Transcript Testing.

UNIT 5 : HIGH VOLTAGE
TESTING AND INSULATION
COORDINATION
5.1 NEED FOR HIGH VOLTAGE
TESTS
Verify the name plate details
Confirm the design specifications
Dr M A Panneerselvam, Professor,
Anna University
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Ascertain the maximum
withstand values (capacity)
Research and development of
insulating materials
5.2 CLASSIFICATION OF HIGH
VOLTAGE TESTS
High voltage tests can be
classified under various heads
Dr M A Panneerselvam, Professor,
Anna University
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as given under:
5.2.1 Based on the type of tests:
Type tests
Routine tests
Special tests
Acceptance tests
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Anna University
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Type tests are tests conducted on
a randomly selected sample out of
a given lot , confirming to the
same rating and specification ,
and manufactured over a given
period of time. These tests are
conducted by a neutral authority
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Anna University
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and witnessed by both
manufacturer and customer.
Routine tests are conducted on
all piece of equipment
generally by manufactures in
their own premises.
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Anna University
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Special tests are certain tests
conducted only on the request of
customer otherwise these tests
are not conducted normally.( Ex.
Impulse test including chopped
waves on transformers) .
Dr M A Panneerselvam, Professor,
Anna University
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Acceptance tests are tests
conducted at the customer’s
premises during receipt of
consignment (equipment). The
customer may specify any test for
this purpose.
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Anna University
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5.2.2 Based on the nature of
voltages:
DC voltage test
AC voltage test
Impulse voltage/Impulse current
test
Switching surge test
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Anna University
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5.2.3 Based on the nature of
breakdown:
Withstand tests
Flashover/Sparkover tests
Puncture tests
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Anna University
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5.2.4 Based on the
environment:
Dry tests
Wet tests
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Anna University
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5.3 ELECTRICAL EQUIPMENT
CONSIDERED FOR TESTING
Power transformers ,distribution
transformers, instrument
transformers,reactors , etc.,
Protective devices like circuit
breakers, lightning arresters , etc.,
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Anna University
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Porcelain insulators and other
types of insulators
Cables , capacitors and other
dielectric materials and
Busducts(isolated and 3 phase
busducts) and other sundry
equipment.
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Anna University
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5.4 STANDARD HIGH VOLTAGE
TESTS
Out of all the equipment listed
above, Power transformers ,
Distribution transformers ,Lightning
arresters , Circuit breakers, Line
insulators and Power cables form
the major components of a power
system.
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Anna University
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The high voltage tests conducted
in general on the above
equipment are classified as
follows:
5.4.1 Direct voltage tests:
Tests on cables and other
dielectric materials.
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Anna University
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5.4.2 Power frequency tests:
Induced overvoltage test and
separate source voltage withstand
test on power transformers, one
minute withstand / flashover test
under dry and wet conditions ,
puncture test on insulators
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Anna University
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power frequency sparkover test
on lightning arresters and dry
and wet withstand tests on
circuit breakers.
5.4.3 Impulse voltage tests:
Withstand test on power
transformers, Impulse sparkover
test and front of wave impulse
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Anna University
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sparkover test(V-T curve) on
lightning arresters, withstand/
flashover tests on insulators and
isolators and withstand/flashover
tests on bus ducts.
5.4.4 Switching surge tests :
Withstand tests on power
transformers, insulators, lightning
arresters , circuit breakers , etc.,
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Anna University
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All the tests mentioned above
can be grouped under two
heads ,viz.,(1) Tests conducted
on external insulation and (2)
Tests conducted on internal
insulation of electrical
equipment.
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Anna University
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As the tests conducted are large
in numbers and are on different
type of equipment , it is not
possible to give the meaning of all
the terminology adopted and they
can be easily verified from the
relevant standards.
