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J S Sastry

Testing of Transformers


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I F YOU HAVE NO DOUBTSYOU HAVE NOT UNDERSTOOD

ANYTHING.

- a Spanish proverb.


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Categories of Tests

1. Tests to prove transformer is built

correctly Voltage Ratio, Polarity,Vector Group, Winding Resistance and

tap change operation.

2. Tests to prove guarantees no-loadand load loss, impedance, oil and

winding temperatures, noise level

3. Tests to prove satisfactory service life -

Dielectric tests and Short-circuit test.


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Schedu le o f Tes ts

Routine Tests Dielectric Tests

Parametric Tests

Type Tests Temperature Rise Test

Impulse Test

Special Tests Short-circuit Test

Magnetic Balance Test

Measurement of Zero sequence Test

Measurement of Noise Level


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Ratio meter (working as potentiometer)

having accuracy 0.1% to be used.

Tolerance allowed : 0.5%.

Ratio also confirms polarity as balance

can not be obtained if winding

connection is reversed.

Ratio & Polar i ty Tes t


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Polar i ty Check Test

V1

V2

HV LV

V2 V1>

for subtractive polarity


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Test fo r Vecto r Group

1U 2u

2v2w2n

1N

1W 1V

Supply 440 V 3-phase supply

across HV terminals 1U-1V-1W.

Terminal Measured Voltage

1V - 2v

1W 2w

1U 2nYy0


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Tes t fo r Vecto r Group

Terminal Measured Voltage

1W 2w

1V 2w

1V 2u

2v 2n

1W 2n

1W 2u

Supply 440 V 3-phase supply

across HV terminals 1U-1V-1W.

Dy1

1U

1V1W

2u

2v2w

1U 2n

1V1W

2u

2v2w


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Kelvin Double Bridge used

25 30 Amps DC required

Ambient/oil temperature to be noted

If oil, average of top & bottom temperatures

to be considered.

Conversion to 750C by:

R75 = Ra x (234.5 + 75)/(234.5 + Ta)

where Ra is resistance at ambient

temperature Ta.

Measu remen t of Wind ing

Resistance


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Measu rement o f Wind ing Resistance

G

P Q

p q

S RX L

External Resistance E

RX = S.Q/P where, RX = Resistance of winding under test

S = Standard Resistance (part of measuring Bridge


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2 KV or 5 KV Insulation Resistance

Tester (Megger) is used.

IR Value at 1 minute and ambient /

average oil temperature to be noted.

15 seconds and 60 seconds or 1 minute

and 10 minute readings also are noted

sometimes to calculate PI of the

insulation system.

Measu remen t o f Insu lat ion

Resistance


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Measu rement of Insulation Resistance

LV

2n 2u 2v 2w

L

G

E

1U 1V 1W

HV

LV

2n 2u 2v 2w

L

G

E

1U 1V 1W

HV

LV

2n 2u 2v 2w

L

G

E

1U 1V 1W

(a) (b) ( c )

(a) IR Value between HV and earth

(b) IR value between LV and earth

(c) IR Value between HV and LV


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Also called Separate Source Voltagewithstand test.

Conducted at 50 Hz for 60 seconds.

All windings other than the winding undertest are shorted and connected to earth.

Checks the Major Insulation of thetransformer.

Values for uniformly insulated windings

are given in the standards. Non-uniformlyinsulated windings with neutral are testedat 38 KV. The transformer shouldwithstand the specified voltage withoutany breakdown.

Power Frequency Test


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LV

2n 2u 2v 2w

1U 1V 1W

HV

LV

2n 2u 2v 2w

1U 1V 1W

HV

Power Frequency Test

~

~


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Usually conducted at 50 Hz from LV sidekeeping HV open.

Core loss consists of Hysteresis and EddyCurrent losses.

Hysteresis loss depends on average valueand eddy current loss on rms value of thesupply voltage.

Hence two voltmeters (average voltmeter

scaled to read 1.11 times the average voltageand rms voltmeter) are used.

Generally 3-wattmeter method is used.

N0-Load Loss Tes t


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N0-Load Loss Test

1U

1V

1W

2v

2w

2uCT

VT3 PhaseVoltage at

Rated Frequency


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Rated voltage as per average voltmeter

(Ua) is applied and readings of rms

voltmeter, no-load currents and lossesof the three phases are noted.

Corrected loss Pm = Pm (1+d) where

Pm = sum of the three wattmeter

readings, and

d = (Ua Urms)/Ua

N0-Load Loss Tes t


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Load loss comprises of I2R loss and stray loss whereR is the resistance of the winding.

I2R loss varies directly with temperature and strayloss varies inversely with temperature.

Measured loss is to be corrected to 750C. Test is carried out by short circuiting usually LV

windings and feeding rated current to HV windings.The voltage measured is the reactance voltage.

3-wattmeter method is normally used formeasurement of losses. Load loss test to beconducted preferably at rated current or at any valuebetween 50 100% rated current.

Load Loss & Impedance Test


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1U

1V

1W

2v

2w

2u

CT

VT3 PhaseControlled Voltage

at Rated Frequency


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Calculation of Load loss at 750C:

Stray loss Sa at ambient / average oil

temperature (Ta) = (Sum of three wattmeter

readings sum of HV & LV I2R losses at Ta) Stray loss S75at 75

0C = Sa x (234.5 + Ta)

(234.5+75)

Load loss at T75 = (Sum of HV & LV I2Rlosses at T75 + S75)



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Calculation of %Impedance Voltage (Iz):

%Iz = (%Ir2 + %Ix2) where,

%Ir = (Load loss at 750C)x100/Rated KVA

of transformer, and

%Ix = (Reactance voltage at rated

current)x100/Rated voltage.



