213366971 Transformer Testing

8/10/2019 213366971 Transformer Testing

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Transformer Testing

For confirming the specifications and performances of an electrical transformer it 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 manufacturerpremises - 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.

Electrical Power Transformer

Working Principle of TransformerIdeal Transformer

Theory of Transformer

EMF Equation of Transformer

Leakage Reactance of Transformer

Equivalent Circuit of TransformerVoltage Regulation of Transformer

Losses in TransformerOpen & Short Circuit Test on Transformer

Auto Transformer

Tertiary Winding of Transformer

Parallel operation of TransformersTransformer Cooling System

Core of Transformer

Transformer Insulating OilTransformer Accessories

Dissolved Gas Analysis of Transformer OilOver Fluxing in Transformer

Three phase transformerCurrent Transformer

Voltage Transformer

Accuracy Limit & Instrument Security Factor Knee Point Voltage of Current Transformer

Earthing or Grounding Transformer

External & Internal Faults in TransformerBackup Protection of Transformer

Differential Protection of Transformer

Restricted Earth Fault Protection

Buchholz Relay in TransformerTransformer Testing

Transformer winding resistance measurement

Transformer Ratio Test

Vector Group Test of Transformer Dielectric Test of Transformer
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Temperature Rise Test of Transformer

Transformer Impulse Test

Type of transformer testing

Tests done at Factory

Type Tests

Routine TestsSpecial Tests

Tests done at Site

Pre Commissioning Tests

Periodic/Condition Monitoring TestsEmergency 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 testsare 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 oftransformerconfirms 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 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 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 the test results

of all the low voltage tests with the factory test reports.
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Type tests of transformer includes

Transformer winding resistance measurementTransformer ratio test

Transformer vector group test

Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Shortcircuit test)Measurement of no load loss and current (Open circuit test)

Measurement of insulation resistance

Dielectric tests of transformerTemperature rise test of Transformer

Tests on on-load tap-changer

Vacuum tests on tank and radiators

Routine tests of transformer include

Transformer winding resistance measurementTransformer ratio test

Transformer vector group testMeasurement of impedance voltage/short circuit impedance (principal tap) and load loss (Shortcircuit test)

Measurement of no load loss and current (Open circuit test)

Measurement of insulation resistanceDielectric tests of transformer

Tests on on-load tap-changer

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

Dielectric Tests

Measurement of zero-sequence impedance of three-phase transformers

Short-Circuit TestMeasurement of acoustic noise level

Measurement of the harmonics of the no-load current

Measurement of the power taken by the fans and oil pumps

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 I2R losses and to

calculate winding temperature at the end of a temperature rise test. It is carried out as a type test
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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 voltage method of measurement of winding resistance.

Bridge method of measurement of winding resistance.

Kelvin bridge method of Measuring Winding Resistance. Measuring winding resistance by Automatic Winding Resistance Measurement Kit.

NB: -Transformer winding resistance measurementshall be carried out at each tap.


The performance of a transformer largely depends upon perfection of specific turns or voltage

ratio of transformer. Sotransformer ratio testis an essential type test of transformer. This testalso performed as routine test of transformer. So for ensuring proper performance of electrical

power transformer, voltage and turn ratio test of transformer one of the vital tests.

The procedure oftransformer ratio testis 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 LVterminals 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 230V AC supply across one of the HV winding terminals and neutral

terminal.

4) Measure the voltage in 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 IV

winding also.

There are three limbs side by side in a core of transformer. One phase winding is wound in onelimb. The voltage induced in different phases depends upon the respective position of the limb in
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the core. The voltage induced 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 phase 115 V 230 V 115 V

Voltage applied at right side phase 50 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 theelectriccurrentrequired to establish flux in the core.

1) First of all keep the tap changer in the lowest position and open all IV & LV terminals.

2) Then apply three phase 415V supply on the line terminals for three phase transformers and

single phase 230V supply on single phase transformers.

