Transformer Testing

 

CPP Electrical 165 MW CPP System Transformer Testing

 

TRANSFORMER TEST

Feorrformcaonncfiers mo infgan etlheectricsap nesr it a p l etrcaifnic sa fotirom hn ad s to go through numbers of testing procedures. Some tests are done at manufacturer premises before delivering the transformer. Mainly two types of transformer testing are done at man anu ufa fact ctur urer er pr pre emi mise ses s - type test of transformer  and routine test of transformer . In addition to that so som me tr tra ansfor orm mer tests are also carried out at the consumer site befo forre commissi sio oning and also periodically in regular & emergency basis through out its service life. 2

 

TRANSFORMER TEST

Feorrformcaonncfiers mo infgan etlheectricsap nesr it a p l etrcaifnic sa fotirom hn ad s to go through numbers of testing procedures. Some tests are done at manufacturer premises before delivering the transformer. Mainly two types of transformer testing are done at man anu ufa fact ctur urer er pr pre emi mise ses s - type test of transformer  and routine test of transformer . In addition to that so som me tr tra ansfor orm mer tests are also carried out at the consumer site befo forre commissi sio oning and also periodically in regular & emergency basis through out its service life. 3

 

Type of transformer testing

Tests done at Factory

1. Routine Test 2. Type Test

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Routine tests of transformer 

Routine tests of transformer is mainly for con confir firmi ming ng ope opera ratio tional nal performance of individual unit in a production lot. Routine tests

are carried out on every unit manufactured.

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Routine tests of transformer 

1. Measurement of insulation resistance 2. Transformer Ratio test 3. Magnetic Balance test 4. Transformer Transformer winding resistance measurement 5. Separate source voltage withstand test (High Voltage tests on HV & LV) 6. Induced Over voltage Withstand test (DVDF test). 7. Short Circui Circuitt Test 8. BDV of transformer oil 9. Transformer vector group test 10.. Tests 10 Tests on OLTC OLTC (if (if Attached) Montag, 7. Dezember 2015

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Type tests of transformer 

THESE TESTS ARE CARRIED OUT ONL ONLY Y ON ONE TRANSFORMER OF THE LOT.

All routine tests Additionally following tests are included in type tests 1. Lightening Impulse test. 2.

Temperature rise test

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Insulation Resistance Test of transformer  Insulation resistance test of transformer is essential type test. 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 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 leads to be connected to LV LV bushing studs and transformer transformer tank earth point to measure Insulation Resistance IR value in between the LV windings and earth. Note : It is unnecessary to perform perform insulation insulation resistance resistance test of transformer 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. Montag, 7. Dezember 2015

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Insulation Resistance Test or Megger Test of transformer  Measurements are to be taken as follows:

For Two Winding Transformer: HV to LV, HV to E, LV to E Three Winding Transformer: HV to LV,, HV 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 1 minute value/ sec s.value secs. value. PolarizationCoefficient Index = 10 = minutes value / 1 15 minute Montag, 7. Dezember 2015

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Insulation Resistance Test of transformer 

IR of Transformer HV-LV HV-EARTH LV-EARTH

VALUE AT 1 MIN.

Insulation Resistance Test of transformer 

PI VALU VALUE E

 

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 tran sformer.. This test also performed as routine test of transformer. So for ensuring proper performance perfor mance of electrical power transformer transformer,, voltage and 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 LV terminals of transformer to find out actual voltage ratio of transformer. We We repeat the test for all tap position separately.

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Transformer Ratio Test Procedure of transformer ratio test 1.F 1. First, the tap changer of transformer is kept in the lowest position and LV terminals are kept open. 2.Th 2. The en ap appl ply y 33-ph phas ase e 415 V supply on HV terminals. Measure the voltages applied on each phase (Phase-Phase) on HV and indu in duce ced d vo volt ltag ages es at LV te term rmin inal als s si simu mult ltan aneo eous usly ly.. 3. After measuring the voltages at HV and LV terminals, the tap changer of transformer should be raised by one position and repea eatt test st.. 4.Repeat the same for each of the tap position separately.

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RATIO TEST:-

•

Transformer Ratio Test

TAP

APPLIED PRIMARY

MEASURED SECONDARY

NO.

