The document provides a comprehensive overview of lightning impulse tests conducted on transformers to assess their insulation strength against lightning-induced voltage surges. It covers the nature of lightning, defines impulse characteristics, describes testing procedures, and outlines equipment and methods for detecting failures during tests. The importance of adhering to specific standards and sequences during testing is emphasized to ensure accurate results and transformer integrity.

1. LIGHTNING IMPULSE TEST
In -precise
Presented by
S.M.G.MUTHAR HUSSAIN
Transformer Engineer

2. CONTENTS
• WHAT IS LIGHTNING?
• WHAT IS IMPULSE?
Typical impulse wave shape and parameters
• WHAT IS LIGHTNING IMPULSE TEST?
• WHY IS LI TEST DONE ON DISTRIBUTION
TRANSFORMERS?
• WHY IS SUCH WAVE SHAPE USED?
• HOW IMPULSE VOLTAGE IS GENERATED?
Generation of Full Wave Impulse
Wave shape adjustment with HV winding
Wave shape adjustment with LV winding
Generation of Chopped wave impulse
• TEST CIRCUIT
• TERMINAL CONNECTIONS
• EARTHING PRACTICES

3. CONTENTS continued…
• FAILURE DETECTION METHODS
• TEST PROCEDURES
Test sequence
Test setup
• EQUIPMENTS REQUIRED FOR TEST
• METHOD OF OPERATION
• MEASURING TECHNIQUES
• INTERPRETATION OF RESULTS
• DIAGNOSIS
General recordings
Voltage recordings-Full wave tests
Current recordings-Full wave tests
Voltage and Current recordings-Chopped wave tests
• FOOT NOTES
• REFERENCES

4. What is Lightning?
• Lightning is a natural phenomenon that occurs in the
atmosphere when two neighboring clouds of opposite charges
strike each other or between a negative cloud and a grounded
object on earth. It is associated with an electrical discharge
and huge sound during a thunderstorm.
• The phenomenon of Lightning introduces lightning surges in
transmission network. The Lightning strokes on transmission
lines are of two type direct strokes and induced strokes. When
a thunder cloud directly discharges on a transmission line
tower or line wire it is called a direct stroke. It is the most
severe form of lightning stroke and is rare on transmission
systems and only induced strokes occurs.
• When a thunderstorm generates negative charges at its
ground end, the earthed objects develop induced positive
charge. The earthed objects of interest are transmission lines
and towers. Normally, it is expected that the lines are
unaffected because they are insulated by strings of insulators.

5. What is Lightning? Continued…
• However because of high field gradients involved the positive
charges leak from the tower along the insulator surfaces to the
line conductors.
• This process may take considerable time in the order of some
hundreds of seconds. When the cloud discharges to some
earthed object other than the line conductor, the transmission
line conductor is left with a high concentration of positive
charge which cannot leak suddenly. The transmission line and
the ground will act as a huge capacitor carrying a positive
charge and hence over voltages occur due to these induced
charges. This would result in a stroke which is commonly
known as an “induced” lightning stroke.
• As a result of a direct stroke or an induced stroke, a large
current impulse is injected into the line conductor which
produces a voltage surge in the line as high as 5,000kV.
However on an average most of the lightning strokes give rise
to voltage surges of less than 1000kV on over head lines.

6. What is Impulse?
• An impulse is defined as a unidirectional voltage
(or current) rising quickly to its peak value and
then decaying slowly to zero. An impulse voltage
is characterized by its peak value Vp, its front
time t f and its tail time or time to half tt.
• Thus an impulse has 2 parts i.e. a rising part
which is usually realized by charging a capacitor
and a decaying part which is realized by
discharging a capacitor.
• Impulse voltages are used to simulate the
stresses imposed on high voltage apparatus due
to lightning or switching surges.