3) Measure the supply voltage andelectric currentin each phase.4) Now repeat the magnetizing current test of transformertest with keeping tap changer innormal 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. Anagreement 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 transformeris to be carried out before DCresistance measurement.

Vector Group Test of TransformerInthree phase transformer,it is essential to carry out avector group test of transformer.Proper

vector grouping in a transformer is an essential criteria forparallel operation of transformers.

There are several internal connection ofthree 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
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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 beforeprocuring oneelectrical power transformer,one should ensure the vector group of the

transformer, whether it will be matched with his or her existing system or not. Thevector group

test of transformerconfirms his or her requirements.

Insulation Resistance Test or Megger Test of transformer

Insulation resistance test of transformeris essential type test. This test is carried out to ensurethe 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 Resistance

IR 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 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 resistance test of transformerper 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

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
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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 voltage

withstand test of transformer, where a single phase power frequency voltage of 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 voltage test of Transformer where, three phase voltage,twice of rated secondary voltage is applied to the secondary winding for 60 second by keepingthe primary of the transformer open circuited. The frequency of the applied voltage should be

double of power frequency too. Here also if no failure of insulation, the test is successful.

In addition toDielectric tests of transformerthere are other type test for checking insulation oftransformer, such as lightning impulse test, switching impulse test and partial discharge test.

Induced voltage test of Transformer

The induced voltage test 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 voltage to the secondary winding. The applied voltage should be twice of

rated voltage of secondary winding in magnitude and frequency.3) The duration of the test shall be 60 second.

4) The test shall start with a voltage lower 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 voltage during test.


Temperature rise test of Transformeris included in type test of transformer. In this test wecheck 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.

Transformer Winding ResistanceMeasurement

Transformer winding resistance measurementis carried out as a type test, routinetest and also as a field test.

In the factory, it helps in determining the following :(a) Calculation of the I

2Rlosses in transformer

(b) Calculation of winding temperature at the end of temperature rise test of transformer.
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(c) As a benchmark for assessing possible damages in the field.It is done at site in order to check for abnormalities due to loose connections, brokenstrands of conductor, high contact resistance in tap changers, high voltage leads andbushings.

Procedure of Transformer winding resistance measurement

For star connected winding, the resistance shall be measured between the line andneutral terminal.

For star connected auto-transformers the resistance of the HV side is measuredbetween HV terminal and IV terminal, then between IV terminal and the neutral.

For delta connected windings, measurement of winding resistanceshall be donebetween pairs of line terminals. As in delta connection the resistance of individualwinding can not be measured separately, the resistance per winding shall be calculated

as per the following formula:

Resistance per winding = 1.5 x Measured value

The resistance is measured at ambient temperature and then converted to resistance at75C for all practical purposes of comparison with specified design values, previousresults and diagnostics.Winding Resistance at standard temperature of 75oC

R75= Rt

235+75

235+t

Rt= Winding resistance at temperature t.t = Winding temperature

Generally transformer windings are immersed in insulation liquid and covered withpaper insulation, hence it is impossible to measure the actual winding temperature in ade-energizing transformer at time of transformer winding resistance measurement.

An approximation is developed to calculate temperature of winding at that condition, asfollows

Temperature of winding = Average temperature of insulating oil.(Average temperature of insulating oil should be taken 3 to 8 hours after de-energizingtransformer and when the difference between top & bottom oil temperatures becomesless than 5oC)

The resistance can be measured by simple voltmeter ammeter method, Kelvin Bridgemeter or automatic winding resistance measurement kit (ohm meter, preferably 25


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Amps kit)

Caution for voltmeter ammeter method: Current shall not exceed 15% of the ratedcurrent of the winding. Large values may cause inaccuracy by heating the winding andthereby changing its temperature and resistance.

NB: - Measurement of winding resistance of transformershall be carried out at eachtap.

Current Voltage Method of measurement of windingresistance

The transformer winding resistances can be measured by current voltage method. Inthis method of measurement of winding resistance, the test current is injected to thewinding and corresponding voltage drop across the winding is measured.

By applying simple Ohm's law i.e. Rx=

V

one can easily determine the value of resistance.