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Induced Over Vo ltage Test

This test is conducted to check the Minor insulationof a transformer.

In case of non-uniformly insulated windings itchecks total insulation (both major and minor).

In case of transformers with uniformly insulatedwindings, 2 times normal voltage is applied to the LVwindings at 2 times normal frequency for 60 secondsduration keeping the HV open.

In case of non-uniformly insulated windings, thevoltage equal to two times or more is applied to LVterminals in such a way that the specified voltage isinduced across the HV terminals.

The duration and frequency can be so adjusted thatthe windings are subjected 6,000 cycles of therequired voltage.


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Induced Over Voltage Test(For Uniformly Distributed winding transformers)

1U

1V

1W

2v

2w

2uCT

VT

3 Phase2 x Rated Voltage

At 2 x Rated

Frequency for

1 Minute


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2u 2v 2w

1U 1V 1W

LV

~

Induced Over Voltage Test(For Non-uniformly Distributed winding transformers)

HV154.33 KV

38.1 KV

at 100 Hz

77.17 KV

Note: Test circuit shown for a

typical 132/33 KV Transformer.

231.5 KV


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For system voltages < 300 KV, Lightning Impulse Testis a type test. For system voltages 300 KV, it is aroutine test.

Chopped wave test is a type test in all cases.

Wave shape of the impulse voltage:

Front: 1.2 s 30% and tail: 50 s 20%. The transformer should withstand the specified voltage

with out any breakdown.

Breakdown is indicated by voltage collapse or changein the neutral current wave shape.

Test sequence: 1 Reduced full wave1 100% Full wave1 Reduced chopped wave If required.2 100% Chopped waves2 100% Full waves

Test for L ightn ing Impu lse


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Test for L ightn ing Impu lse

1U

1V1W

2v

2w

2u

For Neutral Current

Wave shape through

Oscilloscope.

R1 R2

R1 & R2 are external

resistors used for

obtaining wave shape

if required.

Circuit shown for HV


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OutputRf

Rt

Rf

Rt

Rf

Rt

Rf

Rt

Rc

Rc

Rc

C1

C3

C2

Cn

G1

G2

G3

Gn

C1,C2,Cn : Stage CapacitorG1,G2,Gn : Stage GapsRc: Charging Resistance

Rf : Series Resistance (Front)

Rt : Discharge Resistance (Tail)

Impu lse Generato r


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1.00.9

0.5

0.3

0

Time

Voltag

e T1T2

1.0

0.9

0.50.3

0Time

Vol

tage

T1Tc

Impu lse Wave Shapes


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Sw itch ing Impu lse test

Switching over voltages due to switching-on orswitching-off of transformer.

For system voltages > 300 KV, it is routine test.

Switching impulse wave shape to have

specified amplitude of at least 200 s and atotal duration of at least 500 s, but preferably1000 s.

Sequence of impulse voltage application:

1 Reduced full wave2 Full waves Voltage and neutral current wave shapes are

recorded and examined for fault detection.


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1.00.9

0.5

0.3

0

Time

Voltage

Sw itch ing Impulse Wave Shape

200 s

100 s 1000 s(at least 500 )s


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Part ial Discharge Measurement

Causes of Partial Discharges:

Insufficient processing

High stress zones

Sharp points

Cavities in insulation

Bad contacts

Foreign objects inside tank.

Basically a quality control check


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Circuit similar to Open Circuit test (No-load loss test).

Supply 1.5 times rated voltage at higher

frequency for 30 minutes. Check electrical signals generated by

discharges produced with a partial

discharge detector. Discharges measured should be less

than 500 pc.

Part ial Discharge Measurement


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Zero-phase sequence impedance ismeasured on star connected windings withneutral earthed.

This determines the amplitude of current that

will flow in the event of a line to earth fault. Reluctance path for zero sequence flux is

different in 3-limb core and 5-limb core.Hence value of ZPS Impedance depends

upon type of core construction. %Z0 = 3 x voltage measured x rated current

Vph x rated current

Zero -Phase Sequence Tes t


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Zero -Phase Sequence Test

2u 2v 2w

1U 1V 1W

HV

3u 3v 3w

SV

HV

LV

~

1. Short 1U 1V 1W and supply 1U1V1W -1N, with 2u-2v-2w open and tertiary closed.

Measure Z (H T).

2. Short 1U 1V 1W and supply 1U1V1W -1N, with 2u 2v 2w shorted and tertiaryclosed. Measure Z (H /L T).

3. Open 1U 1V 1W and supply 2u-2v-2w -2n, with 2u 2v 2w shorted and tertiaryclosed. Measure Z (L T).

4. Short 1U 1V 1W and supply 1U1V1W -

1N, with 2u 2v 2w shorted and tertiaryopen. Measure Z (H -L).

5. Short 2u-2v-2w and supply 2u-2v-2w 2n,with 1U-1V-1W shorted and tertiary closed.

Measure Z (L/H T).

1N

2n


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Short-Circu i t Tes t

SC current is calculated by using

measured SC impedance of the

transformer plus the system impedance.

Two types of SC Test procedures can be

followed:

Pre-set method: LV winding is shorted prior

to application of power to HV winding. Post-set method: Here one of the windings


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Test calls for 3 shots per phase oneeach at maximum tap, normal tap and

minimum tap.

Impedance measured after the SC Test

should not be more than 2% of thevalue measured before the test.

Short-Circu i t Tes t


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Measu rement of Acous t ic Noise Level

Acoustic noise due to core vibrations

(magnetostriction).

Measured under open circuit test

conditions with all the oil pumps, fans

etc. running.

Test to be conducted as per NEMATRI 1971 and noise levels should notexceed the values specified.


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