7. Typical Impulse wave shape and parameters
t f – Front time is the time taken for the wave to reach its Peak value.
t t – Tail time or time to half of the peak value.
The front time for a standard lightning impulse is 1.2µs while its tail time is 50µs.
Tolerance allowed in peak value is ±3%
The tolerance allowed for front time is ±30% and that for tail time is ±20%

8. What is Lightning Impulse test?
• It is a dielectric test conducted on an electrical
equipment to demonstrate that the insulation will
withstand impulses that strike it externally due to
lightning during its service period. High voltage surges
resembling lightning that are simulated in laboratories
are commonly known as high voltage impulses. These
lightning Impulses are used to test the equipment.
• It is important to note that the distribution of impulse-
voltage stress through the transformer winding may be
very different from the low frequency voltage
distribution.
• Low-frequency voltage distributes itself throughout the
winding on uniform volts-per turn basis. Impulse
voltages are initially distributed on the basis of winding
capacitances.

9. What is Lightning Impulse test? Continued…
• Since the transients are impulses of short rise time, the
voltage distribution along the transformer winding is not
uniform. The initial part of the winding (i.e. close to the
HV line lead) will have higher stress than the winding
part close to the ground end and thus it may be the first
to breakdown in case of high voltage surges. The test is
performed at a voltage ≥ the rated basic lightning
impulse insulation level (BIL).
• Impulse testing of an oil filled distribution transformer is
usually performed using both the full wave (IEC -60076-
2000 Clause 13) and the chopped wave (Clause 14)
impulses with chopping time ranging from
approximately 2 to 6µs. To prevent large over-voltages
being induced in the windings that are not under test,
they are short circuited and connected to ground
through low impedance paths.

10. Why is Lightning Impulse test done
on Distribution transformers?
• Insulation is recognized as one of the most important
constructional elements of a transformer. Its chief
function is to confine the current to useful paths,
preventing it flow into harmful channels. Any weakness
of insulation may result in failure of the transformer. A
measure of the effectiveness with which insulation
performs is the dielectric strength of transformer.
• The purpose of the impulse test is to determine the
ability of the transformer insulation to withstand the
transient voltages due to lightning.
• It is well known that power system components are
subjected to severe over voltage due to internal switching
or external lightning surges. Consequently the integrity
of the individual components, devices, and subsystems
must be checked through high voltage surge testing.

11. Why is such wave shape used?
• From the data compiled by the 1937 AIEE-EEI-
NEMA committee on Insulation Co-ordination
about natural lightning, it was concluded that
system disturbances from lightning can be
represented by three basic wave shapes- full
wave, chopped wave and front-of-waves.
• Later it was recognized that lightning
disturbances will not always have these three
waves but definitely full wave and chopped wave
and hence a standard impulse wave is specified
by IEC with front time 1.2µs ± 30% tolerance
and tail time (time to half) 50µs ± 20% tolerance

12. How Impulse Voltage is generated?
• Lightning Impulse is generated using an Impulse
Generator.
Generation of Full Wave Impulse

13. How Impulse Voltage is generated?
Continued…
• The above figure shows a simple full wave impulse
generating circuit.
It consists of 2 capacitors C1 (larger) and C2 (smaller).
Capacitor C1 is used for charging and Capacitor C2 for
discharging. It has 2 Resistors R1 and R2.
R1 is connected in series with C1 through a HV switch
(e.g. a sphere spark gap). R2 is connected parallel to C2.
The output is taken across the parallel combination of
R2 and C2.This is a single stage generator.
• In this circuit, R1 and C2 determine the front time while
C1 and (R1+R2) determine the tail time of an impulse.
The values of these parameters are selected to generate
the desired impulse wave form for the required
application.

14. How Impulse Voltage is generated?
Continued…
• When very high impulse voltages are needed, multistage
generators are used. The basic idea of a multistage
impulse generator circuit is to charge several stage
capacitors in parallel and then discharge them in series.
• Where standard impulse shape cannot be obtained
because of low winding inductance or high surge
capacitance to earth wider tolerances may be accepted by
agreement between purchaser and supplier.
• Technical papers written on the subject of “Surge
generator characteristics for transformer testing” show
that the time to crest (front time t f) of an impulse wave is
affected by the series inductance, series resistance and
load capacitance. The tail of the wave (tail time t t) is
controlled by the generator capacitance, load resistance,
load inductance and also load capacitance.

