2. Lightning Phenomena
Lightning require the formation of
certain type of clouds to be presence
in their generation.
This type of clouds is known as
cumulonimbus clouds more
specifically is the lightning generating
type.
3. Lightning Phenomena
Based on Ice Splinter Theory, moistures in the
atmosphere combining with precipitating particles which
are suspended in air are force higher up into the altitude
by updraft.
Every km going higher up in altitude there is decreased
about 0.4oC.
So the moistures experienced super cooling and formed
crystalline ice with double layered ice structures.
During the process of super cooling, H2O undergo
ionization break up into anions (H+) and cations (OH-).
The existences of temperature gradient in the super
cooling moisture structure allowing the migration of H+
ions and OH- ions.
However due to lighter weight of H+ ions they become
more mobile allowing them to move towards the outer
shell, leaving the heavier ions OH- in the inner shell of
the ice structures
4. Lightning Phenomena
Figure (a) show the physical relationship of
moisture, precipitating particle with the
updraft pushing the matters mentioned high
in the latitude
5. Lightning Phenomena
Figure (b) show the formation of ice crystal with
two-layered structure of positively charged
externally and negative charged internally
6. Lightning Phenomena
Figure (c) : Splintering process of the ice crystal to
form minute splinters pushing the positive charged
splinters higher up in the altitude
7. Lightning Phenomena
• Figure d shows the lower part of a thundercloud is usually
negatively charged. The upper portion area is usually
positively charged. Lightning from the negatively charged
area of the cloud generally carries a negative charge to Earth
and is called a negative flash. A discharge from a positively-
charged area to Earth produces a positive flash
8. Stroke Formation
There are a number of different types of
lightning strokes.
These include strokes within clouds, strokes
between separate clouds, strokes to tall
structures, and strokes that terminate on
the ground.
The positive and negative strokes
terminating on the ground are the types of
most interest in designing shielding
systems.
8
9. Stroke Formation
Stepped Leaders
The actual stroke development occurs in a two-
step process.
The first step is ionization of the air surrounding the
charge center and the development of stepped
leaders which propagate charge from the cloud into
the air.
Current magnitudes associated with stepped
leaders are small (in the order of 100 A) in
comparison with the final stroke current.
The stepped leaders progress in a random
direction in discrete steps from 10 to 80 m in
length. 9
11. Stroke Formation
Return Stroke
The second step in the development of a lightning
stroke is the return stroke.
The return stroke is the extremely bright streamer
that propagates upward from the earth to the cloud
following the same path as the main channel of the
downward stepped leader.
This return stroke is the actual flow of stroke
current that has a median value of about 24 000 A .
The velocity of the return stroke propagation is
about 10% the speed of light.
11
14. Correlations between Return stroke current magnitude
and
strike distance
Return stroke current magnitude and strike distance (length of
the last stepped leader) are interrelated. A number of
equations have been proposed for determining the striking
distance.
S is the strike distance to ground in (m)
I is the return stroke current in (kA)
The equation stroke current,
14
15. Keraunic Level
• Keraunic level is defined as the average number
of thunderstorm days or hours for a given
locality.
• A daily keraunic level is called a thunderstorm-
day and is the average number of days per year
on which thunder will be heard during a 24 hours
period.
• The average annual keraunic level for locations
can be determined by referring to keraunic maps.
15
17. Ground Flash Density
• Ground flash density (GFD) is defined
as the average number of strokes per
unit area per unit time at a particular
location.
• It is usually assumed that the GFD to
earth, a substation, or a transmission
or distribution line is roughly
proportional to the keraunic level at
the locality.
17
18. Ground Flash Density
If thunderstorm days are to be used as a basis, it is
suggested that the following equation be used:-
Where
◦ Nk is the number of flashes to earth per square kilometer
per year
◦ Nm is the number of flashes to earth per square mile per
year
◦ Td is the average annual keraunic level, thunderstorm days
18
19. 19
The mapping of the lightning ground flash density of Peninsula Malaysia(courtesy of
TNBR)
20. EMPIRICAL DESIGN METHODS
Two classical design methods have
historically been employed to protect
substations form direct lightning
strokes:-
◦ Fixed angles
◦ Empirical curves
The two methods have generally
provided acceptable protection
20
21. Fixed Angles
The fixed-angle design method uses
vertical angles to determine:-
• the number
• position
• height of shielding wires or masts
The angles used are determined by:-
• the degree of lightning exposure
• the importance of the substation being protected
• the physical area occupied by the substation
• The value of the angle alpha that is
commonly used is 45°. Both 30° and 45°
are widely used for angle beta.
21
24. Empirical curves
From field studies of lightning and laboratory model
tests, empirical curves have been developed to
determine number, position, and height of shielding
wires and masts (Wagner et al., 1941; Wagner,
1942; Wagner, McCann, Beck, 1941).
The curves were developed for shielding failure
rates of 0.1, 1.0, 5.0, 10, and 15%.
A failure rate of 0.1% is commonly used in design.
The empirical curve method has also been referred
to as the Wagner method.
24
25. Figure (a) Single lightning mast protecting single ring of object — 0.1% exposure. Height of mast above protected object,
y, as a function of horizontal separation, x , and height of protected object, d .