I

Procedure of current voltage method of measurement of windingresistance

1) Before measurement the transformer should be kept in OFF condition withoutexcitation at least for 3 to 4 hours. During this time the winding temperature will become

equal to its oil temperature.


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2) Measurement is done with D.C.

3) To minimize observation errors, polarity of the core magnetization shall be keptconstant during all resistance readings.

4) Voltmeter leads shall be independent of the current leads to protect it from highvoltages which may occur during switching on and off the current circuit.

5) The readings shall be taken after theelectric currentand voltage have reachedsteady state values. In some cases this may take several minutes depending upon thewinding impedance.

6) The test current shall not exceed 15% of the rated current of the winding. Largevalues may cause inaccuracy by heating the winding and thereby changing itsresistance.

7) For expressing resistance, the corresponding temperature of the winding at the timeof measurement must be mentioned along with resistance value. As we said earlier thatafter remaining in switch off condition for 3 to 4 hours, the winding temperature wouldbecome equal to oil temperature. The oil temperature at the time of testing is taken asthe average of top oil and bottom oil temperatures of transformer.

8) For star connected three phase winding, the resistance per phase would be half ofmeasured resistance between two line terminals of the transformer.

9) For delta connected three phase winding, the resistance per phase would be 0.67times of measured resistance between two line terminals of the transformer.

10) This current voltage method of measurement of winding resistanceoftransformer should be repeated for each pair of line terminals of winding at every tapposition.
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Bridge Method of measurement of winding resistance

The main principle of bridge method is based on comparing an unknown resistance witha known resistance. Whenelectric currentsflowing through the arms of bridge circuitbecome balanced, the reading of galvanometer shows zero deflection that means atbalanced condition noelectric currentwill flow through the galvanometer. Very smallvalue of resistance ( in milli - ohms range) can be accurately measured by Kelvin Bridgemethod whereas for higher value Wheatstone bridge method of resistancemeasurement is applied. In bridge method of measurement of winding resistance, theerrors is minimized.

The resistance measured by Kelvin

Bridge;

Rx= RsRB

RA

( RA= Rave RB= Rb)
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All other steps to be taken during transformer winding resistance measurement in thesemethods are similar to that of current voltage method of measurement of windingresistance of transformer, except the measuring technique of resistance.

The resistance measured by

Wheatstone Bridge;

Rx= RA

B


The performance of a transformer largely depends upon perfection of specific turns orvoltage ratio of transformer. So transformer ration testis an essential type test oftransformer. The voltage should be applied only in the high voltage winding in order toavoid unsafe voltage.

Ratio test of transformer andcheck of phase displacement

Actually the no load voltage ratio of transformer is equal to the turn ratio. So ratio testof transformer

Procedure of transformer ration test

1) First, the tap changer of transformer is kept in the lowest position and LV terminalsare kept open.2) Then apply 3-phase 415 V supply on HV terminals. Measure the voltages applied oneach phase (Phase-Phase) on HV and induced voltages at LV terminalssimultaneously.3) After measuring the voltages at HV and LV terminals, the tap changer of transformer


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should be raised by one position and repeat test.4) Repeat the same for each of the tap position separately.

The above transformer ratio testcan also be performed by portable TransformerTurns Ratio (TTR) Meter. They have an in built power supply, with the voltages

commonly used being very low, such as 8-10 V and 50 Hz. The HV and LV windings ofone phase of a transformer are connected to the instrument, and the internal bridgeelements are varied to produce a null indication on the detector.

Let's have a discussion on Transformer Turns Ratio (TTR) Meter method of turn ratiotest of transformer

A phase voltage is applied to the one of the windings by means of a bridge circuit andthe ratio of induced voltage is measured at the bridge. The accuracy of the measuringinstrument is < 0.1 %

Theoretical turn ratio =

HV winding voltage

LV winding voltage

This theoretical turn ratio is adjusted on the transformer turn ratio tested or TTR by theadjustable transformer as shown in the figure above and it should be changed until abalance occurs in the percentage error indicator. The reading on this indicator impliesthe deviaton of measured turn ratio from expected turn ratio in percentage.