15. How Impulse Voltage is generated?
Continued…
• Wave shapes Adjustment with HV winding:
• The surge capacitance of the transformer under test being constant, the series
resistance may have to be reduced in an attempt to obtain the correct front time T1
(1.2µs ± 30%).
• Wave shapes Adjustment with LV winding:
• With the low voltage windings the virtual time to half value T2 (50µs ± 20%) may not
be achievable because of the low inductance making the wave shape oscillatory.
• This problem can be solved to some extent by
• Use of large capacitance within the generator
• By multiple parallel stage operation of generator
• By adjustment of the series resistor
• By impedance earthing rather than direct earthing resulting in a significant increase
in the effective inductance i.e. earthing the non tested terminals through resistors
not exceeding the surge impedance of the cable (400Ω) such that in all circumstances
the voltage to earth appearing at the non tested terminals does not exceed 75% of the
rated lightning impulse withstand voltage of that terminal for STAR connected
windings and 50% of the rated lightning impulse withstand voltage of that terminal
for DELTA connected windings (because of opposite polarity voltages to earth on
delta terminals)

16. How Impulse Voltage is generated?
Continued…
• In direct earthing only leakage inductance is involved.
• In impedance earthing main inductance becomes more predominant and
this makes the effective inductance 100 to 200 times greater than direct
earthing.
• Lightning Impulse test can done by either apply Full Wave (LI) or Chopped
wave (LIC).
• Generation of chopped Wave Impulse
• When a full wave surge occurs on a power network and a flashover takes
place across a bushing or an insulator etc., the voltage instantaneously falls
to zero resulting in a chopped impulse wave. The voltage chopping can take
place either on the front, at the peak or on the tail of a surge. To simulate
such a chopped surge wave, a rod-rod chopping gap is normally placed in
parallel with the test object across the impulse generator. The distance of
the chopping gaps can be adjusted to control the width of the applied
chopped wave during the chopped impulse testing. Triggered chopping gaps
are often used to control the chopping time. Chopped impulse testing is
required in some applications.
• Switching Impulse (SI) is done on power transformers with Um≥72.5kV
and where Lightning Impulse becomes a Routine test. SI is usually required
for transformers subjected to ACLD (AC induced over voltage test with long
duration) with Partial Discharge Measurement.

17. Test Circuit
• Main Circuit:
• Impulse Generator
• Wave shaping Components
• Test object
• Measuring circuits
•
• Voltage Measuring Circuit:
• Voltage divider (Resistance type, Capacitance type, Compensated type)
• Coaxial Cable
• Digital Impulse Measuring system (DIMS)
• Peak Voltmeter (if available)
•
• Chopping Circuit if Applicable:
• Rod-Rod Chopping gap
•
• Test Calibration:
• Before a test an overall check of the test circuit and measuring system may be performed at
a voltage lower than the reduced voltage level (50% of the rated full voltage). In this check
voltage may be determined by means of sphere gap or by comparative measurement with
another approved device. When using a sphere gap it should be recognized that this is only
a check and does not replace the periodic calibration of the approved measuring system.
After any check has been made it is essential that neither the measuring nor the test circuit
is altered except for the removal of any devices for checking. i.e. The impulse circuit and
measuring connections shall remain unchanged during calibration (Reduced wave at 50%
of test voltage)

18. Typical impulse test circuit

19. Terminal Connections
• Normally the non-tested terminals of the phase winding
under test are earthed and the non-tested phase
windings are shorted and earthed. However in order to
improve the wave tail T2, resistance earthing of the non-
tested windings may be advantageous and in addition
the non-tested line terminals of the winding under test
may also be resistance earthed.
• If a terminal has been specified to be directly earthed or
connected to a low impedance cable in service, then that
terminal should be directly earthed during the test or
earthed through a resistor with an ohmic value not in
excess of the surge impedance of the cable (400Ω).
• Earthing through a low impedance shunt for the purpose
of impulse response current measurements may be
considered the equivalent of direct earthing.
• No Impulse test on the neutral terminal is
recommended. During Impulse test on a line terminal
the neutral shall be connected directly to earth.

