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7/24/2019 Trafo Tests

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Transformer Testing | Type Testand Routine Test of Transformer

Under Electrical TransformerFor confirming the specifications and performances of anelectrical power transformerit has to go through numbers oftesting procedures. Some tests are done at manufacturer

premises before delivering the transformer. Mainly two types oftransformer testingare done at manufacturer premises-typetest of transformerand routine test of transformer. Inaddition 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.

Type of Transformer TestingTests done at factory

1. Type tests

. !outine tests

". Special testsTests done at site

1. #re-commissioning tests

. #eriodic$condition monitoring tests

". %mergency tests

Type Test of TransformerTo prove that the transformer meets customers specifications anddesign e'pectations( the transformer has to go through differenttesting procedures in manufacturer premises. Some transformertestsare carried out for confirming the basic design e'pectation

of that transformer. These tests are done mainly in a prototypeunit not in all manufactured units in a lot. Type test oftransformerconfirms main and basic design criteria of a

production lot.

Routine Tests of TransformerRoutine tests of transformeris mainly for confirmingoperational performance of individual unit in a production lot.

!outine tests are carried out on every unit manufactured.

Special Tests of Transformer
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Special tests of transformeris done as per customerre)uirement to obtain information useful to the user during

operation or maintenance of the transformer.

Pre Commissioning Test of TransformerIn addition to these( the transformer also goes through someother tests( performed on it( before actual commissioning of the

transformer at site. The transformer testingperformed beforecommissioning 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 thetest results of all the low voltagetests with the factory testreports.

Type tests of transformer includes

1. Transformer winding resistance measurement

. Transformer ratio test.

". Transformer vector group test.

*. Measurement of impedance voltage$short circuit impedance

+principal tap, and load loss +Short circuit test,.

. Measurement of no load loss and current+pen circuit test,./. Measurement of insulation resistance.

0. ielectric tests of transformer.

2. Temperature rise test of transformer.

3. Tests on on-load tap-changer.

14. 5acuum tests on tan6 and radiators.

Routine tests of transformer include

1. Transformer winding resistance measurement.

. Transformer ratio test.

". Transformer vector group test.

*. Measurement of impedance voltage$short circuit impedance

+principal tap, and load loss +Short circuit test,.

. Measurement of no load loss and current+pen circuit test,

/. Measurement of insulation resistance.

0. ielectric tests of transformer.

2. Tests on on-load tap-changer.
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3. il pressure test on transformer to chec6 against lea6ages

past 7oints and gas6ets.

That means !outine tests of transformer include all the type tests

e'cept temperature rise and vacuum tests. The oil pressure test

on transformer to chec6 against lea6ages past 7oints and gas6ets

is included.

Special Tests of transformer include

1. ielectric tests.

. Measurement of 8ero-se)uence impedance of three-phase

transformers

". Short-circuit test.

*. Measurement of acoustic noise level.

. Measurement of the harmonics of the no-load current.

/. Measurement of the power ta6en by the fans and oil pumps.

0. Tests on bought out components $ accessories such as

buchhlo8 relay( temperature indicators( pressure relief devices(oil preservation system etc.

Transformer Winding ResistanceMeasurementTransformer winding resistance measurementis carried out to

calculate the I! losses and to calculate winding temperature atthe 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 ensurehealthiness of a transformer that is to chec6 loose connections(bro6en strands of conductor( high contact resistancein tapchangers( high voltageleads and bushings etc.

There are different methods for measuring of transformer

winding( li6ewise

9urrent voltagemethod of measurement of winding resistance.

:ridge method of measurement of winding resistance.

;elvin bridge method of Measuring


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Measuring winding resistance by =utomatic 5 and ?5

terminals of transformer to find out actual voltage ratio of

transformer. 5 winding terminals and neutral terminal.*. Measure the voltage in two other >5 terminals in

respect of neutral terminal.

. !epeat the test for each of the three phases.In case of auto transformer( magnetic balance test of transformer

should be repeated for ?5 winding also.There are three limbs side by side in a core of transformer. nephase winding is wound in one limb. The voltage induced in

different phases depends upon the respective position of the limbin the core. The voltage induced in different phases of
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transformer in respect to neutral terminals given in the tablebelow.

eft side p!ase Central p!ase Rig!t side p

=@ :@ 9@

5oltage applied at left side phase "4 5 124 5 4 5

5oltage applied at central phase 11 5 "4 5 11 5

5oltage applied at right side phase 4 5 124 5 "4 5

Magneti"ing Current Test of TransformerMagneti"ing current test of transformer is 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 currentre)uired to establish flu' in the

core.

