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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 TransformerEarthing or Grounding Transformer
External & Internal Faults in Transformer
Backup Protection of Transformer
Differential Protection of Transformer
Restricted Earth Fault Protection
Maintenance of Transformer
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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 Tests
Special Tests
Pre Commissioning Test
Winding Resistance MeasurementRatio 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 transformerit has to go through numbers of testing procedures.Some tests are done at manufacturer premises before delivering the
transformer. Mainly two types of transformer testingare done at
manufacturer premises-type test of transformerand routine test of
transformer. In addition to that some transformer testsare also
carried out at the consumer site before commissioning and also
periodically in regular & emergency basis through out its service life.
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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
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
-
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manufactured units in a lot. Type test of transformerconfirms main
and basic design criteria of a production lot.
Routine Tests of Transformer
Routine tests of transformeris 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 transformeris done as per customer requirement to
obtain information useful to the user during operation or maintenanceof 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 testingperformed before commissioning the
transformer at site is called pre-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 voltagetests
with the factory test reports.
Type tests of transformer includes
Spring Contact ProbesExperts in Probe Production Probes for All Types of Contacts
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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.
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)
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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 joints and gaskets is
included.
Special Tests of transformer include
1. Dielectric tests.
2. Measurement of zero-sequence impedance of three-phase
transformers
Neutral Grounding CubicleNER / NGR up to 55 kV [email protected]
Design a TransformerFree samples 24 hours Easy forms on-line TRY IT!
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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 measurementis carried out to
calculate the I
2
R losses and to calculate winding temperature at theend 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 resistancein tap changers, high voltageleads
and bushings etc.
There are different methods for measuring of transformer winding,likewise
Current voltagemethod of measurement of winding resistance.
Bridge method of measurement of winding resistance.
Kelvin bridge method of Measuring Winding Resistance.
Measuring winding resistanceby Automatic Winding
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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 voltageratio 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 powertransformer, voltageand turn ratio test of transformer
one of 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 voltageratio 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.
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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 transformershould be repeated for LV winding also.
There are three limbs side by side in a core of transformer. One phase
winding is wound in one limb. The voltageinduced 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
phase230 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 tap changers. These conditions
change the effective reluctance of the magnetic circuit, thus affecting
the currentrequired to establish flux in the core.
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1. First of all keep the tap changer in the lowest position and open all
IV & LV terminals.
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 andphase 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.
3. Megger leads to be connected to HV bushing studs and transformer
tank earth point to measure insulation resistanceIR value in between
the HV windings and earth.
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4. Megger leads to be connected to LV bushing studs and transformer
tank earth point to measure insulation resistanceIR 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 as because 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 resistance
test 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 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
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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, firstone 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 secondarywinding 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.
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.
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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 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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