20. Lightning Impulse test terminal connections and
applicable methods of failure detection

21. Earthing Practices
• During Impulse testing Zero potential cannot be
assumed throughout the earthing system due to the high
values and rates of change of impulse currents and
voltages and the finite impedances involved. Therefore
selection of proper earth is important.
• The Current return path between the test object and the
impulse generator should be of low impedance. It is a
good practice to firmly connect this current return path
to the general earth system of the test room preferably
close to the test object. This point of connection should
be used as reference earth and to attain good earthing of
the test object it should be connected to the reference
earth by one or several conductors of low impedance.
• The voltage measuring circuit which is a separate loop of
the test object carrying only the measuring current and
not any major portion of the impulse current flowing
through the windings under test should also be
effectively connected to the same reference earth.

22. Failure Detection Methods
• The fault in winding insulation is detected by general
observations of noise, smoke, etc. during the impulse voltage
application. Moreover, the inspection of voltage and current
oscillograms give more accurate indication of the failure
especially for the partial failure. A partial or complete failure
of winding appears as a partial or complete collapse of the
applied impulse voltage. However, the impulse voltage may
not show a small partial failure since the sensitivity of the
voltage waveform method is low and this method does not
detect faults which occur on less than 5% of the total winding.
The failure detection in such cases is enhanced by current
oscillogram which usually shows a record of the impulse
current flowing through a resistive shunt or a high bandwidth
current transformer connected between the neutral and the
ground or between the low voltage winding and the ground.

23. Failure Detection Methods continued…
• The current oscillogram usually consists of a high frequency
oscillation, a low frequency disturbance and a current rise due to
reflections from the ground end of the windings. When a major fault
such as breakdown between turns or between one turn and the
ground occurs, high frequency pulses are observed in the current
oscillogram and the wave shape changes. For local failure such as a
partial discharge only high frequency oscillations are observed
without a change of wave shape. To detect any failure, voltage and
current oscillographs for the full wave impulses are compared with
the initial records corresponding to the reduced full wave.
• Similarly chopped impulses are compared as well. The IEC test
criteria states that “the absence of significant differences between
voltage and current transients recorded at reduced voltage and
those recorded at full test voltage constitutes evidence that the
insulation has withstood the test.” If there are doubts about the
interpretation of possible discrepancies between oscillograms, three
subsequent impulses at full test voltage shall be applied or the whole
impulse test on the terminal shall be repeated.

24. Test Procedures
• Lightning Impulse test (LI) is performed on transformers whose winding
terminals are brought out for accessibility. For Oil immersed transformers
the test voltage is normally of Negative polarity because this reduces the
risk of erratic external flash over in the test circuit. Bushing spark gaps may
be removed or their spacing increased to prevent their spark over during
the test.
• The test impulse shall be a full standard lightning impulse 1.2µs ± 30% /
50µs ± 20% waveform. However there are cases where this standard
impulse wave shape cannot be reasonably obtained because of low winding
inductance or high capacitance to earth. The resulting impulse shape is then
often oscillatory. Therefore in such cases wider tolerances may be permitted
by agreement between the parties. The amplitude of opposite polarity of an
oscillatory impulse should not exceed 50% of the first amplitude.
• The test is usually performed at a voltage ≥ the rated basic lightning
impulse insulation level (BIL).
• Impulse testing of an oil filled transformer is usually performed using both
the full wave (Clause 13) and the chopped wave (Clause 14) impulses with
chopping time ranging from 2 to 6 µs as per the requirements of IEC-
60076-3 (2000 edition), Clauses 13 and 14. To prevent large over-voltages
being induced in the windings, those are not under test are short circuited
and connected to ground through low impedance paths.

