during periodic maintenance

(7) Oil Temperature Indicator:
Thermocouple is placed in the pocket provided with
the tank near hot oil
Both average reading and hot spot temperature
gauges can use a bulk-type detecting unit that is
immersed in the oil either near the top of the oil
level
or near the windings at the spot that is expected to
be the hot test.
A capillary tube is connected to the bulb and
brought out of the tank.
The temperature indication is provided either by a,
linear marking on the tube itself, or by a dial-type
indicator.

(7) Oil Temperature Indicator:
(1) The lowest setting usually actuates external cooling
fans that will come on at a preset temperature level.
The fans will shut off once the temperature has been
reduced to the prescribed level.
(2) The contacts can also be set to actuate remote
alarms that will alert maintenance personnel of the
condition of the transformer. These devices must be
reset even though the temperature has returned to
normal.
(3) The highest and most critical contact setting on the
temperature gauge is connected to a relay or a circuit
breaker that will trip out and de-energize the
transformer.

(8) Winding Temperature
Indicator(Hot spot indicator) with
alarm and tripping contacts:
Thermocouples are placed in the tank near hot oil
Winding Temperature Indicator : The Winding is the
component with highest temperature within the
transformer and, above all, the one subject to the
fastest temperature increase as the load increases.
 Thus to have total control of the temperature
parameter within transformer, the temperature of the
winding as well as top oil, must be measured

 An indirect system is used to measure winding
temperature, since it is dangerous to place a sensor
close to winding due to the high voltage. The indirect
measurement is done by means of a Built-in Thermal
Image.
 Winding Temperature Indicator is equipped with a
specially designed Heater which is placed around the
operating bellows through which passes a current
proportional to the current passing through the
transformer winding subject to a given load.

 Winding Temperature is measured by connecting
the CT Secondary of the Transformer through a shunt
resistor inside the Winding Temperature Indicator to
the Heater Coil around the operating Bellows
 It is possible to adjust gradient by means of Shunt
Resistor
 In this way the value of the winding temperature
indicated by the instrument will be equal to the one
planned by the transformer manufacturer for a given
transformer load.

(9) Marshalling kiosk (control
cabinet):
Placed on the side of the tank
 Control cables, power cables between transformers,
coolers, control room, auxiliary supply switch gear etc.
are via control cabinet
 Start , stop switches for fan motors, cooler pump
motors etc.
 Temperature indicators and temperature controllers

(9) Marshalling kiosk (control
cabinet):

(10) Surge Arrestors:
One per phase is used
One terminal is connected to the lead of the
transformer terminal
Other terminal is connected to earth
Lightning and switching surges are discharged to
earth
Silicon carbide and ZnO surge arrestors

Maintenance of Power Transformer
Maintenance of transformer:Maintenance of Power
Transformer.docx

Sudden short-circuit withstand test
Special test
Conducted at short circuit testing station
Ability of a transformer to withstand external short-
circuits is proved by conducting these tests
Transformer must withstand mechanical and thermal
stresses caused by repeated external short circuit
currents
Due to short circuit currents, the transformer
winding is subjected to radial electromagnetic forces
The radial forces produce hoop stress on outer
winding and compressive stress on inner winding
 axial forces tend to collapse the winding, fracture
the end rings, bending of conductor between stresses

Designed to withstand short circuit forces

Short circuit testing of Power
Transformers
Special test
Large amounts of power are required
As per IEC 60076-5, Power Transformers are
categorized as:
Category I - Up to 2500kVA
Category II – From 2501kVA to 100MVA
Category III – Above 100MVA
Symmetrical short circuit current = measured short
circuit impedance of transformer + the system
impedance

Requirements prior to short
circuit test
Transformer should pass all routine tests as
specified in IEC 60076-I
Lighting Impulse test is not to be required to be
conducted prior to short circuit test
At the beginning of short-circuit test, the average
winding temperature shall be preferably between
10oC to 40oC