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Anna University
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Depending upon the country in
which the equipment is going
to be installed it can be tested
as per the standards of that
country. (Ex., Indian standard,
British standard, NEMA
standard etc.,)
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Anna University
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5.5 LIST OF IMPORTANT
INDIAN STANDARDS
USED IN HIGH VOLTAGE
TESTING
1)Method for high voltage
measurement by means of
sphere gaps (one sphere
earthed ) :IS 1876-1961
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Anna University
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2)Method of high voltage testingPart I : General definitions and
test requirements : IS 2071(Part
I)-1974
3) Method of high voltage testing
– Part II:Test procedures :IS
2071(Part II) 1974
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Anna University
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4) Method of high voltage
testing-Part III: Measuring
devices:IS 2071(Part III)1976
5) Specification for power
transformers-Part III :Insulation
levels and dielectric tests : IS
2026(Part III) -1981
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Anna University
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6) Specification for lightning
arresters for alternating current
systems-Part I :Non linear type
lightning arrester : IS 3070 (Part I
) -1985.
7) Specification for alternating
current circuit breakers - Part II :
IS 2516 (Part II/ Sec.2) -1965.
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Anna University
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8) Direct voltage tests on
cables: IS -698.
(Cables are generally tested
with DC voltages to avoid
heavy charging currents and
loading of generator)
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Anna University
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When specific standard is not
available for testing of newly
developed equipment the tests
can be conducted in general
agreement with existing
standards based on the type
of test and the nature of
insulation.
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Anna University
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5.6 GENERAL
REQUIREMENTS OF HIGH
VOLTAGE TESTING
Prior to starting of the actual test
on a given equipment there are
certain important prerequisites
and details to be looked into , viz,
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Anna University
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(1) Arrangement of test object
(2) Conditioning of test object
and (3) Correction factors for
atmospheric conditions ( air
density and humidity).
5.6.1 Arrangement of test
object:
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Anna University
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Clearance to extraneous
structures and general
disposition of the object , viz,
height above ground level , the
arrangement of high voltage lead
etc. is of importance. If the
clearance to extraneous
structures is at least 1.5 times
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Anna University
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the flashover distance between
the electrodes of the test object
it is found to be sufficient.
5.6.2 Conditioning of test
object:
Depending upon the
requirements of testing ,the tests
are conducted under dry or wet
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Anna University
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conditions. The rain conditions
are simulated using spray of
water through nozzles falling on
the test object at angle of 45 0 to
the vertical axis. The
characteristics of the spray for
tests with alternating voltages are
given in the following slide.
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Anna University
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ARRANGEMENT FOR WATER SPRAY DURING WET TEST
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Anna University
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Average precipitation rate for
measurement of vertical component
= 3 ± 10 % mm/min
Limit for individual measurement
= 3 ± 25 % mm/min
Resistivity of collected water
= 100 ± 10 % ohm-m
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Anna University
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Duration of wet withstand test =
60 sec.
The characteristics of water spray
for switching surge test for
voltages above 500 kV are :
Vertical component = 1 to 1.5
mm/min. and Horizontal
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Anna University
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component = 1 to 1.5 mm/min.
Limit for individual
measurements = 0.5 to 2.0
mm/min
Resistivity of water=100± 15 %
ohm-m.
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Anna University
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5.6.3 Atmospheric correction
factors:
As the high voltage tests are
invariably conducted at
conditions other than standard
temperature, pressure and
humidity the voltage values
should be multiplied by
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Anna University
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atmospheric correction factors,
viz,
Air density correction factor , kd
Humidity correction factor , kh
Standard atmospheric conditions
(STP) are : Temperature = 20 0 C ,
Pressure = 760 mm of Hg. (1013
millibars) and absolute humidity
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Anna University
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=11 gm/m3.
For tests conducted on external
insulations the voltage at room
temp. V(RTP) is related to V(STP)
as below:
V (RTP) = V (STP) kd/kh
In case of sphere gap test, only air
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Anna University
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density correction factor is
applied and the humidity
correction factor is not applied.
