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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
1Prepared by S Arun M.Tech Insulation coordination
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.
2Prepared by S Arun M.Tech Insulation coordination
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.
3Prepared by S Arun M.Tech Insulation coordination
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.
4Prepared by S Arun M.Tech Insulation coordination
• 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.
5Prepared by S Arun M.Tech Insulation coordination
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.
6Prepared by S Arun M.Tech Insulation coordination
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.
7Prepared by S Arun M.Tech Insulation coordination
• 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.
8Prepared by S Arun M.Tech Insulation coordination
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.
9Prepared by S Arun M.Tech Insulation coordination
• 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.
10Prepared by S Arun M.Tech Insulation coordination
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
11Prepared by S Arun M.Tech Insulation coordination
• 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.
12Prepared by S Arun M.Tech Insulation coordination
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.
13Prepared by S Arun M.Tech Insulation coordination
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.
14Prepared by S Arun M.Tech Insulation coordination
• 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.
Prepared by S Arun M.Tech Insulation coordination 15

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Insulation coordination

  • 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 1Prepared by S Arun M.Tech Insulation coordination
  • 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. 2Prepared by S Arun M.Tech Insulation coordination
  • 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. 3Prepared by S Arun M.Tech Insulation coordination
  • 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. 4Prepared by S Arun M.Tech Insulation coordination
  • 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. 5Prepared by S Arun M.Tech Insulation coordination
  • 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. 6Prepared by S Arun M.Tech Insulation coordination
  • 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. 7Prepared by S Arun M.Tech Insulation coordination
  • 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. 8Prepared by S Arun M.Tech Insulation coordination
  • 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. 9Prepared by S Arun M.Tech Insulation coordination
  • 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. 10Prepared by S Arun M.Tech Insulation coordination
  • 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 11Prepared by S Arun M.Tech Insulation coordination
  • 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. 12Prepared by S Arun M.Tech Insulation coordination
  • 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. 13Prepared by S Arun M.Tech Insulation coordination
  • 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. 14Prepared by S Arun M.Tech Insulation coordination
  • 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. Prepared by S Arun M.Tech Insulation coordination 15