Figure (b) Two lightning masts protecting single object, no overlap — 0.1% exposure. Height of mast above protected
object, y , as a function of horizontal separation, s , and height of protected object, d.
Figure (a) Figure (b)
25
26. Empirical curves
Areas Protected by Lightning Masts
If two masts are used to protect an area, the data derived
from the empirical curves give shielding information only for
the point B, midway between the two masts, and for points on
the semicircles drawn about the masts, with radius x, as
shown in Figure a.
The locus drawn by the semicircles around the masts, with
radius x, and connecting the point B, represents an
approximate limit for a selected exposure rate.
Any single point falling within the crosshatched area should
have <0.1% exposure.
Points outside the cross-hatched area will have >0.1%
exposure.
Figure b illustrates this phenomenon for four masts spaced at
the distance s as in Figure a.
26
27. Areas protected by multiple masts for point exposures
shown in Figure a with two lightning masts, Figure b with
four lightning masts.
Figure a Figure b
27
28. Empirical curves
The protected area can be improved by moving the
masts closer together, as illustrated in figure below.
The protected areas are, at least, as good as the
combined areas obtained by superimposing those
of the previous figure .
The distance s′ is one half the distance s in the
previous figure.
The size of the areas with an exposure greater
than 0.1% has been significantly reduced.
28
29. Areas protected by multiple masts for point exposures shown in Figure
a with two lightning masts, Figure b with four lightning masts with the
distance s′ is one half the distance s in the previous figure.
Figure a Figure b
29
30. ELECTROGEOMETRIC MODEL (EGM)
Shielding systems developed using fixed
angles and empirical curves of determining
the necessary shielding for direct stroke
protection of substations have historically
provided a fair degree of protection.
However, as voltage levels (and therefore
structure and conductor heights) have
increased over the years, the classical
methods of shielding design have proven
less adequate.
This led to the development of the
electrogeometric model. 30
31. ELECTROGEOMETRIC
MODEL(EGM)
Striking Distances To A Mast/Shield
Wire
Where
◦ Sm is the strike distance in meters
◦ Sf is the strike distance in feet
◦ I is the return stroke current in kiloamperes
◦ K is a coefficient to account for different striking
distances to a mast, a shield wire, or the ground 31
32. ELECTROGEOMETRIC
MODEL(EGM)
Mousa (1988) gives a value of k = 1
for strokes to wires or the ground
plane and a value of k = 1.2 for
strokes to a lightning mast.
The EGM theory shows that the
protective area of a shield wire or
mast depends on the amplitude of the
stroke current.
32
33. ELECTROGEOMETRIC
MODEL(EGM)
Allowable Stroke Current
• Bus insulators are usually selected to
withstand a basic lightning impulse level
(BIL).
• Insulators may also be chosen according to
other electrical characteristics, including
negative polarity impulse critical flashover
(CFO) voltage.
• Flashover occurs if the voltage produced by
lightning stroke current flowing through the
surge impedance of the station bus
exceeds the withstand value. 33
34. ELECTROGEOMETRIC
MODEL(EGM)
Where
◦ IS is the allowable stroke current (kA)
◦ BIL is the Basic lightning Impulse Level (kV)
◦ CFO is critical flashover voltage of the insulation being
considered (kV)
◦ ZS is the surge impedance of the conductor through which the
surge is phasing (ohm)
◦ 1.1 is the factor to account for the reduction of stroke current
terminating on a conductor as compared to zero impedance earth
34
35. ELECTROGEOMETRIC
MODEL(EGM)
Application Of The EGM By The Rolling Sphere
Method
• Rolling sphere method employs the simplifying assumption
that the striking distance to the ground, a mast or a wire are
the same.
• Use of the rolling sphere method involves rolling an
imaginary sphere of radius S over the surface of a substation.
• The sphere rolls up and over lightning masts, shield wires,
substation fence, and other grounded metallic objects that
can provide lightning shielding.
• A piece of equipment is said to be protected from a direct
stroke if it remains below the curved surface of the sphere.
• Equipment that touches the sphere or penetrates its surface
is not protected.
35
37. Examples
1. Determine the allowable stroke current
(kA) through a substation equipment BIL
of 850 kV, if given surge impedance is 350
ohm.
2. A 69 kV substation is protected by a
lightning mast and has a basic insulation
level (BIL) of 350 kV. Calculate the strike
distance (Sm) in meters (m) if the surge
impedance of the conductor is 300 Ω.
37
38. Insulation Co-ordination
Introduction
• Insulation coordination is the process of
coordinating the insulation level of electrical
equipment and its associated surge
arresters with the expected overvoltage that
occur on the power system.
• It is selection of suitable insulation levels of
various components in any electrical
system and their rational arrangement.
• Substation equipment require properly
applied surge arresters for reliable
operation. 38
39. Insulation Co-ordination
• It is required to ensure:-
1) Insulation shall withstand all normal stresses and majority of
abnormal ones
2) Efficient discharge of over voltages due to internal or external
causes
3) B/D shall be only due to external causes
4) B/D shall be at such places where least damage is caused
5) Safety of operating personnel and public
• The purpose of insulation coordination is to reduce the risk of
insulation failure of key electrical equipment.