Peak value of short-circuit
current

category 1: single-phase transformers
Number of tests shall be three, the duration of
each test being 0.5 second with a tolerance of ± 10
percent .
Unless otherwise specified each of the three
tests on a single-phase transformer with tapping's
is made in a different position of the tap-changer,
that is, one test in the position corresponding to
the highest voltage ratio, one test on the principal
tapping and one test in the position corresponding
to the lowest voltage ratio .

category 1: three-phase transformers
 Total number of tests shall be nine, that is three
tests on each limb, the duration of each test being
0.5 second with a tolerance of ± 10 percent
Unless otherwise specified the tests on each limb
of a transformer with tapping’s are made in
different positions of the tap-changer, that is, three
tests in the position corresponding to the highest
voltage ratio on one of the outer limbs, three tests
on the principal tapping on the middle limb and
three tests in the position corresponding to the
lowest voltage ratio on the other outer limb

Detection of Faults and the Evaluation
of the Results of the Short-Circuit Test
Transformers of category 1 — All the routine tests
shall be repeated
The dielectric routine tests shall be at 75 percent of
the original test value unless a higher value has been
agreed between the manufacturer and the purchaser

The transformer is deemed to have passed the short-
circuit tests if,
1.The routine tests have been successfully repeated .
2.The results of the short-circuit tests, measurements
during short-circuit tests and out of tank inspection do
not reveal any defects (displacements, deformations of
windings, connections or supporting structures, or
traces of discharge),
3. The short-circuit reactance measured after the tests
differs from that measured in the original state by not
more than specified percent ( 2%)

If the three conditions for passing the short-circuit
tests have been met, the transformer is restored to its
original state and any further routine tests necessary to
prove fitness for service are repeated before dispatch.
If any of the three conditions have not been met, it
may be necessary to dismantle the unit as far as is
required to establish the cause of the change of the
conditions.

Transformers of category 2 and 3
Total number of tests –nine, three tests on each limb,
the duration of each test being 0.25 second with a
tolerance of ± 10 percent
The transformer is deemed to have passed the short-
circuit
tests if,
1.The routine tests have been successfully repeated.

Transformers of category 2 and 3
2.The results of the short-circuit tests,
measurements during short-circuit tests and out of
tank inspection do not reveal any defects
(displacements, deformations of windings,
connections or supporting structures, or traces of
discharge),
3. The short-circuit reactance measured after the
tests differs from that measured in the original state
by not more than specified percent ( 2% for category
II and 1% for category III)

Efficiency
The efficiency of transformer is the ratio of its output to input.
The efficiency changes with load (both magnitude and quality of the
load). The losses in transformers are
a) No-load losses: includes both hysteresis loss and eddy current loss. As
the core flux in a transformer remains practically constant at all loads, the
core loss is also called as constant loss. The input power of a transformer
under no-load, measures the core loss.
b) Load losses: This loss is mainly due to holmic resistance of the
transformer winding. Copper loss also includes the stray loss occurring in
the mechanical structure and winding conductor due to the stray fluxes.
c) Full load copper loss is measured by the short circuit test.
Loading conditions of transformer:
•Decided by permissible temperature rise of windings and oil.
•Permissible oil temperature is 650C and hot spot temperature of the
winding is 800C at rated current.
•Life expectancy reduces if the transformer is overloaded for longer
duration.

Regulation
% Regulation =
(V2 at No Load - V2 at Full Load/ V2 at full load)x100
% regulation of a loaded transformer at any given power factor =
(R cosΦ+X sinΦ)+( X cosΦ- R sinΦ)2/200
where R= %age resistive drop,
X= %age reactive drop.
cos Φ power factor
Note: Both SC and OC test results are used to predict the
efficiency and regulation of transformer at any desired load &
power factor.