Air density factor ‘ d ‘ is given by,
d = 0.289 b / 273+ td
Here ‘b’ is pressure in millibars
and ‘ td ‘ is the dry bulb temp. 0C
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Anna University
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When pressure is given in mmHg , d = 0.386 b/ 273+ td
The table in next slide, gives the
values air density correction
factor for the calculated value
air density factor ‘d’.
Dr M A Panneerselvam, Professor,
Anna University
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TABLE SHOWING AIR DENSITY CORRECTION FACTOR AND AIR
DENSITY FACTOR
Note : FOR CHENNAI WEATHER CONDITIONS AIR DENSITY
CORRECTION FACTOR IS EQUAL TO AIR DENSITY FACTOR
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Anna University
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Humidity correction factor(kh):
COMPUTATION OF
ABSOLUTE
HUMIDITY
FROM DRY AND
Dr M
A Panneerselvam,
Professor,
Anna University TEMPERTURES
WET BULB THERMOMETER
42
HUMIDITY CORRECTION FACTORS
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Anna University
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5.6.4 Calibration of Impulse
Generator:
Calibration means getting the
relationship between the input
charging voltage and output
voltage of the Impulse
Generator for a given connected
load.
Dr M A Panneerselvam, Professor,
Anna University
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Let us take testing of insulators for
use in 132 kV system : The
maximum system voltage is132 x
1.1 = 145 kV .
Assuming the system is effectively
earthed the Power frequency
withstand voltage level = 230 kV
and the impulse voltage withstand
level = 550 kV.
Dr M A Panneerselvam, Professor,
Anna University
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To test the insulator for an
impulse voltage of 550 kV we
should know the corresponding
input charging level of the
Impulse Generator and hence
we do the calibration of the
generator.
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Anna University
46
The calibration is generally done
at a voltage of 50 to 75 % of the
test voltage.
In this case since the test
voltage is 550 kV the calibration
is done at a level of 275 kV to 413
kV.
Let the Impulse Generator has 6
stages with a charging voltage of
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Anna University
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200 kV. When the generator is
connected for full series the
output voltage of the generator
is approximately 200 x 6 = 1200
kV. The test object is connected
to the Impulse Generator as
shown in the next slide.
Dr M A Panneerselvam, Professor,
Anna University
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MULTISTAGE IMPULSE GENERATOR CONNECTED TO
Dr M AMEASURING
Panneerselvam, Professor,
POTENTIAL DIVIDER,
SPHERES AND LOAD
Anna University
49
As the calibration has to be done
at about 275 kV and referring to
sphere gap standard , for 50 cm
dia. spheres , a gap set for 15 cm
the disruptive discharge voltage is
374 kV (which is 68 % of the test
level) at STP for positive impulse.
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Anna University
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Let us assume the
following atmospheric
conditions during the testing:
Dry bulb temp. = 30 deg. C ,
Wet bulb temp. = 24 deg. C and
Pressure = 758 mm of Hg.
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Anna University
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The air density factor
= 0.386x758 / 273+30= 0.965
For Chennai weather conditions
Air density correction factor
= Air density factor , kd.
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Anna University
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Hence, the disruptive discharge
voltage for a gap setting of 15 cm
for 50 cm dia. spheres = 374 x kd
= 374 x 0.965 = 361
kV ( Note: Humidity correction is
not applied for sphere gaps).
Dr M A Panneerselvam, Professor,
Anna University
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To get an output voltage of 361
kV , the charging level to the first
stage of the Impulse Generator is
approximately = 361 / 6 = 61 kV.
Starting from a charging voltage
of 55 kV to 65 kV, the 50 %
disruptive discharge voltage for
Dr M A Panneerselvam, Professor,
Anna University
54
this setting is obtained using
average or up and down
method. Let the 50 % disruptive
discharge voltage so obtained is
63 kV. That is, a charging voltage
of 63 kV gives an output voltage
of 361 kV at RTP conditions.
