4. 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
5. 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
7. 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
8. 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
9. 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
10. 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
11. • Separate source voltage withstand test
• Induced over voltage withstand test
• Impulse voltage withstand test
Dielectric test
12. 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
13. 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
14. Testing facilities
• Impulse generator
• Discharge circuit
• Recording system
• Voltage divider for measurement
Standard Impulse wave
Impulse withstand test
15. 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.
19. 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.
21. MINOR INSULATION -conductor turns,
layers
●
enamel, paper
●
oil ducts, paper cylinders
MAJOR INSULATION- winding, core
●
press board cylinders, oil ducts
BUSHINGS - porcelain , condenser
22. 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
23. 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
24. 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
25. 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 %
26. Oscillogram of dynamic short-circuit test on
3Phase 11/0.433kV 500kVA Distribution
transformer
27. 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.
28. 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
29. 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)
30. 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