VOLTAGE (IN VOLT)

VOLTAGE (IN VOLT)

1U1V 1 2

414.3 416.2

1V1W 1W1U

2U2V

2V2W

2W2U

416

126.1

126.4

126.1

415.3

415 415.1

129.7

129.6

415.3

416.7

415.3

132.7

132.4

132.1

4

415.4

414.2

415.2

134.9

135.4

135.3

413.8

415.1

415.6

138.8

139.1

U

V

W

3.29

3.29

3.29

3.21

3.20

3.21

3.13

3.15

3.14

3.08

3.06

3.07

2.98

2.98

2.99

129.3

3

5

RATIO

138.9

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RATIO TEST:-

•

Magnetizing Current Test

MAGNETIZING CURRENT Tap No AP APPL PLIE IED D VO VOLT LTAG AGE E IN IN H.V H.V SI SIDE DE Tap No

MEAS ME ASUR URED ED CU CURR RREN ENTT IN IN H.V H.V SIDE (mA)

3

1U1V

1V1W

1W1U

1U

1V

1W

417

419

416

33.1mA

25.1mA

33.7mA

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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 apply single phase phase 230V AC supply supply across one of the HV winding terminals and neutral terminal. 4.Measure the voltage in two other HV terminals ter minals in respect of neutral terminal. 5.Repeat the test for each of the three phases.

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Magnetic balance test of transformer  Left side phase

Central phase

Right side phase

RN

YN

BN

Voltage applied at left side phase

230

180

50

Voltage applied at central phase

115

230

115

50

180

230

Voltage applied at right phase

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measurement test of Winding resistance measurement transformer  Transformer winding resistance measurement is 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 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. Not ote: e: - Tran ans sfo form rmer er wi win ndi ding ng res esis isttan ance ce mea easu sure rem men entt sha hall ll be carried out at each tap.

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measurement test of Winding resistance measurement transformer  Proc oced edur ure e of Tra rans nsfo form rmer er wi wind ndin ing g re resi sist stan ance ce me meas asur urem emen ent: t:-Pr For star connecte ted d wi win nding, the resistanc nce e shal alll be measured betwee een n the line and neutral term rmiinal al.. For delta connected windings, measurement of winding resistance shall be done between pairs of line terminals. As in delta connection the resistance of individual winding 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

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measurement test of Winding resistance measurement transformer  HV SIDE (Phase to phase) VALUE IN milli ohm TAP NO.

1U1V

1V1W

1W1U

3

83.5 m ohm

83.3 m ohm

83.5 m ohm

HV SIDE (Phase to Neutral ) VALUE IN milli ohm TAP NO.

1U1N

1V1N

1W1N

3

42.3 m ohm

42.3 m ohm

43.1 m ohm

L.V SIDE: SIDE: VALUE IN milli ohm 2U2V

2V2W

2W2U

5.8 m ohm

5.7 m ohm

5.7 m ohm

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Separate source voltage withstand test Seperate source voltage withstand test (High (High Voltage Voltage tests on HV & LV)LV)This test checks the insulation property between Primary to earth, Secondary to earth and between Primary & Secondary. Secondary. HV high voltage test : LV LV winding connected together and earthed. HV winding connected together together and given 28 KV ( for 11KV 11KV transformer) for 1 minute. LV high Voltage test : HV winding win ding connected together and earthed. LV LV winding connected together and given 3 KV for 1 minute. Equipment used : High Hi gh Voltage Voltage tester ( 100KV & 3KV)

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Separate source voltage withstand test

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Dielectric Tests of Transformer  Procedure

1.All three line terminals of the winding to be tested are connected together. 2.Other wi win nding terminals which are not under test and also tank of the transf tran sfor orm mer sh shou ould ld be co conn nnec ecte ted d to ea earrth th.. 3. Then a single-phase power frequency voltage of shape approximately sinusoidal is applied for 60 seconds to the terminals of  the th e wi win ndi ding ng un unde derr te tes st. 4.The test shall be performed on all the wi win ndings one by one. 5. The test is successful if no break down in the dielectric of the insu in sula lati tion on oc occu curs rs du duri ring ng te test st..

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Dielectric Tests of Transformer  The values of test voltage for different fully insulated winding are furnished below in the table. Nominal system voltage rating for equipment

Highest system voltage rating for equipment

Rated short duration power frequency withstand voltage

415V

1.1 KV

3 KV

11 KV

12 KV

28 K V

33 KV

36 KV

70 K V

132 KV

145 KV

230 / 275 KV

220 KV

245 KV

360 / 395 KV

400 KV

420 KV

570 / 630 KV

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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 bet ween wind winding ings s Procedure of induced voltage test of transformer:1.Kee 1.K eep p th the e pr prim imar ary y wi win ndi ding ng of tra rans nsfo form rmer er op ope en ci cirrcu cuiite ted. d. 2. Apply three phase voltage to the secondary winding. The applied voltage should be twice of rated voltage of secondary winding in magnit mag nitude ude and fre freque quency ncy.. 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 duri du ring ng te test st..