Loading Conditions of Transformer
•The loading of transformer is decided by permissible
temperature rise of windings and oil.
•Permissible oil temperature is 650C and hot spot temperature of
the winding is 800C at rated current.
•As load on the transformer varies according to the load curve,
loading becomes an important operating problem.
•Life expectancy reduces if the transformer is overloaded for
longer duration.

Tap Changers
• Fast growth of electrical load and distribution network voltage
variation is normal.
•Maintain system voltage within the specified limit for the better
health of electrical equipments.
• The system voltage may be adjusted by changing the tapings on
the power transformer.
• The variation in voltage may be brought in either by step or step
less control.
• Variation is generally achieved by means of tapings on the power
transformer, as of the smaller currents to be dealt with, are
normally located on the higher-voltage winding.
•Tap Changer for Distribution Transformers.flv

Tap Changers
Off circuit tap changer:
•The economic method of changing the
turns ratio of a transformer is the use of
off-circuit tap changer.
•It is necessary to de-energize the
transformer before changing the tap.
•A mechanical lock is provided to
prevent unauthorized operation and
inadvertent operation.
•The transformers are normally
provided with off-circuit taps with ± 2.5
percent and ±5 percent on the side.
•The station transformers are preferably
provided with OLTC with ± 10% in steps
of 1.25 percent on the side.

Tap Changers
On Load Tap Changers (OLTC):
•OLTC are employed to change turns ratio of
transformer to regulate system voltage while
the transformer is delivering normal load.
•All forms of OLTC circuit possess impedance,
which is introduced to avoid short circuiting of
tapping section during tap changer operation.
•The OLTC can in general, be classified as
resistor or reactor type.
•Motor drive unit is initiated by a push button
or voltage control relay, tap selector changes
tap.
•The diverter switch diverts the current.
•The tap changers function is without
interruption in load current.
•Function Principle of the OLTC -
YouTube (360p).mp4

Causes for Troubles & Failure in power
Transformer
1. Failure in Magnetic circuit:
Due to failure of insulation bolts used for clamping the core &
yoke, circulating current flows and core is heating
2. Vibrations in core: Loose core clamping bolts between core
results vibration of core leads to weaken the core insulation.
3. Excessive core heating, high magnetizing currents & high in
rush current : Temperature rise of winding.
4. Failure of core bolt insulation: Moisture absorption leads to
dielectric failure.
5. Over fluxing of power transformer, melting of core bolts,
burning of core bolt insulation etc: Generator transformer fails.

Causes for Troubles & Failure in power Transformer
and Preventive Action

Causes for Troubles & Failure in power
Transformer and Preventive Action

Noise in the Transformer
1. Magnetostriction: Core laminations starts vibrating at
harmonic frequency. (humming noise)
2. Mechanical vibrations developed by the laminations.
3. Mechanical vibrations of the tank: leakage flux pass through
the tank walls produce vibration.
4. Noise by fans used for forced air cooling.
5. Noise in cooling system :oil pumps.

Maintenance of Transformer
1. Regular/routine inspection & minor works.
 No need of opening the transformer cover.
 Visual inspection
• Checking oil level
• Cooling system
• Control circuit
• Oil leakages.
 BVD of oil sample and sample gas from Buchholz relay.
 Cleaning the porcelain surface, tighten the bolts.
 Replacement of silica gel.
 Repair of external fitments and oil filtration.
Note: operating personnel in power station and substation.

2. Medium repairs (testing & reconditioning).
• Dismantling of the transformer taken for inspection/repair
of the core coil. Assembly.
• Example: repair of conservator tank, tap changer,
cooling system
Note: Defects located in routine inspection & could not be
corrected are taken for medium maintenance work

3. Major repairs work
• For serious internal fault / 8 to 10 years of prolonged
service.
• Complete dismantling of transformer for replacement of
winding & insulation, tightening of core laminations.
• Work is carried out by trained personnel.
• Well planned.
• Well equipped repair workshop.

Unit 1-2-b

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Unit 1-2-b