Transformer Testing Guide: Key Procedures and Standards

Testing
of
Transformer
K.N.Srinivasan

Traceabllity
Accuracy
Precision
Probability
Std. deviation
Standards
Accreditation

Insulation tests
Short ckt test
Power rating
Other tests

Overview
A MODERN TRANSFORMER IS A
COMPLEX ELECTROMAGNETIC
APPARATUS
TRANSFORMERS SERVE FOR
TRANSMISSION AND DISTRIBUTION
OF ELECTRICAL ENERGY
TRANSFORMERS (DISTRIBUTION)
ARE THE LAST MAJOR LINK
BETWEEN UTILITY & CONSUMER

Introduction
TRANSFORMERS ARE STATIC
ALTERNATING CURRENT MACHINES
PRINCIPLE IS BASED ON MUTUAL
INDUCTION BETWEEN WINDINGS
THROUGH A COMMON CORE
OIL-IMMERSED AND DRY TYPES
POWER AND DISTRIBUTION TYPES
SPECIAL TYPES

Transformer on load
ϕ
V1
I0
E2
I1I’2
I2
E = - 4.44 φmf N

 Measurement of winding resistance
 Measurement of voltage ratio
 Polarity test
 Measurement of no-load loss & no-load current
 Measurement of load-loss & short-circuit impedance
 Measurement of insulation resistance
 Dielectric tests
Routine tests

 Measurement of winding resistance
 Measurement of voltage ratio
 Polarity test
 Measurement of no-load loss & no-load current
 Measurement of load-loss & short-circuit impedance
 Measurement of insulation resistance
 Dielectric tests
 Temperature-rise test
Type tests

 Dielectric test
 Short-circuit test
 Measurement of zero-sequence
impedance of three phase transformers
 Measurement of acoustic noise level
 Measurement of harmonics of the no-load
current
 Measurement of power taken by the fans
& oil pumps
Special tests

• Separate source voltage withstand test
• Induced over voltage withstand test
• Impulse voltage withstand test
Dielectric test

Terminals of windings under test connected to HV terminal of the
source . All other windings, core, frame and tank are earthed.
Duration of test is 60 secs.
Highest system voltage(kV) Power frequency voltage(kV)
1.1 3
3.6 10
7.2 20
12 28
24 50
36 70
52 95
72.5 140
Separate source voltage withstand test

 This test is useful in determining
layer insulation and inter turn
insulation of windings.
 HV is kept open & twice the rated
voltage at twice the rated frequency
is applied to the LV for a duration of
60 secs.
Induced over voltage withstand test

Testing facilities
• Impulse generator
• Discharge circuit
• Recording system
• Voltage divider for measurement
Standard Impulse wave
Impulse withstand test

Wave shape
 Standard wave shape of impulse voltage is
1.2/50µsec.
 t1 is the rise time = 1.2µsecs.
 t2 is the tail time = 50 µ secs.
 Tolerance on t1 is ±30% & ±20% on t2.
Impulse generator
 Impulse wave is generated by discharge of
capacitors charged in parallel into a wave shaping
circuit.

Connection
Fig.7.3.1.Connection diagram
lmpulse Application : (Negative Polarity).
Oscillograms : 7.3.1 Reduced full wave 100 kV (Peak)
7.3.2. 100% full wave 170 kV (Peak)
7.3.3 Reduced Chopped wave 103 kV (Peak)
7.3.4. 100% chopped wave 170 kV (Peak)
7.3.5. 100% chopped wave 170 kV (Peak)

Oscilograms contd.
Fig 7.3.3 Connection diagram
Generator capacitance 0.0625 uF
Series resistance 160 ohms
Tail resistance 1040 ohms
Voltage divider 1200 pF in parallel with 100 ohms
Impulse applications : (Negative Polarity)
Oscillogram No. 7.3.15. Reduced full wave 43 kV (Peak)
7.3.16. 100% full wave - 75 kV (Peak)
7.3.17. 100% full wave 75 kV (peak)
Comparison of oscillograms 15 & 16 indicates that the second voltage wave
oscillogram has collapsed and the corresponding point of the neutral current
there are high frequency oscillations. This indicates an arc over across major
portion of the transformer winding as a result of failure of the transformer
insulation.