Dr M A Panneerselvam, Professor,
Anna University
55
Before the calibration procedure
starts it is important to check
the wave form of the impulse
voltage and if it is not as per
specification the parameters (R1
and R2)are changed to get the
proper wave shape of 1 .2 / 50 μs.
Dr M A Panneerselvam, Professor,
Anna University
56
Once the calibration is over ,
the test voltage levels to be
applied at RTP conditions are
calculated. i.e.,
V(RTP) = V(STP) x kd/kh
Humidity correction factor is
obtained from the dry bulb
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Anna University
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and wet bulb temperatures.
For td = 30 deg.C and tw= 24
deg.C the humidity is obtained
from the graph as 18.5 gm/m3 .
From another graph the
humidity correction factor for
power frequency, positive and
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Anna University
58
negative impulses are obtained
as 0.910, 0.930 and 0.940
respectively.
Hence the positive impulse to be
applied at (RTP) conditions =
550 x 0.965 / 0.930 = 571 kV and
for negative impulse =550x0.965 /
0.940 = 565 kV
Dr M A Panneerselvam, Professor,
Anna University
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For power frequency tests, the
cascaded transformer is
calibrated and the relation
between the input voltage and
the output voltage is obtained.
The power frequency withstand
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Anna University
60
level at STP for 145 kV insulator
is 230 kV (RMS) .
The test voltage to be applied at
RTP conditions
=230 x 0.965 / 0.910 = 244 kV
(RMS).
Dr M A Panneerselvam, Professor,
Anna University
61
5.7 TESTS ON EXTERNAL
INSULATIONS(WITHSTAND/
PUNCTURE/FLASHOVER TESTS
UNDER DRY/WET CONDITIONS)
5.7.1 DC voltage tests:
The voltage should be applied to the
test object starting with a low value
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Anna University
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to prevent overvoltage due to
switching transient but not so
slowly as to cause unnecessary
stress on the test object. The
voltage is maintained for a
specific time (as per standards)
and then rapidly reduced. The
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Anna University
63
test object is supposed to have
withstood the test if no discharge
occurs.To obtain the disruptive
discharge voltage , the voltage is
raised as described above till the
disruptive discharge occurs and
the level is noted.
Dr M A Panneerselvam, Professor,
Anna University
64
5.7.2 AC voltage tests:
The voltage should be applied as
specified earlier (under DC test).
The requirements during voltage
application are generally met if
the rate of rise above 75% of the
test voltage is about 2 % per
second. It should be maintained
Dr M A Panneerselvam, Professor,
Anna University
65
for specified time (generally 60
sec.) and then rapidly decreased.
The test object is supposed to
have withstood the test if there is
no disruptive discharge. For wet
tests water spray as per
specification should be used.
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Anna University
66
The voltage is increased further
to obtain the disruptive discharge
level.
5.7.3 Impulse voltage tests:
Generally 15 impulses of the
rated withstand voltage and of
specified wave shape and polarity
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Anna University
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are applied. The test object is
supposed to have withstood if
no disruptive discharge occurs
in the non self restoring
insulation and not more than
one disruptive discharge
occur in self restoring
insulation. In addition the
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Anna University
68
oscillograms of the applied
voltages are recorded and
there should be no difference
between the first and the
fifteenth impulse and both
should be exactly identical and
look one and the same.
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Anna University
69
5.7.4 Switching surge tests:
As the system voltage increases
beyond 400 kV, switching surge
magnitude increases and
becomes more severe. The
general wave shape for the test is
250/2500 microseconds. The test
Dr M A Panneerselvam, Professor,
Anna University
70
procedure for switching surges
is the same as that of lightning
impulse tests. Depending upon
the particular type of
equipment tested the wave
shapes vary and 200/2000
microseconds or 300/3000
microseconds waves are used.
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Anna University
71
The impulse , switching surge
and power frequency test
voltage levels for testing
equipment at different system
voltages are given in the next
slide.
