Basic for Insulation coordination including details on Section of lighting arrestors and BIL. Excursively for GTU students.

1. Insulation
Coordination
Prepared by :
Shivam Patel
GCET

2. Content
 Introduction.
 Definitions.
 Requirement of Insulation.
 Determination of line insulation.
 BIL & insulation level selection of various equipment's.
 Selection of lighting arrestors.
 Procedure for Insulation Coordination
 Data Required for Insulation Coordination Study

3. Introduction
 The selection of suitable values for the insulation levels of the
various components in any electrical system and their arrangement
in a rational manner is called insulation coordination.
 The insulation level of an apparatus is defined as that combination
of voltage values (both power frequency and impulse) which
characterize it insulation with regard to its capability of
withstanding the dielectric stress.
 In its simplest form, Insulation Coordination is 'The selection of
insulation strength‘.
 The overall aim of insulation coordination is to reduce to an
economically and operationally acceptable level the cost and
disturbance caused by insulation failure. In insulation coordination
method, the insulation of the various parts of the system must be
so graded that flash over if occurs it must be at intended points.

4. Insulation
Coordination –
Definition
 IEEE 1313.1 – The selection of the insulation strength of equipment
in relation to the voltages, which can appear on the system for
which the equipment is intended and taking into account the
service environment and the characteristics of the available
protective devices
 IEC 60071-01 – Selection of the dielectric strength of equipment in
relation to the operating voltages and over-voltages which can
appear on the system for which the equipment is intended, taking
into account the service environment and the characteristics of thE
available preventing and protective devices.

5. Definitions.
1.Dry flashover voltage (Dry FOV) : of an insulation defines that power
frequency voltage which will cause flash-over of the insulation.
2.Wet flashover voltage (Wet FOV) : of an insulation defines that power
frequency voltage which will cause flashover of the insulation when sprayed
with water of resistance 9,000 — 11,000 ohms-cm drawn from a source of
supply at temperature within 10°C of the ambient temperature in the
neighbourhood of the insulation under test, and directed at an angle of 45°,
the volume of water being equivalent to a precipitation of 0.305 cm per
minute.
3.Impulse flashover voltage (Impulse FOV) defines that voltage which will
cause flashover of the insulation when subjected to a 1.5 x 40 t sec
(according to American standards) or a 1 x 50 µ sec (according to British
standards) or a 12 x 50 kt sec (according to Indian standards) impulse.

6. Definitions.
4. Impulse spark over volt-time characteristic :
This characteristic is obtained by plotting on the abscissa the time
which elapses between the moment the voltage wave is applied and
the moment of the spark over and on the ordinate the voltages at the
moment of the spark over occurring on the wave front or the wave
peak, and the crest of the voltage for spark overs occurring on the
wave tail.

7. An Example of
Insulation
Coordination
 • Tower type - steel
 • Shielding wire - Yes
 • Line BIL = 1050 kV
 • Transformer BIL = 850 kV
 • Breaker BIL = 1050 kV
 • Switch & post insulators BIL
 = 900 kV
 • Arrester
 – Continuous voltage = 190 kV
 – Discharge voltage = 600 kV
 • Separation distance ≤ 3 m
 between arrester & x’mer
 (as per IEEE C62.22)

8. Requirement
of Insulation.
1. Highest Power Frequency System Voltage
• The AC power network has different nominal power frequency voltage
level like 400V, 3.3KV, 6.6k, etc. When the system is lightly loaded the
power frequency voltage at the receiving end of the line rises.
• The equipment of the power system is designed and tested to withstand
under highest power frequency system voltage (440 V, 3.6K, 7.2K,etc.)
without internal or external insulation failure.
2. Temporary Power Frequency Overvoltages
• The temporary overvoltage in the power system can be caused by the load
throw off, faults, resonance, etc. Their frequency is about 50 Hz and of
lesser peaks, the lesser rate of rising and longer duration (second or even
minutes).
• The protection against temporary power frequency overvoltage is
provided by IDMT relay.

9. Requirement
of Insulation.
3.Transient Overvoltage Surge.
 The transient over voltage surges in the power system can be caused by
lightning, switching, restrikes travelling waves, etc. The surge of the power
system has the high peak, high rate of rising and last for a few tens/hundreds
of microseconds and are therefore called the transients.
 This surge can cause spark over voltage and flash over at sharp corners,
between phase and earth, at the weakest point, the breakdown of
gaseous/liquid/solid insulation, failure of a transformer and rotating
electrical machines.

10. Requirement
of Insulation.
4. Withstand Levels of the equipment
• The basic insulation level is the reference level expressed in impulse crest
voltage with a stranded wave not higher than 1.2/50 μs.
• The apparatus or equipment should be capable of withstanding test wave
above BIL.
• Insulation coordination implies the selection of the insulation of equipment
with regards to its application to minimize the undesired incident due to
voltage stresses (caused by the overvoltage in the system) within the system.
• Insulation breakdown means the correlation of the insulation breakdown of
the various components of a power system to the insulation of the protective
devices used for the protection of that equipment against overvoltage.

11. Determination
of line
insulation
 The insulation of a line has to be based upon the consideration or
lightning and switching surges and power frequency over-voltages.
 In the case of high voltage lines of 132 kV and above, these can be
made particularly lightning proof by
 (i) efficient sledding
 (ii) low tower footing impedances.
 Good shielding is obtained when the shielding angle is about 30° and
similarly, optimum conditions are generally obtained when the tower-
footing impedance is reduced to about 10 ohms.
 The line insulation must be sufficient to prevent a flashover from the
power frequency over voltages and the switching surges, taking into
account all the local unfavorable circumstances which decreases the
flashover voltage (rain, dust, insulator pollution, etc). It is usual to
adopt the following over-voltage factors.