Deviation in percentage = Measured turn ratio - expected turn ratioX 100 %


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Expected turn ratio

Out-of-tolerance, ratio test of transformercan be due to shorted turns, especially ifthere is an associated high excitation current. Open turns in HV winding will indicate

very low exciting current and no output voltage since open turns in HV winding causesno excitation current in the winding means no flux hence no induced voltage. But openturn in LV winding causes, low fluctuating LV voltage but normal excitation current in HVwinding. Hence open turns in LV winding will be indicated by normal levels of excitingcurrent, but very low levels of unstable output voltage. The turn ratio test oftransformeralso detects high resistance connections in the lead circuitry or highcontact resistance in tap changers by higher excitation current and a difficulty inbalancing the bridge.

Vector Group Test of Transformer

The vector group of transformeris an essential property for successfulparalleloperation of transformers.Hence everyelectrical power transformermust undergothrough vector group test of transformerat factory site for ensuring the customerspecified vector group of transformer.

The phase sequence or the order in which the phases reach their maximum positivevoltages, must be identical for two paralleled transformers. Otherwise, during the cycle,each pair of phases will be short circuited.

The several secondary connections are available in respect of various primary three phase

connection in a thethree phase transformer.So for same primary applied three phase voltagethere may be different three phase secondary voltages with various magnitudes and phases for

different internal connection of the transformer. Let's have a discussion in detail by example for

better understanding.

We know that, the primary and secondary coils on any one limb have induced emfs thatare in time - phase. Let's consider two transformers of same number primary turns andthe primary windings are connected in star. The secondary number of turns per phasein both transformers are also same. But the first transformer has star connectedsecondary and other transformer has delta connected secondary. If same voltages areapplied in primary of both transformers, the secondary induced emf in each phase will

be in same time-phase with that of respective primary phase, as because the theprimary and secondary coils of same phase are wound on the same limb in thecore oftransformer.In first transformer, as the secondary is star connected, the secondary linevoltage is 3 times of induced voltage per secondary phase coil. But in case of secondtransformer, where secondary is delta connected, the line voltage is equal to inducedvoltage per secondary phase coil. If we go through the vector diagram of secondary linevoltages of both transformer, we will easily find that there will be a clear 30 oangulardifference between the line voltages of these transformers. Now, if we try to run these
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transformers in parallel then there will be a circulating current flows between thetransformers as because there is a phase angle difference between their secondary linevoltages. This phase difference can not be compensated. Thus two sets of connectionsgiving secondary voltages with a phase displacement can not be intended forparalleloperation of transformers.

The following table gives the connections for which from the view point of phasesequence and angular divergences, transformer can be operated parallel. According totheir vector relation, allthree phase transformers are divided into different vector groupof transformer. Allelectrical power transformers of a particular vector group can easilybe operated in parallel if they fulfill other condition forparallel operation of transformers.

GROU

PConnection Connection

0

(0o)

Yy0 Dd0

1

( 30o)

Yd1 Dy1
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6( 180o)

Yy6 Dd6

11

( - 30o)

Yd11 Dy11


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Procedure of vector group test of transformer

Lets have a YNd11transformer.

1) Connect neutral point of star connected winding with earth.

2) Join 1U of HV and 2W of LV together.

3) Apply 415V, three phase supply to HV terminals.

4) Measure voltages between terminals 2U1N, 2V1N, 2W1N, that meansvoltages between each LV terminal and HV neutral.

5) Also measure voltages between terminals 2V1V, 2W1W and 2V1W.

For YNd11transformer, we will find,2U1N > 2V1N > 2W1N2V1W > 2V1V or 2W1W

The vector group test of transformerfor other group can also be done in similar way.

Dielectric Test of Transformer

The dielectric test of transformeris generally performed in two different steps,likewise, separate source voltage withstand test and induced voltage withstand test oftransformer, which we have discussed one by one below.

