transformer testing _ type test and routine test of transformer _ electrical4u
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electrical4u.comOnline Electrical Engineering
Transformer Testing | Type
Test and Routine Test ofTransformer
More related topics:
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Transformer winding resistance measurement Transformer Ratio Test
Vector Group Test of Transformer
Dielectric Test of Transformer
Temperature Rise Test of Transformer
Transformer Impulse Test
Sweep Frequency Response Analysis
Tan Delta or Dissipation Factor Test
Over Fluxing in Transformer
Earthing or Grounding Transformer
External & Internal Faults in Transformer
Backup Protection of Transformer
Differential Protection of Transformer
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Restricted Earth Fault Protection
Maintenance of Transformer
Distribution Transformer
Air Core Transformer
High Voltage Transformer
Installation of Power Transformer
Commissioning of Power Transformer
This page is all about:
Type of Transformer Testing
Type Test
Routine TestsSpecial Tests
Pre Commissioning Test
Winding Resistance Measurement
Ratio Test
Magnetic Balance Test
Procedure of Magnetic Balance Test
Magnetizing Current Test
Vector Group Test
Insulation Resistance Test or Megger Tes
Procedure of Insulation Resistance Test
Dielectric Tests of Transformer
Induced Voltage Test of Transformer
Temperature Rise Test of Transformer
For confirming the specifications and performances of an
electrical power transformer it has to go through numbers of
testing procedures. Some tests are done at manufacturer
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premises before delivering the transformer. Mainly two types
of transformer testing are done at manufacturer premises-
type test of transformer and routine test of transformer.
In addition to that some transformer tests are also carried
out at the consumer site before commissioning and also
periodically in regular & emergency basis through out its
service life.
Type of Transformer Testing
Tests done at factory
1. Type tests
2. Routine tests
3. Special tests
Tests done at site
1. Pre-commissioning tests
2. Periodic/condition monitoring tests
3. Emergency tests
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Type Test of Transformer
To prove that the transformer meets customers specifications
and design expectations, the transformer has to go through
different testing procedures in manufacturer premises. Some
transformer tests are carried out for confirming the basic
design expectation of that transformer. These tests are done
mainly in a prototype unit not in all manufactured units in a
lot. Type test of transformer confirms main and basic
design criteria of a production lot.
Routine Tests of Transformer
Routine tests of transformer is mainly for confirming
operational performance of individual unit in a production lot.
Routine tests are carried out on every unit manufactured.
Special Tests of Transformer
Special tests of transformer is done as per customer
requirement to obtain information useful to the user during
operation or maintenance of the transformer.
Pre Commissioning Test of Transformer
In addition to these, the transformer also goes through some
other tests, performed on it, before actual commissioning of
the transformer at site. The transformer testing performed
before commissioning the transformer at site is called pre-
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commissioning test of transformer. These tests are done to
assess the condition of transformer after installation and
compare the test results of all the low voltage tests with the
factory test reports.
Type tests of transformer includes
1. Transformer winding resistance measurement
2. Transformer ratio test.
3. Transformer vector group test.
4. Measurement of impedance voltage/short circuit impedance
(principal tap) and load loss (Short circuit test).
5. Measurement of no load loss and current (Open circuit test).
6. Measurement of insulation resistance.
7. Dielectric tests of transformer.
8. Temperature rise test of transformer.
9. Tests on on-load tap-changer.
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10. Vacuum tests on tank and radiators.
Routine tests of transformer include
1. Transformer winding resistance measurement.
2. Transformer ratio test.
3. Transformer vector group test.
4. Measurement of impedance voltage/short circuit impedance
(principal tap) and load loss (Short circuit test).
5. Measurement of no load loss and current (Open circuit test)
6. Measurement of insulation resistance.
7. Dielectric tests of transformer.
8. Tests on on-load tap-changer.
9. Oil pressure test on transformer to check against leakages
past joints and gaskets.
That means Routine tests of transformer include all the type
tests except temperature rise and vacuum tests. The oil
pressure test on transformer to check against leakages past
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joints and gaskets is included.