• The insulation levels of substation equipment should be co-
ordinate with the protective levels of the surge arresters and
spark gaps.
39
40. Insulation Level
• Insulation Level is defined by the values of test
voltages which the insulation of equipment under
test must be able to withstand.
• Basic Lightning Impulse Insulation Level ( BIL) :
◦ The electrical strength of insulation expressed in terms of
the crest value of a standard lightning impulse under
standard atmospheric conditions.
• Basic Switching Impulse Insulation Level (BSL) :
◦ The electrical strength of insulation expressed in terms of
the crest value of a standard switching impulse.
◦ BSL may be expressed as either statistical or conventional.
40
41. Insulation Level
Lightning Impulse Voltage
The BIL of substation apparatus and equipment
is verified by applying a standard lightning
impulse having a 1.2/50 μs waveshape.
Additional test on some equipment consist of the
application of a 1.2/50 μs wave chopped at either
2 or 3 μs.
For example, a circuit breaker must withstand
the application of a 1.2/50 μs impulse chopped at
3 μs, having a crest of 1.15 times the BIL and the
application of a 1.2/50 μs impulse chopped at 2
μs, having a crest of 1.29 times the BIL.
41
42. Insulation Level
Busings must withstand the application of
1.2/50 μs impulse, chopped at 3 μs, having a
crest of 1.15 times the BIL.
Test on transformers include a 3 μs chopped
wave test at 1.10 times the BIL.
Thus the lightning impulse strength of all
insulations is defined by use of the standard
lightning impulse waveshape.
42
44. Insulation Level
t1 is the equivalent time to crest based
on the time taken to rise 10-90% of
the crest.
t2 is the time between the origin of 10
– 90 % virtual front and the point
where drops to half value
44
.
45. Insulation Level
o Insulation Level Of An Equipment
Basic Insulation level is a term which includes
the following characteristics of an equipment
:-
Power frequency voltage withstand level.
Lightning impulse voltage withstand level.
Switching impulse voltage withstand level.
These withstand levels together characterize
the insulation level of a substation equipment.
Each substation equipment has its rated
insulation levels.
45
46. Insulation Level
These are specified in the respective standard of that
equipment.
The substation insulation co-ordination is governed by
impulse withstand values, lightning voltage withstand levels
for voltages up to 220 kV and switching impulse for voltage of
345 kV and above.
46
47. Insulation co-ordination of a
substation
Insulation co-ordination of a substation
involves the following :-
To determine the appropriate insulation of
transformers and other equipments.
To select the settings of co-ordinating gap.
To select the correct surge arresters at various
locations.
Insulation co-ordination consists of steps
taken to prevent damage to electrical
equipment due to over voltage and to localize
the flashovers to discharge over voltage at
such points where they will cause minimum
damage.
47
48. Insulation co-ordination of a
substation
This is achieved by correlating the insulation
strengths of line, switchgear, transformer,
machines etc. and the characteristics of the
overvoltage protective devices such as rod
gaps, surge arresters.
The impulse withstand level of system
components and the impulse protective level
to protective devices are correlated.
The insulation level of an equipment is
expressed in term of curve of the specified
impulse wave called impulse withstand level.
48
49. Insulation co-ordination of a
substation
Consider for example at 132 kV incoming
line.
Basic insulation level of 550 kV is chosen.
The line insulation can withstand standard
impulse wave of 860 kV crest value.
The breakdown voltage of line lightning
arrester is 500 kV.
Transformer impulse voltage withstand is 550
kV.
Therefore, a high voltage surge coming from
transmission line will be discharge to earth by
lightning arrester.
The residual voltage less than breakdown
voltage, the transformer insulation is
protected. 49
50. Insulation co-ordination of a
substation
The insulation coordination between
equipment and a surge arrester
50
51. Insulation co-ordination of a
substation
The following points should kept in mind while
co-ordinating the insulation in the substation:-
Surge arrester should have the lower protective level
characteristics. The residual voltage should be less
than transformer insulation strength.
The voltage time curves of individual components
must be taken into account. To withstand voltage
characteristic of all the equipments should lie above
the protective characteristic of the surge arrester with
enough margin.
The insulation co-ordination should be based on
worst possible atmospheric condition as regards rain,
humidity, pollution etc.
The protective device such as surge arrester should
give protection against wave of both polarities.
51
52. Insulation co-ordination of a
substation
Margin Protection
The Margin of Protection (MP) is the difference between
arrester discharge characteristics and equipment withstand
level, at any given instant of time.
◦ Where:-
BILequip : the BIL of the equipment
Var : the discharge of voltage of arrester
The margin of protection for the equipment protection and
should not be less than 0.20 (20%)
52
100
(%) x
BIL
V
BIL
MP
equip
ar
equip
54. Example
A Substation transformer 66 kV/150 kV,
have BIL of 275 kV/ 650 kV. A lightning
arrester is installed on transformer terminal
of 66 kV, has discharge voltage of 250 kV.
Calculate the protection margin in
percentage.
54