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Anna University
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Dr M A Panneerselvam, Professor,
Anna University
73
POWER FREQUENCY AND IMPULSE TEST VOLTAGE
Dr MFOR
A Panneerselvam,
WITHSTAND LEVELS
SYSTEMProfessor,
VOLTAGES < 245 kV
Anna University
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INSULATION LEVELS FOR SYSTEM VOLTAGES > 245 kV
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Anna University
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SPECIMEN OSCILLOGRAMS TAKEN DURING IMPULSE
TEST ON 11 kV INSULATOR
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Anna University
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5.8 POWER FREQUENCY /
IMPULSE VOLTAGE
WITHSTAND/FLASHOVER
TESTS ON THREE PHASE
CIRCUIT BREAKERS /
ISOLATORS
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Anna University
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5.8.1 Modes of connections :
High voltage tests on 3 phase
circuit breakers / isolators are
conducted by connecting them
in 5 different modes as given
below:
With breaker/isolator contacts
closed:
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Anna University
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Mode 1 : Phase ‘RY’ connected
together and earthed with frame
and high voltage applied to
‘B’pahase.
Mode 2: Phase ‘YB’ connected
together and earthed with frame
and high voltage applied to ‘R’
phase.
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Anna University
79
Mode 3: Phase ‘BR’ connected
together and earthed with frame
and high voltage applied to ‘Y’
phase.
With breaker / isolator contacts
open:
Mode 4: One side of each pole
connected together and earthed
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Anna University
80
and the other side of the poles
connected together and applied
with high voltage.The frame is
isolated.
Mode 5: Same as ‘Mode 4’ but
high voltage lead and earth
connections inter changed.
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Anna University
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TESTING OF 3 PHASE CIRCUIT BREAKER / ISOLATOR
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Anna University
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OSCILLOGRAMS TAKEN DURING IMPULSE TEST ON 11 kV AIR
BREAK SWITCH
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Anna University
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TESTING OF 3 PHASE 230 kV DISCONNECTING SWITCH
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Anna University
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TESTING OF 500 kV DISCONNECT SWITCH
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Anna University
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5.9 IMPULSE VOLTAGE
WITHSTAND TEST(INCLUDING
CHOPPED WAVES) ON
POWER TRANSFORMERS /
DISTRIBUTION
TRANSFORMERS
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Anna University
86
5.9.1 introduction :
In case of power transformers both
primary and secondary windings
are subjected to impulse test as
both the windings are HV windings.
They are exposed to open
atmosphere and hit by lightning
strokes (Ex. 66 kV/ 11 kV).
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Anna University
87
But, the distribution transformers
are subjected to impulse test only
on high voltage winding. The low
voltage winding is generally
connected to distribution network
and are not likely to be hit by
lightning (Ex.11 kV/ 420 V).
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Anna University
88
IMPULSE TESTING OF 3 PHASE TRANSFORMER
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Anna University
89
Unless and otherwise stated ,
impulse test on transformers
are conducted generally with
negative polarity impulse.
The sequence of voltage
applications given below:
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Anna University
90
(i)One full wave at reduced BIL
recorded with lesser time base.
(ii) One full wave at reduced
BIL(same as (i)) recorded with
higher time base.
Dr M A Panneerselvam, Professor,
Anna University
91
(iii) One full wave at 100 % BIL
recorded with higher time base.
(iv) One chopped wave at reduced
BIL recorded with lesser time base..
Dr M A Panneerselvam, Professor,
Anna University
92
(v) & (vi) Two chopped waves at
100 % BIL recorded with lesser
time base.
(vii)&(viii) Two full waves at 100
% BIL recorded with higher time
base
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Anna University
93
TYPICAL OSCILLOGRMS TAKEN DURING IMPULSE TEST
ON TRANSFORMER
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Anna University
94
TYPICAL OSCILLOGRMS TAKEN DURING
IMPULSE TEST ON TRANSFORMER
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Anna University
95
Oscillograms taken during the impulse
voltage withstand test on
100kVA, 11kV/433V three phase
Distribution Transformer
Plate 1
: Reduced BIL waves (56.25kV at STP) recorded on 10s /
division.