/. First of all 6eep the tap changer in the lowest position

and open all I5 & ?5 terminals.

0. Then apply three phase *1 5 supply on the line

terminals for three phase transformers and single phase "4 5supply on single phase transformers.

2. Measure the supply voltageand currentin each phase.

3. @ow repeat the magneti"ing current test of

transformertest with 6eeping tap changer in normal position.

14. =nd repeat the test with 6eeping the tap at highest

position.

Aenerally there are two similar higher readings on two outer limbphases on transformer core and one lower reading on the centrelimb phase( in case of three phase transformers. =n agreement to

within "4B of the measured e'citing currentwith the previoustest is usually considered satisfactory. If the measured e'citingcurrentvalue is 4 times higher than the value measured during

factory test( there is li6elihood of a fault in the winding whichneeds further analysis.Caution: T!is magneti"ing currenttest of transformer is to

#e carried out #efore $C resistancemeasurement%
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&ector 'roup Test of TransformerIn three phase transformer( it is essential to carry out a vector

group test of transformer. #roper vector grouping in a transformer

is an essential criteria for parallel operation of transformers.There are several internal connection of three phase transformerare available in mar6et. These several connections gives various

magnitudes and phase of the secondary voltageC the magnitudecan be ad7usted for parallel operation by suitable choice of turnratio( but the phase divergence can not be compensated. So wehave to choose those transformer for parallel operation whose

phase se)uence and phase divergence are same. =ll thetransformers with same vector ground have same phase

se)uence and phase divergence between primary and secondary.So before procuring one electrical power transformer( one shouldensure the vector group of the transformer( whether it will bematched with his or her e'isting system or not. The vector group

test of transformer confirms his or her re)uirements.

(nsulation Resistance Test or Megger Testof Transformer(nsulation resistance test of transformer is essential type

test. This test is carried out to ensure the healthiness of over all

insulation system of an electrical power transformer.

Procedure of (nsulation Resistance Test of Transformer

11. First disconnect all the line and neutral terminals of the

transformer.

1. Megger leads to be connected to ?5 and >5 bushing

studs to measure insulation resistanceI! value in between the

?5 and >5 windings.

1". Megger leads to be connected to >5 bushing studs and

transformer tan6 earth point to measure insulation resistance

I! value in between the >5 windings and earth.

1*. Megger leads to be connected to ?5 bushing studs and

transformer tan6 earth point to measure insulation resistance

I! value in between the ?5 windings and earth.

NB : (t is unnecessary to perform insulation resistancetestof transformer per p!ase )ise in t!ree p!ase transformer%
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(R *alues are ta+en #et)een t!e )indings collecti*ely as#ecause all t!e )indings on ,& side are internally

connected toget!er to form eit!er star or delta and also allt!e )indings on & side are internally connected toget!er

to form eit!er star or delta%Measurements are to be ta6en as followsDFor auto transformerD >5-I5 to ?5( >5-I5 to %( ?5 to %. For twowinding transformerD >5 to ?5( >5 to %( ?5 to %. Three winding

transformerD >5 to I5( >5 to ?5( I5 to ?5( >5 to %( I5 to %( ?5 to%.il temperature should be noted at the time of insulationresistance test of transformer. Since the I! value of

transformer insulating oil may vary with temperature. I! values

to be recorded at intervals of 1 seconds( 1 minute and 14minutes.

ere also if no failure of

insulation( the test is successful. In addition to dielectric tests of
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transformerthere are other type test for chec6ing insulation of

transformer( such as lightning impulse test( switching impulse test

and partial discharge test.

(nduced &oltage Test of Transformer

The induced voltagetest of transformer is intended to chec6 the

inter turn and line end insulation as well as main insulation to

earth and between windings-

1. ;eep the primary winding of transformer open

circuited.

1/. =pply three phase voltage to the secondary winding.

The applied voltage should be twice of rated voltage ofsecondary winding in magnitude and fre)uency.

10. The duration of the test shall be /4 second.

12. The test shall start with a voltagelower than 1$" the

full test voltage( and it shall be )uic6ly increased up to desired

value.

The test is successful if no brea6 down occurs at full test voltage

during test.

Temperature Rise Test of TransformerTemperature rise test of transformeris included in type test of

transformer. In this test we chec6 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 chec6 oil temperaturerise as well as winding temperature rise limits of an electrical

transformer.
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