Separate source voltage withstand test of transformer

This dielectric testis intended to check the the ability of main insulation to earth andbetween winding.


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Procedure

1) All three line terminals of the winding to be tested are connected together.

2) Other winding terminals which are not under test and also tank of the transformer

should be connected to earth.

3) Then a single-phase power frequency voltage of shape approximately sinusoidal isapplied for 60 seconds to the terminals of the winding under test.

4) The test shall be performed on all the windings one by one.

6) The test is successful if no break down in the dielectric of the insulation occurs duringtest.

In thistransformer testing,the peak value of voltage is measured, that is why thecapacitor voltage divider with digital peak voltmeter is employed as shown in thediagram above. The peal value multiplied by 0.707 (1/2) is the test voltage.

The values of test voltage for different fully insulated winding are furnished below in thetable.

Nominal systemvoltage rating

for equipment

Highest systemvoltage rating

for equipment

Rated short durationpower frequency withstand

voltage

415V 1.1 KV 3 KV

11 KV 12 KV 28 KV
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33 KV 36 KV 70 KV

132 KV 145 KV 230 / 275 KV

220 KV 245 KV 360 / 395 KV

400 KV 420 KV 570 / 630 KV

Winding with graded insulation, which has neutral intended for direct earthing,

is tested at 38KV

Induced voltage test of Transformer

The induced voltage test of transformeris intended to check the inter turn and lineend insulation as well as main insulation to earth and between windings

1) Keep the primary winding of transformer open circuited.

2) Apply three phase voltage to the secondary winding. The applied voltage should betwice of rated voltage of secondary winding in magnitude and frequency.

3) The duration of the test shall be 60 second.

4) The test shall start with a voltage lower than 1/3 the full test voltage, and it shall bequickly increased up to desired value.

The test is successful if no break down occurs at full test voltage during test.


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Temperature rise test of Transformeris included in type test of transformer. In this test we

check whether the temperature rising limit of transformer winding and oil as per specification ornot.

Temperature rise test for top oil of transformer

1) First the LV winding of the transformer is short circuited.

2) Then one thermometer is placed in a pocket in transformer top cover. Other twothermometers are placed at the inlet and outlet of the coolerbankrespectively.

3) The voltage of such value is applied to the HV winding that power input is equal to noload losses plus load losses corrected to a reference temperature of 75oC.

4) The total losses are measured by three watt-meters method.

5) During the test, hourly readings of top oil temperature are taken from thethermometer already placed in the pocket of top cover.

6) Hourly readings of the thermometers placed at inlet and outlet of the cooler bank arealso noted to calculate the mean temperature of the oil.

7) Ambient temperature is measured by means of thermometer placed around the

transformer at three or four points situated at a distance of 1 to 2 meter from and half-way up the cooling surface of the transformer.

8) Temperature rise test for top oil of transformer should be continued until the top oiltemperature has reached an approximate steady value that means testing would becontinued until the temperature increment of the top oil becomes less than 3

oC in one

hour. This steady value of top oil is determined as final temperature rise of transformerinsulating oil.
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9) There is another method ofdetermination of oil temperature. Here the test in allowed to be continued until the top oiltemperature rise does not vary more than 1oC per hour for four consecutive hours. Theleast reading is taken as final temperature rise of the oil.

During temperature rise test for top oil of transformer we make the LV windingshort circuited and apply voltage to the HV winding. So for full load ratedelectriccurrentflows in the transformer, the supply voltage required will much less thanrated transformer voltage. We know that core loss of a transformer depends uponvoltage. So there will not be any considerable core loss occurs in the transformerduring test. But for getting actual temperature rise of the oil in a transformer, wehave to compensate the lack of core losses by additional copper loss in thetransformer. For supplying this total losses, transformer drawselectric currentfrom the source much more than its rated value for transformer.