100% BIL waves (75kV at STP) recorded on 10s / division
Plate 2
: Reduced chopped wave (60kV at STP) recorded on one
s./division
Plate 3
: First 100% chopped waves (75kV at STP) recorded on one
s./division
Second 100% chopped waves (75kV at STP) recorded on
one s./div.
Plate 4
:100% BIL waves (75kV at STP) recorded on 10s / division
100% BIL waves (75kV at STP) recorded on 10s / division
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Anna University
96
OSCILLOGRAMS TAKEN DURING IMPULSE VOLTAGE WITHSTAND
TEST ON 250kVA, 11kV/433V, THREE PHASE DISTRIBUTION
TRANSFORMER
PHASE A
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Anna University
97
PHASE B
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Anna University
98
PHASE C
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Anna University
99
5.10 FAILURE REPORTS
Case 1 : 100kVA, 11kV/433V Al WOUND TRANSFORMER
(shows the progressing failure in the winding insulation)
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Anna University
100
Case II : 315kVA, 22kV/433 V Cu
transformer
wound
Dr M A Panneerselvam, Professor,
Anna University
three phase distribution
101
Case iii : 22kV metering Unit (CT)
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Anna University
102
Case iv : 12KV, 630A, 20KA, SF6 Load Break Fault Make switch
FIRST AND FIFTEENTH POSITIVE AND NEGATIVE IMPULSE VOLTAGES
APPLIED AT 75 KV IN MODE I TO MODE IV.
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Anna University
103
5.10 INSULATION
COORDINATION
Insulation Coordination is the
correlation of the
characteristics of the protective
devices and the equipment
connected in the power system
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Anna University
104
such that the equipment and the
system are well protected by the
protective devices in the event
of excessive over voltages.
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Anna University
105
5.10.1 Fixing of BIL for power system
operating at 132 kV:
Dr M A Panneerselvam, Professor,
Anna University
106
Let us first consider that the
system is not effectively earthed,
in which case the coefficient of
earthing (COE) is unity.
The rating of the lightning arrester
is chosen as 132 x 1 = 132 kV
Dr M A Panneerselvam, Professor,
Anna University
107
The discharge voltage for this
rating of the arrester = 132 x 3
(taken from arrester manual)= 396
kV.
Allowing 40 % excess for reflection
and refraction at the terminal end,
the voltage at the transformer
terminal = 396 x 1.4 = 555 kV.
Dr M A Panneerselvam, Professor,
Anna University
108
Further allowing a margin of
about 60 kV the insulation level
becomes = 555 + 60 = 615 kV
Choosing the next slab in the
insulation level, the BIL is
selected as 650 kV.
Dr M A Panneerselvam, Professor,
Anna University
109
Let us now assume the system
to be an effectively earthed
system, for which the COE is
taken as less than 75 %.
Working on the same line as
earlier, the rating of the
lightning arrester is chosen
as 132x0.75 = 99 kV.
Dr M A Panneerselvam, Professor,
Anna University
110
The residual voltage for this
rating from the arrester manual
= 99 x 3 = 297 kV . Allowing 40 %
excess for reflection and
refraction , the voltage at the
transformer terminal rises to =
297 x 1.4 = 416 kV.
Dr M A Panneerselvam, Professor,
Anna University
111
Allowing a margin of 60 kV ,
insulation level becomes
= 416 + 60 = 476 kV
Choosing the higher slab of
insulation the BIL is selected as
550 kV.
This level is known as reduced BIL.
Dr M A Panneerselvam, Professor,
Anna University
112
Hence, it is seen that as the
system becomes effectively
earthed the insulation level is
reduced resulting in great
saving of insulation and cost.
Dr M A Panneerselvam, Professor,
Anna University
113