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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 for parallel operation of transformers. There are several internal connection of three phase transformer are 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 vec ecto torr gro rou up tes estt of tr tran ansf sfo orm rmer er co con nfi firm rms s hi his s or he herr req equi uire rem men ents ts..

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BDV of Transformer oil Dielectric strength of transformer oil is also known as breakdown volta oltage ge of tr tran ansf sfo ormer rmer oil oil or  or B BDV of transformer oil. Break down voltage is measured by observing at what voltage, sparking strants between two electrods immerged in the oil, separated by specific gap. low value of BDV indica indi cate tes s pr pres esen ence ce of mo mois istu ture re co cont nten entt an and d condu onduct ctiing subs substtance ances s in the the oil. oil. For measuring BDV of transformer oil, portable BDV measuring kit is generally available at site. In this kit, oil is kept in a pot in which one pair of  electrodes are fixed with a gap of 2.5 mm (in some kit it 4mm) between them.

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BDV of Transformer oil Slowly rising voltage is applied between the electrodes. The rate of rising voltage is controlled at 2 KV/s and observe the voltage at which sparking starts between the electrodes. That means at which voltage dielectric strength of transformer oil between the electrodes has been broken down.

This measurement is taken 3 to 6 times in the same sample of oil and the average value of these reading is taken. BDV is an important and popular test of transformer oil, as it is the primary indicator of the health of oil and it can be easily carried out at the site. Dry and clean oil gives BDV results, better than the oil with moisture content and other conducting impurities. Minimum breakdown voltage of transformer oil or dielectric strength of transformer oil at which this oil can safely be used in transformer, is considered as 30 KV.

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Specific Resistance of Transformer Oil This is another important property of transformer oil. This is a measure of DC resistance between two opposite sides of one cm3 block of oil. Its unit is taken as ohm-cm at a specific temperature. With increase in temperature the resistivity of oil decreases rapidly. Just after charging a transformer after long shut down, the temperature of the oil will be at ambient temperature and oduring full load, the temperature will be very high and may go up to 90 C at an overload condition. So resistivity of the insulating oil must be high at room temperature and also it should have good value at high temperature as well. That is why specific resistance or resistivity of transformer oil should be measured at 27oC as well as 90oC. Minimum standard specific resistance of transformer oil at 90oC is 35 × 1012 ohm –  –cm cm and at 27oC it is 1500 × 1012 ohm –cm.  –cm.

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Water Content in Transformer Oil

Moisture or water content in transformer oil is highly undesirable as it affects adversely the dielectric properties of the oil. The water content in oil also affects the paper insulation of the core and winding of a transformer. Paper is highly hygroscopic in nature. Paper absorbs the maximum amount of water from oil which affects paper insulation property as well as reduced its life. But in a loaded transformer, oilthe becomes hotter, hence water in content oil increases, as a result, paper releases waterthe andsolubility increaseof the water in transformer oil. Thus the temperature of the oil at the time of taking a sample for the test is very important. During oxidation, acids are formed in the oil the acids give rise the solubility of water in the oil. Acid coupled with water further decompose the oil forming more acid and water. This rate of degradation of oil increases. The water content in oil is measured as ppm (parts per million unit). The water content in oil is allowed up to 50 ppm as recommended

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Vector Group Test Test of Transformers

Vector group identification of 3-ph transformers are necessary in order to select transformers to be operated in parallel because the same or compatible vector groups of transformers alone can be paralleled. The Vector Group of Transformer is an important property for successful operation of transformers. The phase sequence or the order in which the phases reach their maximum positive voltages, must be identical for two paralleled transformers. Otherwise, during the cycle, each pair of phases will be short circuited. Dd0 Delta connected HV winding, delta connected LV winding, no phase shift between HV and LV. Dyn11 Delta connected HV winding, star connected LV winding with neutral brought out, LV is leading HV with 30¦  YNd5 Star connected HV winding with neutral brought out, delta connected LV winding, LV lags HV with 150¦

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Vector Group Test of Transformer  Procedure of vector group test of transformer:Let’s have a Dyn11 transformer. 1.Connect neutral point of star connected winding with earth. 2. Join 1U of HV and and 2W of LV LV together. together. 3. Apply 415V, 415V, three phase supply to HV terminals. terminals. Applied voltage in HV side (1U & 2U shorted):1U1V:1V1W:1W1U:Measured voltage:1U2N + 1V2N =1U1V 1V2V = 1V2W 1W2W < 1W2V

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Vector Group Test of Transformer 

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