25. Test Sequence
• Following sequence of impulse voltage applications is specified for oil filled
transformers as per IEC 60076-3, if test is required for full and chopped
waves i.e. Clause 13 and 14:
• One or more reduced full wave impulse at 50-75% of BIL.
• One full wave impulse of 100% BIL.
• One or more reduced chopped wave impulse at 50-75% of BIL.
• Two chopped impulses at 110% BIL and
• Two full wave impulses at 100% BIL.
•
• If the chopped wave testing is not required sequences 3 and 4 mentioned
above are ignored. Thus for full wave test according to Clause 13 only, one
or more full wave reduced impulse and three full wave impulses at 100%
BIL are applied. Since the insulation is of non-self restoring type, the
transformer must withstand all the three impulses at 100% of BIL.
• If the LI test is performed as per ANSI C57 test procedures, the test voltage
applications should have the following sequence
• One reduced full wave impulse at 50-70% of BIL.
• One reduced chopped wave impulse at 50-70% of BIL.
• Two chopped wave impulses ≥ 115% BIL and
• One full wave impulse at 100% BIL
• The other criterion is more or less similar to the IEC requirements given
above.

26. Test set up
• The schematic diagram showing typical connections
for the impulse testing of a three phase
DELTA/STAR distribution transformer is given in
Fig 11.10. Here winding UW is under test with full
impulse voltage application. Moreover, it is ensured
that windings UV and VW are subjected only to half
of the test voltage with the help of an external
voltage divider. In case of STAR/STAR winding
connections, each HV winding is tested separately
and the other two windings are short circuited and
grounded. In transformer testing, it is essential to
record the waveforms of the applied voltage and the
resulting current transients through the winding
under test. Sometimes, the transferred voltages in
the secondary winding and/or the neutral currents
are also recorded.

27. Arrangement of a transformer for Impulse
voltage test

28. Equipments required for test
• Transformer to be tested
• Triggered sphere spark gaps
• Impulse generator
• Voltage divider (Resistive or Capacitive)
• Dual Channel digital impulse measuring system
(DIMS)
• Components ( High Bandwidth CT, Current
shunt )
• Coaxial cable & Connection wires

29. Method Of Operation
• Energize the circuit from the output of the HVAC transformer
through a half wave negative polarity rectifier.
• Adjust the sphere gap’s separation so that it can breakdown at
the rated full impulse voltage level or slightly higher than that.
• Increase the HVAC and thus the HVDC charging voltage
slowly till the breakdown of the spark gap is possible.
• Then apply a trigger pulse so that a spark occurs in the
spheres and an impulse is generated.
• Apply one or more full wave impulse voltage at reduced
magnitude of 50 to 75% of BIL for calibration purpose
between the phases UW (φ U) of a DELTA connected winding
(HV Winding)
• Measure the impulse (peak value) at reduced voltage using the
voltage divider and DIMS and record its waveform
parameters through one channel.
• Measure the impulse response current (winding current)
passing through the high bandwidth CT or Current shunt
connected between the “W” terminal and ground using the
second channel of the DIMS.

30. Method Of Operation continued…
• Do not alter either the test circuit or the measuring
circuit after calibration.
• Now apply three full wave impulses of 100% BIL in
succession between phases UW (φ U).
• Measure the impulse (peak value) at full rated voltage
and impulse current using the two channels of DIMS and
record its waveform parameters.
• Repeat the above procedure from 5 to 10 for the other
two phases VU (φ V) and WV (φ W) and record its
voltage and current waveforms.
• Compare the wave shapes of voltages and currents
recorded between reduced and full impulse voltage levels
or between successive records at rated test voltage.
• If no significant difference is found between voltage and
current transients recorded at reduced voltage and those
recorded at full test voltage it is evident that the
insulation has withstood the test and passed the LI test.