Special Tests of transformer include
1. Dielectric tests.
2. Measurement of zero-sequence impedance of three-phase
transformers
3. Short-circuit test.
4. Measurement of acoustic noise level.
5. Measurement of the harmonics of the no-load current.
6. Measurement of the power taken by the fans and oil pumps.
7. Tests on bought out components / accessories such as
buchhloz relay, temperature indicators, pressure relief
devices, oil preservation system etc.
Transformer Winding Resistance
Measurement
Transformer winding resistance measurement is carried out to
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calculate the I2R losses and to calculate winding temperature
at the end of a temperature rise test. It is carried out as a type
test as well as routine test. It is also done at site to ensure
healthiness of a transformer that is to check loose
connections, broken strands of conductor, high contact
resistance in tap changers, high voltage leads and bushings
etc.
There are different methods for measuring of transformer
winding, likewise
Current voltagemethod of measurement of windingresistance.
Bridge method of measurement of winding resistance.
Kelvin bridge method of Measuring Winding
Resistance.
Measuring winding resistanceby Automatic Winding
ResistanceMeasurement Kit.
NB:- Transformer winding resistancemeasurement shall be
carried out at each tap.
Transformer Ratio Test
The performance of a transformer largely depends upon
perfection of specific turns or voltage ratio of transformer. So
transformer ratio test is an essential type test of transformer.
This test also performed as routine test of transformer. So
for ensuring proper performance of electrical power
transformer, voltage and turn ratio test of transformer one of
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the vital tests.
The procedure of transformer ratio test is simple. We just
apply three phase 415 V supply to HV winding, with keeping
LV winding open. The we measure the induced voltages at
HV and LV terminals of transformer to find out actual voltage
ratio of transformer. We repeat the test for all tap position
separately.
Magnetic Balance Test of Transformer
Magnetic balance test of transformer is conducted only on
three phase transformers to check the imbalance in the
magnetic circuit.
Procedure of Magnetic Balance Test of Transformer
1. First keep the tap changer of transformer in normal position.
2. Now disconnect the transformer neutral from ground.
3. Then apply single phase 230 V AC supply across one of the
HV winding terminals and neutral terminal.
4. Measure the voltagein two other HV terminals in respect of
neutral terminal.
5. Repeat the test for each of the three phases.
In case of auto transformer, magnetic balance test of
transformer should be repeated for LV winding also.
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There are three limbs side by side in a core of transformer.
One phase winding is wound in one limb. The voltage
induced in different phases depends upon the respective
position of the limb in the core. The voltageinduced in
different phases of transformer in respect to neutral terminals
given in the table below.
Left side
phase
Central
phase
Right side
phase
AN BN CN
Voltage applied at left side
phase
230 V 180 V 50 V
Voltage applied at central
phase115 V 230 V 115 V
Voltage applied at right
side phase50 V 180 V 230 V
Magnetizing Current Test of
Transformer
Magnetizing current test of transformeris performed to
locate defects in the magnetic core structure, shifting of
windings, failure in turn to turn insulation or problem in tapchangers. These conditions change the effective reluctance of
the magnetic circuit, thus affecting the currentrequired to
establish flux in the core.
1. First of all keep the tap changer in the lowest position and
open all IV & LV terminals.
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2. Then apply three phase 415 V supply on the line terminals
for three phase transformers and single phase 230 V supply
on single phase transformers.
3. Measure the supply voltageand currentin each phase.
4. Now repeat the magnetizing current test of
transformertest with keeping tap changer in normal
position.
5. And repeat the test with keeping the tap at highest position.
Generally there are two similar higher readings on two outer
limb phases on transformer core and one lower reading on the
centre limb phase, in case of three phase transformers. An
agreement to within 30% of the measured exciting current
with the previous test is usually considered satisfactory. If the
measured exciting current value is 50 times higher than the
value measured during factory test, there is likelihood of a
fault in the winding which needs further analysis.
Caution: This magnetizing current test of transformer is
to be carried out before DC resistancemeasurement.
Vector Group Test of Transformer
In three phase transformer, it is essential to carry out a vector
group test of transformer. Proper vector grouping in a
transformer is an essential criteria forparallel operation of
transformers.