 MINOR INSULATION -conductor turns,
layers
●
enamel, paper
●
oil ducts, paper cylinders
 MAJOR INSULATION- winding, core
●
press board cylinders, oil ducts
BUSHINGS - porcelain , condenser

Power & distribution transformers must be constructed to
withstand mechanical stresses caused by external faults.
These stresses are produced due to electromagnetic forces
as a result of very high current in the windings during
short-circuits.
Short-circuit test

 Prior to the short-circuit, transformer
is subjected to routine tests.
Asymmetrical current
 The peak current that transformer is
required to withstand = Isc(peak)=K Isc
Dynamic ability to withstand short-circuit
Value of K :
x/r 1 1.5 2 3 4 5 6 8 10 >14
K 1.51 1.64 1.76 1.95 2.09 2.19 2.27 2.38 2.46 2.55

Short-circuit may be
a) Preset or
b) Post set
Duration of each test
being 0.5 sec. ( > 5 MVA 0.25 sec)
For 1φ Transformers
No. of tests = Three
one test with tap at highest voltage ratio, one test in principal taping &
one test in
lowest voltage ratio
Short-circuit testing procedure

3φ Transformers
 No. of tests = Nine
 Three tests with tap at highest voltage ratio, three
tests in principal taping & three tests in lowest voltage
ratio
Tolerances on
 Asymmetrical current = ±5 %
 Symmetrical current = ±10 %
 Test duration = ±10 %

Oscillogram of dynamic short-circuit test on
3Phase 11/0.433kV 500kVA Distribution
transformer

 All the routine tests shall be repeated
 Dielectric test shall be at 75%
 Transformer shall be untanked
 Reactance measured after s.c. test shall not
differ by more than 2% for circular coils & 7.5%
for non-circular coils.
Analysis of the results
SC Test contd.

This is to ascertain that transformer & its cooling arrangements
are effectively designed so that temp. rise of winding & cooling
medium does not exceed the permissible limits.
Temp. rise limits for Dry type transformers
Class of insulation Temp. rise °C
A 50
E 65
B 70
F 90
H 115
C 140
Temperature rise test

Temp. rise limits for oil immersed transformers
Part Temp. rise °C
External cooling medium
Air Water
Winding 55 60
(temp.rise by resistance method)
Top oil 50 55
(temp.rise by thermometer method)

a) Direct loading
b) Back-to-Back method
c) Short-circuit method
Loading methods
Duration of test
Top oil temperature-rise does not vary more than
10
C/hour during 4 consecutive hourly readings

Partial Discharge Tests
100pC
500pC

Sound pressure level 20 log10 d/0.00002 dB
d Newton per square meter
Noise Level

Highest voltage
for equipment
Um
kV
(r.m.s. value)
Rated short duration
power frequency
withstand voltage
kV
(r.m.s. value)
Rated
lightning impulse
withstand voltage
kV
(peak value)
3.6 10 20
40
7.2 20 40
60
12 28 60
75
95
17.5 38 75
95
24 50 95
125
145
36 70 145
170
52 95 250
72.5 140 325
123 (185)
230
450
550
145 (185)
230
275
(450)
550
650
170 (230)
275
325
(550)
650
750
245 (275)
(325)
360
395
460
(650)
(750)
850
950
1050

Highest
voltage for
equipment
Um
kV
(r.m.s. value)
Standard switching impulse
withstand voltage
--------------------------------------
-------------------
Longitudinal Phase-to-earth
Phase-to-phase
Insulation
(ratio to the
(note 1)
phase-to-earth
kV kV
peak value)
(peak value) (peak value)
Standard
lightning impulse
withstand voltage
kV
(peak value)
300 750 750 1,50 850
950
750 850 1,50 950
1050
362 850 850 1,50 950
1050
850 950 1,50 1050
1175
420 850 850 1,60 1050
1175
950 950 1,50 1175
1300
950 1050 1,50 1300
1425
525 950 950 1,70 1175
1300
950 1050 1,60 1300
1425
950 1175 1,50 1425
1550
765 1175 1300 1,70 1675
1800
1175 1425 1,70 1800
1950
1175 1550 1,60 1950
2100

Transformer Testing Guide: Key Procedures and Standards

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