Temperature rise limits of transformer when it is oil immersed, given in the table below
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Temperature rise

limit

for air as

cooling medium

Temperature rise

limit

for water as

cooling medium

Condition

Winding

55oC 60oC When oil circulation is natural

60oC 65oC When oil circulation is forced

Top Oil

50oC 55oCWhen transformer is sealed &

equipped with conservator tank

45oC 50oC

When transformer is neither

sealednor equipped with conservator

tank

NB: These temperature rises limits mentioned in the above table are the temperaturerise above the temperature of cooling medium. That means these are the differencebetween winding or oil temperature and temperature of cooling air or water.

Winding temperature rise test on transformer

1) After completion of temperature rise test for top oil of transformer the current isreduced to its rated value for transformer and is maintained for one hour.

2) After one hour the supply is switch off and short circuit and supply connection to theHV side and short circuit connection to the LV side are opened.

3) But, the fans and pumps are kept running (if any)


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4) Then resistance of thewindings are measured quickly.

5) But there is always a minimum 3 to 4 minutes time gap between first measurement ofresistance and the instant of switching off the transformer, which can not be avoided.

5) Then the resistances are measured at the same 3 to 4 minutes time intervals over aperiod of 15 minutes.

6) Graph of hot resistance versus time is plotted, from which winding resistance (R2) at

the instant of shut down can be extrapolated.

7) From this value, 2, the winding temperature at the instant of shut down can bedetermined by the formula given below

2=R2

(235 + t1) - 235R1

Where, R1is the cold resistance of the winding at temperature t 1.

For determining winding temperature rise we have to apply the above discussedindirect method. That means hot winding resistance is measured and determinedfirst and then from that value we have to calculate the winding temperature rise,by applying resistance temperature relation formula. This is because unlike oilthe winding of transformer is not accessible for external temperaturemeasurement.


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Impulse Test of Transformer

Lighting is a common phenomenon in transmission lines because of their tall height.This lightning stroke on the line conductor causes impulse voltage. The terminal

equipment of transmission line such as power transformer then experiences thislightning impulse voltages. Again during all kind of online switching operation in thesystem, there will be switching impulses occur in the network. The magnitude of theswitching impulses may be about 3.5 times the system voltage.

Insulation is one of the most important constituents of a transformer. Any weakness inthe insulation may cause failure of transformer. To ensure the effectiveness of theinsulation system of a transformer, it must confirms the dielectric test. But the powerfrequency withstand test alone can not be adequate to demonstrate the dielectricstrength of a transformer. That is why impulse test of transformerperformed on it.Both lightning impulse testand switching impulse testare included in this category

of testing.

Lightning Impulse

The lightning impulse is a pure natural phenomenon. So it is very difficult to predict theactual wave shape of an lightning disturbance. From the data compiled about naturallightning, it may be concluded that the system disturbance due to natural lightningstroke, can be represented by three basic wave shapes.

1) Full wave2) Chopped wave and

3) Front of wave

Although the actual lightning impulse disturbance may not have exactly these threeshapes but by defining these waves one can establish a minimum impulse dielectricstrength of a transformer.If lighting disturbance travels some distance along the transmission line before itreaches the transformer, its wave shape may approach to full wave.If during traveling, if flash-over occurs at any insulator of the transmission line, after thepeak of the wave has been reached, the wave may become in form of chopped wave.If the lightning stroke directly hits the transformer terminals, the impulse voltage risesrapidly until it is relieved by a flash over. At the instant of flash - over the voltage

suddenly collapses and may form the front of wave shape.The effect of these wave forms on the transformer insulation may be different from eachother. We are not going here in detail discussion of what type of impulse voltage waveforms causes what type of failure in transformer. But whatever may be the shape oflightning disturbance voltage wave, all of them can cause insulation failure intransformer. So lighting impulse test of transformeris one of the most important typetest of transformer.


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Switching Impulse

Through studies and observations reveal that the switching over voltage or switchingimpulse may have front time of several hundred microseconds and this voltage may beperiodically damped out. The IEC - 600060 has adopted for their switching impulse test,

a long wave having front time 250 s and time to half value 2500 s with tolerances.

213366971 Transformer Testing

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