This document discusses insulation coordination, which involves selecting insulation levels for electrical system components and arranging them rationally. Proper insulation coordination ensures reliability by minimizing failures from overvoltages while reducing costs. Key aspects covered include determining line and equipment insulation levels, selecting surge arrestors, and the ideal characteristics of protective devices to coordinate insulation withstand. The relationship between insulation, protective device characteristics, and location of surge arrestors is also examined.

1. Insulation Coordination
Contents
•Introduction
•Necessity of insulation coordination
•Insulation problem
•Ideal requirement of protective devices
•Volt-time characteristics
•Line insulation
•Surge arrestor selection
•Location of lightning arrestor
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2. Introduction
• The selection of suitable values for the
insulation levels of the varies component in
any electrical system and their arrangement in
a rational manner is called insulation
coordination.
• Insulation level of an apparatus
– It is defined as that combination of voltage values
(power frequency and impulse frequency) which
characterise its insulation with regards to its
capability of withstanding the dielectric stress.
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3. Necessity of insulation coordination
• To ensure reliability and continuity to the
utility concern.
• To minimize the no of failure of lines and
substations due to over voltages.
• To minimize the cost involved in the design,
installation and operation of protective
devices.
• To maintain the flashover should be minimum
for insulators so that the system disturbances
are less.
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4. Insulation problem
• It occurs due to following
– Determination of line insulation
– Selection of Basic Impulse Insulation level (BIL)
and the insulation of other equipment
– Selection of lightning arrester.
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5. • Lightning Impulse Withstand level (or) BIL
– Basic impulse level is established for each system
nominal voltage for different equipment. Various
equipment should have their BIL value above the
system protection level using lightning arresters by
a suitable margin.
• For voltage < 400kv
– If BIL is chosen correctly, the equipment will have
an adequate switching surge level
• For voltage > 400kv
– Switching surge magnitude are higher than the
lightning over voltage, then certain condition to
be adopted.
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6. Ideal requirement for a protective devices
• It should not usually flashover for power
frequency over voltages.
• The volt-time char of the devices must be lie
below the withstand voltage of insulation.
• It should be capable of discharging high
energy contained in surges and recover
insulation strength quickly.
• It should not allow power frequency follow on
current to flow.
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7. Volt time characterises
• Proper insulation coordination would ensure
that the volt time char of the equipment will
lie above the volt time char of the protective
devices as shown.
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8. • Case (i): Assuming the surge voltage rise is
shown in curve 1,
– The rate of rise of surge is less than the critical
slope (curve X)
– The rod gap flashes and protects the transformer
insulation.
• Case (ii): Assuming the surge voltage rise is
shown in curve 2,
– The rate of rise of surge is greater than the critical
slope (curve X)
– The rod gap cannot protects the transformer and
only surge arrester can protect the transformer.
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9. Line insulation
• The insulation of line is based on the
consideration of lightning and switching surges
and power frequency over voltages.
• The line insulation must be provided to prevent a
flash over due to power frequency and switching
surges and also considering rain, dust, insulator
pollution factor.
• Sometimes one or two disc in the string insulator
may be defective.
• So, lines upto 220kv, one extra disc must be
provided and for upto 400kv, two extra disc must
be provided.
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10. • The line insulation is not directly related to the
insulation level of the station equipment.
• But the impulse flash over of the line
insulators determines the highest surge
voltage that can travel into the station from a
distance.
• Over head guard wires are used to protect the
line and should be kept at proper distance
above secondary line conductor.
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11. Surge arrester selection
• The selection of surge arrester voltage rating
foe EHV and UHV system depends on
– The rate of rise voltage
– Operating char
• The following types of arresters are used for
this purpose
– Silicon carbide arresters with spark gap
– Silicon carbide arrester with current limiting gap.
– Zinc oxide or gapless metal oxide arrester
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12. • Advantage of ZnO arrester for EHV system
– Simple in consideration
– Flat V-I char over a wide current range
– The absence of a spark gap that produces steep
voltage gradients when sparking occurs.
• Disadvantage of ZnO arrester
– Continues flow of power frequency current.
– Power loss is high.
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13. Location of lightning arrester
• Lightning arrester should be installed on the
transformer terminal, considering the
following factor
– Shielding against the direct strokes
– BIL of the transformer winding
– BIL of the circuit breaker, isolator and other
substation equipment.
– Residual discharge voltage of lightning arrester
– No of line entering into the substation
– Construction of over head line
– Layout of substations.
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14. Equipment insulation level and insulation
coordination of substation
• The component of the substation are transformer,
circuit breaker, isolators, instrument transformer.
• Insulation coordination of substation involves the
number of location of surge arrester to be decided to
minimize the overall cost.
• To protect the transformer in high voltage substation,
the arrester must be provided between transformer
and circuit breaker.
• The protective level of the substation insulation
depends on,
– The station location
– The protective of arrester
– Line shielding used.
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15. • Conclusion
Here came to know about how the insulation level can be
chosen such that find withstanding capability.
• Reference
1. “High voltage engineering ” by M S Naidu and V Kamaraju,
Tata McGraw Hill Education, 5th edition.
2. “High voltage engineering ” by M.Jeraldin Ahila, A.R.S.
Publication, 1st edition.
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