Transformer Testing _ Type Test and Routine Test of Transformer _ Electrical

7/25/2019 Transformer Testing _ Type Test and Routine Test of Transformer _ Electrical

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sformer Testing | Type Test and Routine Test of Transformer | Electrical4u

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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


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 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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Transformer Testing _ Type Test and Routine Test of Transformer _ Electrical