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There are several internal connection of three phase
transformerare available in market. These several
connections gives various magnitudes and phase of the
secondary voltage the magnitude can be adjusted for parallel
operation by suitable choice of turn ratio, but the phase
divergence can not be compensated. So we have to choose
those transformer for parallel operation whose phase sequence
and phase divergence are same. All the transformers with
same vector ground have same phase sequence and phase
divergence between primary and secondary. So before
procuring one electrical power transformer, one should ensure
the vector group of the transformer, whether it will be
matched with his or her existing system or not. The vector
group test of transformer confirms his or her requirements.
Insulation Resistance Test or Megger
Test of Transformer
Insulation resistancetest of transformeris essential type
test. This test is carried out to ensure the healthiness of over
all insulation system of an electrical power transformer.
Procedure of Insulation Resistance Test of Transformer
1. First disconnect all the line and neutral terminals of the
transformer.
2. Megger leads to be connected to LV and HV bushing studs to
measure insulation resistanceIR value in between the LV and
HV windings.
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3. Megger leads to be connected to HV bushing studs and
transformer tank earth point to measure insulation resistance
IR value in between the HV windings and earth.
4. Megger leads to be connected to LV bushing studs and
transformer tank earth point to measure insulation resistance
IR value in between the LV windings and earth.
NB : It is unnecessary to perform insulation resistancetest
of transformer per phase wise in three phase transformer.
IR values are taken between the windings collectively asbecause all the windings on HV side are internally
connected together to form either star or delta and also all
the windings on LV side are internally connected together
to form either star or delta.
Measurements are to be taken as follows:
For auto transformer: HV-IV to LV, HV-IV to E,
LV to E.
For two winding transformer: HV to LV, HV to E, LV to
E.
Three winding transformer: HV to IV, HV to LV, IV to
LV, HV to E, IV to E, LV to E.
Oil temperature should be noted at the time of insulation
resistancetest of transformer. Since the IR value of
transformer insulating oil may vary with temperature.
IR values to be recorded at intervals of 15 seconds, 1 minute
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and 10 minutes.
With the duration of application of voltage, IR value
increases. The increase in IR is an indication of dryness of
insulation.
Absorption coefficient = 1 minute value/ 15 secs. value.
Polarization index = 10 minutes value / 1 minute value.
Dielectric Tests of Transformer
Dielectric tests of transformeris one kind of insulation test.
This test is performed to ensure the expected over all
insulation strength of transformer. There are several test
performed to ensure the required quality of transformer
insulation, dielectric test is one of them. Dielectric tests of
transformeris performed in two different steps, first one
called Separate source voltagewithstand test of transformer,
where a single phase power frequency voltageof prescribed
level, is applied on transformer winding under test for 60
seconds while the other windings and tank are connected to
the earth and it is observed that whether any failure of
insulation occurs or not during the test. Second one is induced
voltagetest of Transformer where, three phase voltage, twice
of rated secondary voltageis applied to the secondary
winding for 60 second by keeping the primary of the
transformer open circuited. The frequency of the applied
voltageshould be double of power frequency too. Here also if
no failure of insulation, the test is successful.
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In addition to dielectric tests of transformerthere are other
type test for checking insulation of transformer, such as
lightning impulse test, switching impulse test and partial
discharge test.
Induced Voltage Test of Transformer
The induced voltagetest of transformer is intended to check
the inter turn and line end insulation as well as main
insulation to earth and between windings-
1. Keep the primary winding of transformer open circuited.
2. Apply three phase voltageto the secondary winding. The
applied voltageshould be twice of rated voltageof
secondary winding in magnitude and frequency.
3. The duration of the test shall be 60 second.
4. The test shall start with a voltagelower than 1/3 the full test
voltage, and it shall be quickly increased up to
desired value.
The test is successful if no break down occurs at full test
voltageduring test.
Temperature Rise Test of
Transformer
Temperature rise test of transformeris included in type test of
transformer. In this test we check whether the temperature
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rising limit of the transformer winding and oil as per
specification or not.
In this type test of transformer, we have to check oil
temperature rise as well as winding temperature rise limits of
an electrical transformer.
Objective Questions on Transformer (MCQs)
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