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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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Load Loss & Impedance Test
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)
Load Loss & Impedance Test
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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.
Load Loss & Impedance Test
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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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