1. LIGHTNING AND INSULATIONS COORDINATION
CHAPTER 6.
LIGHTNING AND INSULATIONS COORDINATION
LIGTHNING PARAMETER
SHILEDING PARAMETER
ALLOWABLE STROKE CURRENT
ELECTROMAGNETIC MODEL (EGM)
INSULATION LEVEL AND
MARGIN PROTECTION

2. Correlations between Return stroke current magnitude and
Lightning Parameters
Strike distance
Stroke current magnitude
Keraunic Level
Ground Flash Density
Correlations between Return stroke current magnitude and
strike distance
rs = 10 I
Love (1993)
rs = 8
IEEE (1985)
rs = 3.3
Suzuki (1981)
rs : the strike distance to ground in (m)
I : the return stroke current in (kA)

3. Keraunic level is defined as the average number
The equation stroke current,
I = rs
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.

4. Annual frequency of thunderstorm days in the world

5. 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 distributions
line is roughly proportional to the keraunic level at the
locality.
. The general form of the ground flash density
Ng = k
Td : the number of thunderstorm days per year,
the keraunic level.
k and a : Constants have been proposed.

6. The equations that suggested to determine Ground Flash Density,
Ng = 0.12 Td
Ng = 0.036
The relationship between thunderstorms hours per year Th
and ground flash density as suggested by CIGRE,
Ng = Th

7. Fixed angles for shielding wires
The value of the angle
that is commonly used is 450.
Both 300 and 450 are widely
used for angle .
Fixed angles for shielding wires

8. Fixed angles for mast

9. Allowable stroke current
Flashover occurs if the voltage produced by lightning
stroke current flowing through the surge impedance
Of the station bus exceeds the withstand value, = = =

10. IS : The allowable stroke current (kA)
BIL : The Basic lightning Impulse Level (kV)
CFO : Critical flashover voltage of the insulation
being considered (kV).
ZS : The surge impedance of the conductor
through which the surge is phasing (ohm)
1.1. : The factor to account for the reduction
of stroke current terminating on a conductor
as compared to zero impedance earth

11. Principle of the rolling sphere method
The Electrogeometric Model (EGM) by the rolling
sphere method.
Principle of the rolling sphere method

12. rs = 8 k I 0.065 The relationship between strike distance rs
and return stroke current I (for the striking distance S )
in the rolling sphere method
rs = 8 k I 0.065
k = 1 for strokes to wires or the ground plane and
= 1.2 for a stroke to lightning mast

13. Definitions according to IEEE std 1313-1996;
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.
Conventional BIL : The crest value of standard lightning
impulse for which the insulation shall not exhibit
disruptive discharge when subjected to a specific number
of applications of this impulse under specified conditions,
applicable specifically to non-self-restoring insulations.

14. Lightning over-voltage: a type of transient over-
voltage in which a fast front voltage is produced
by lightning or fault. Such over-voltage is usually
unidirectional and of very short duration.
Lightning over-voltage: a type of transient over-
voltage in which a fast front voltage is produced
by lightning or fault. Such over-voltage is usually
unidirectional and of very short duration.

15. Standar 1.2 x 50 Lightning impulse voltage Theoretical

16. PRACTICAL Standard Impulse

17. Maximum residual voltage of Lightning Arrester
Arrester Rating
F.O.W
10 kA Light and heavy-duty
and 5 kA
kVrms
kV/µs
Std kV peak
F.O.W kV peak 84
700
302
347 96
790
324
371
102
830
343
394
108
870
363
418
120
940
400
463
126
980
420
485
138
1030
460
530
150
1080
500
577
174
1160
570
660
186
1180
610
702 Vr
Var
F.OW : From of wave voltage impulse spark over
Var : Voltage discharge of Arrester

18. Determine of Voltage rating of the lightning Arrester
Vr = Vn x 1.1. x Grounding coefficient (Cg)
Cg : (Effective grounded)
Cg : (Solid grounded).
For system voltage 133 kV effective grounded,
= 133 x 1.1. x 0.8 = Volt

19. Insulation strength of equipments to impulse wave is
determined by the Basic Lightning Impulse Level (BIL).
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 .
Some insulation is also tested with a chopped wave
CCW, a chopped wave has the same characteristic
as the 1.2 x 50
wave, but the wave save is chopped
off after 2 or 3

20. Margin Protection
Insulation coordination is the process of determining
the proper insulation levels of various components
in a power system and their arrangement.
Insulation coordination involves the following:
Determination of line insulations
Selection of the BIL and insulation levels of
other apparatus, and
Selection of lightning arresters.

21. The insulation coordination between equipment
(e.g., transformer) and a surge arrester
1. Transformer with 550 kVBIL
2. Line insulation of 9 suspension
3. Disconnecting switch
4. Bus insulation of 10
suspensions units

22. The Margin of Protection (MP) is The difference
between arrester discharge characteristics and
equipment withstand level, at any given instant of time
BILequip : the BIL of the equipment and
Var : the discharge of voltage of arrester
The margin of protection for the equipment
protection and should not be less than 0.20 (20%)

23. Insulation coordination
Between equipment and
surge arrester

24. Problem:
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.
(i) Calculate the protection margin in percentage.
(ii) Is protection system installed has requirement of
margin protection.

25. Solution MP (%) = { (275 – 250) / 275 } x 100 % = 9 %
Protection System not fulfill requirement because less than 20 %