31. Measuring Techniques
• Impulse response current measurement is done
through low impedance shunt.
• Digital Recordings:
• Oscillograph / Digital records obtained during
Calibration and tests shall clearly show the
applied lightning impulse voltage, impulse shape
(front time, time of half value, amplitude) and
impulse response current wave with the help of
at least 2 independent recording channels.

32. Interpretation Of Results
• Assessment of test results is primarily based on
the comparison of wave shapes of voltages and
currents recorded between reduced and full
impulse voltage levels or between successive
records at rated test voltage.

33. DIAGNOSIS
General Recordings
Discrepancy Possible Cause Check & Remedy
Variation in wave shapes of
Voltages and current records
Disturbances due to test circuit,
measuring circuit and earthing
methods
Check and if so eliminate or
minimize their effect
Variation in the amplitude of
current records with high
frequency initial oscillations
Difference in firing times of the
individual stages of a multi stage
generator
Check and if so correct the firing
timing to be the same
Variation in the amplitude of
current records with high
frequency initial oscillations
Discharge circuits are not
coincident in time
Check and if so make new
settings of the discharge gaps on
generator
Logical and Progressive change
with increasing voltage levels
Core earthing or non-linear
elements (surge arrestors)
disturbances
Check and if so eliminate by
solid core earthing and removing
non-linear resistors.
Even after eliminating the above sources of discrepancies, any variations in the
wave shape of voltage and current records between the reduced and rated test
voltage or between successive records at rated test voltage which cannot be
proved to have originated from the test circuit or in non-linear resistors within
the test object, it is evident that the insulation has failed during the test.

34. DIAGNOSIS
Voltage Recordings-Full wave tests
Discrepancy Possible Cause Check & Remedy
Direct earth fault near the terminal
under test resulting in rapid and
total collapse of the voltage
Defective Insulators. Low Line
leads to earth clearance.
Check insulators for any defect
such as crack, dust and spots and
replace it. Line leads from inside to
be well protected from earth.
Total flash over across the winding
under test resulting in slower
collapse of voltage
Insufficient insulation at the initial
part of the winding and line leads
not well insulated.
Additional insulation between the
first 2 layers and last 2 layers. Start
and Finish line leads to be extra
insulated.
Part flash over across the winding
results in reduced impedance,
decrease of time to half &
characteristic oscillations in
voltage wave at the moment of
flashover
Insufficient/ Weak layer insulation
of the winding
Check the weak point and
increase the layer insulation
accordingly
Less extensive faults detected as
high frequency oscillations
Breakdown between Coil to Coil or
turn to turn insulation
Impulse response current
recordings to be done

35. DIAGNOSIS
Current Recordings-Full wave tests
Discrepancy Possible Cause Check & Remedy
Major change in amplitude and frequency
in Current records
Winding breakdown within the tested
winding, between windings or to earth
Increased layer insulation, sufficient Coil
to Coil Clearance, adequate phase
barriers, improved line to earth
clearances.
Significant increase with change in the
superimposed frequency in neutral
current
Fault within the tested winding Check for the break down point and do
insulation co-ordination
Significant decrease with change in the
superimposed frequency in neutral
current
Fault from the tested winding to an
adjacent winding or to earth
Adequate Core to Winding insulation,
sufficient Coil to Coil Clearance, adequate
phase barriers, improved line to earth
clearances
Instantaneous decrease in amplitude
with change in polarity and basic
frequency of capacitively transferred
current
Fault in the tested winding or to earth Check for the break down point and do
insulation co-ordination and improve line
to earth clearance.
Instantaneous increase in amplitude and
basic frequency in same polarity of
capacitively transferred current
Fault from the tested winding to an
adjacent winding
Adequate Core to Winding insulation,
sufficient Coil to Coil Clearance, adequate
phase barriers.
Small, local, jagged disturbances spread
over 2µs or 3µs
Severe Partial discharge or insulation
breakdown between turns, coils or
connections
Vacuum Oil filling to be done. Avoid sharp
edges in insulation materials, copper bus
bars, core clamping and tie rods