12. Determination
of line
insulation
 It can be worked out to see that lines working at voltages 132 kV and above are
immune to lightning provided. of course, it proper shielding and low tower footing
resistance are provided.
 Coordination of Line Insulation Willi Station Insulation. The line Insulation is not
directly related with the insulation level of the station equipment. It, however, enters
into coordination with station insulation in that the impulse flashover of the line
insulators determines the highest surge voltage that can travel into the station from a
distance.
 High voltage surges of steep front originating on the unshielded section of line will
have their front sloped off in travelling over the shielded line into the station

13. BIL &
insulation level
selection of
various
equipment's
 For each system voltage basic impulse insulation level has been fixed by
most of the national and international standards.

14. BIL &
insulation level
selection of
various
equipment's
 The major substation equipment viz, transformers, breakeis.
isolating switches, current trans formers, potential transformers
are manufactured for the same insulation level, expect where
transformers may be manufacturing for a lower step of insulation
level in consideration of the economy possible.
 Sometimes, where the lightning arresters are installed right on the
terminals of transformers, some of the substation equipment may
fall outside the protective zone determined from the Withstand
level of the equipment. discharge voltage of the lightning arresters
and the distance between the equipment and the lightning
arrester, and such equipment may be arranged with one step
higher B.l.L.
 In general the insulation level of substation equipment such as
circuit breakers, snatches, bus bars, instrument transformers is
assumed 10% higher than the transformer B.l.L. insulation level
across the open poles of disconnect switches may be kept 10 -15%.
higher than that provided between the poles and earth.

15. Selection of
lighting
arrestors.
 For simplify, the process of making an arrester application may be reduced
to seven steps.
1. To determine the magnitude of the power frequency phase to ground
voltage expected at the proposed arrester location during phase to
ground fault, or other abnormal conditions which cause higher voltages
to ground than normal.
2. To make a tentative selection of the power frequency voltage rating of
the arrester. This selection may have to be reconsidered after step (6) ls
completed.
3. To select the impulse current likely to be discharged through the tax-
tester. W
4. To determine the maximum arrester discharge voltage for the impulse
current and type of arrester selected.
5. To establish the full-wave impulse voltage with stand level of the
equipment to be protected.
6. To make certain that the maximum arrester discharge voltage is below
the full wave impulse, withstand level of the equipment insulation to
be protected, by adequate man-gm.
7. To establish the separation limit between the arrester and the
equipment to be protected.

16. Power System
Over-Voltages
 The power frequency voltage which may appear across an arrester
becomes greater than maximum operating phase to ground voltage for
one or more of the following reasons :
1. System Faults
2. Regulation
3. Over spaced
4. Switching transients
5. Interaction of transformer magnetizing reactance and line capacitance.
6. Excitations of induction motors by shunt capacitor banks.
7. Use of salient pole machines without damper windings.
 For the purpose of selection of voltage rating of lightning arrester three
types of earthling are considered viz.
1. Effectively earthed
2. Non-effectively earthed and
3. Isolated neutral l. Effectively earthed system

17. Procedure for
Insulation
Coordination
 Transient analysis & simulation
 • Origin and level of over-voltages
 • Statistical distribution of over-voltages
 • Protective level of arresters
 • Insulation characteristics
 • Determine contamination severity
 • Verification of data and assumptions
 • Determine coordination factor Kc
 • Determine altitude correction factor Ka
 • Determine safety factor Ks
 • Determine test conversion factor Ktc
 • Determine level and range of Uw (for
 both internal and external)

18. Insulation
Coordination
for
Transmission
Lines
 Transmission Line Insulation Coordination Involves
 Shielding angle of the shielding wire
 Clearance of conductors
 Selection of the type and length of insulators
 Points to remember
 Shielding Failure Flashover Rate (SFFOR) and Back Flashover Rate
(BFR) are two typical design criteria – typical SFFOR is 0.05 f/100km-
yr and BFR is 1 f/100km-yr
 The higher the tower and voltage, the smaller the shielding angle
 BFR impacts substation insulation requirements
 Contamination influences creep age distance (mm/kV) and
consequently the number of insulator units
 Generally, from an insulation perspective, transmission line reliability
performance is 10% of substation reliability criterion

19. Insulation
Coordination
for Substations
 Substation Insulation Coordination Involves
 Determination of BILs for major equipment or equipment group
 Location of shielding masts and/or shielding wires
 Clearance of conductors
 Surge arresters – Rating, number & locations
 Points to remember
 MTBF (and BFR) determine if line-entrance arresters are required
 Transformers and cables should always be primary concerns
 Protective margin is generally for non-self-restoring insulation
 Cost of equipment failure generally determines sequence of failure
 Gap configuration can change CFO level by ±30%
 Lightning flash can be multiple strokes – longitudinal insulation
 There may be back-and-forth calculations/adjustments required

20. Over-Voltage
Protection
Devices

21. Data Required
for Insulation
Coordination
Study
 The data collected for a study is dependent on the purpose of the
study, but the following are examples of required data.
1. Collect BIL, CFO data of all insulation
2. Collect Arrester characteristics and installed locations if
applicable.
3. Obtain one line diagram of system with distances between all
insulators and arresters.
4. Insulator counts and locations in particular if a switching study is
the goal.
5. Region of the country
6. Lightning data for area of analyses.
7. Ground resistances where possible.