36. DIAGNOSIS
Voltage and Current Recordings-
Chopped wave tests
Discrepancy Possible Cause Check & Remedy
Change in frequency of Voltage
and Current recordings after
chopping
Flash over in the return loop to
the laboratory earth or an
internal failure in the test object
Increase clearance to earth to
avoid flash over. Improved
Insulation co-ordination
necessary
Failure of the chopping gap to
chop or any external part to
spark over
Failure either in the test circuit or
in the test object
Check the test circuit for any
such discrepancy or identify the
breakdown point in the test
object and do insulation co-
ordination
Fault occurring before chopping Condition resembles a full wave
test failure due to insulation
breakdown
Increased layer insulation,
sufficient Coil to Coil Clearance,
adequate phase barriers,
improved line to earth
clearances

37. FOOT NOTES
• Use of surge diverters/arrestors is to limit the transferred over voltage
transients.
• Transients - Impulses of short rise-time.
• Effective impedance – It is the total impedance between the impulse
terminal of the transformer and ground.
• 400Ω is the maximum Surge impedance of a transmission line.
• SI is done on power transformers with Um ≥ 72.5kV and where LI becomes
a Routine test.
• SI is usually required for transformers subjected to ACLD with Partial
discharge measurement.
• No Impulse test on the Neutral terminal is recommended. During Impulse
test on a line terminal, the neutral shall be connected directly to earth.
• For Oil immersed transformers the test voltage is normally of Negative
polarity because this reduces the risk of erratic external flash over in the
test circuit.
• In direct earthing only leakage inductance is involved.
• In impedance earthing main inductance becomes more predominant and
this makes the effective inductance 100 to 200 times greater than direct
earthing.
• Voltage to earth appearing at the non tested terminals in a DELTA Winding
is 50% of the rated lightning impulse withstanding voltage of that terminal
is because of opposite polarity voltages to earth on delta terminals.

38. FOOT NOTES Continued…
• Impulse response current measurement is done through low impedance shunt.
• A triggered spark gap is needed in order to initiate the generation of the
impulse.
• 1.2µs is for front time (T1) to reach at least 50% of the full impulse voltage level.
• 50µs is for the tail time (T2) to sustain without collapsing.
• Impulse current is normally the most sensitive parameter in failure detection.
Therefore the recorded current waves are the main criteria of the test result.
• When recording the winding current the recording should continue till the
inductive peak reaches which will permit examination of the wave to determine
whether there is a change in inductance due to any shorting of turns as a result
of insulation failure.
• Apply one impulse of a voltage 50 -75% of the full test voltage for calibration
purpose and three subsequent impulses at full voltage on each of the line
terminals of the tested winding (Example HV winding) in succession. In a
DELTA connected 3φ transformer, the other line terminals of the winding shall
be earthed directly or through low impedance not exceeding the surge
impedance of the connecting line.
• If the winding has a neutral terminal (Example LV winding STAR connected),
the neutral shall be earthed directly or through low impedance such as a current
measuring shunt.
• The tank shall be earthed.
• In case of a transformer with tertiary winding the terminals of the windings not
under test shall be earthed directly or through low impedance.

39. REFERENCES
• Fundamentals of High Voltage Engineering by
Abdulrhman Al-Arainy, Mohammad Iqbal Qureshi,
Nazar Malik
• Experiments in High Voltage Engineering by
Abdulrhman Al-Arainy, Abderrahmane Beroual,
Nazar Malik
• International Standard IEC 60076-3 Second edition
2000-03
• International Standard IEC 60076-4 First edition
2002-06
• IEEE Guide for Transformer Impulse Tests IEEE
Std C57.98-1993