12-1
12.1  Lightning Stroke Protection
Substation design involves more than installing apparatus, protective devices, and equipment. The
significant monetary investment and required reliable continuous operation of the facility requires
detailed attention to preventing surges (transients) from entering the substation facility. These
surges can be switching surges, lightning surges on connected transmission lines, or direct strokes
to the substation facility. The origin and mechanics of these surges, including lightning, are dis-
cussed in detail in Chapter 10 of The Electric Power Engineering Handbook (CRC Press 2001). This
section focuses on the design process for providing effective shielding (that which permits lightning
strokes no greater than those of critical amplitude [less design margin] to reach phase conductors
[IEEE Std. 998-1996 (R2002)]) against direct lightning stroke in substations.
12
Direct Lightning Stroke
Shielding of Substations*
12.1 Lightning Stroke Protection..........................................................12-1
e Design Problem
12.2 Lightning Parameters ....................................................................12-2
Strike Distance Stroke Current Magnitude Keraunic Level
Ground Flash Density Lightning Detection Networks
12.3 Empirical Design Methods ...........................................................12-5
Fixed Angles Empirical Curves
12.4 e Electrogeometric Model (EGM) ...........................................12-7
Whiteheads EGM Recent Improvements in the EGM
CriticismoftheEGM A Revised EGM Application
oftheEGM bytheRolling Sphere Method Multiple Shielding
Electrodes • ChangesinVoltage Level • Minimum Stroke Current
ApplicationofRevised EGM byMousa and Srivastava Method
12.5 Calculation of Failure Probability ..............................................12-19
12.6 Active Lightning Terminals ........................................................12-19
References ..................................................................................................12-19
Robert S. Nowell
(retired)
Commonwealth
Associates, Inc.
*
A large portion of the text and all of the gures used in this chapter were prepared by the Direct Stroke Shielding
of Substations Working Group of the Substations Committee—IEEE Power Engineering Society, and published as
IEEE Std. 998-1996 (R2002), IEEE Guide for Direct Lightning Stroke Shielding of Substrates, Institute of Electrical and
Electronics Engineers, Inc., 1996. e IEEE disclaims any responsibility of liability resulting from the placement or use
in the described manner. Information is reprinted with the permission of the IEEE. e author has been a member of the
working group since 1987.
12-2 Electric Power Substations Engineering
12.1.1  The Design Problem
e engineer who seeks to design a direct stroke shielding system for a substation or facility must con-
tend with several elusive factors inherent in lightning phenomena, namely:
e unpredictable, probabilistic nature of lightning
e lack of data due to the infrequency of lightning strokes in substations
e complexity and economics involved in analyzing a system in detail
ere is no known method of providing 100% shielding short of enclosing the equipment in a solid
metallic enclosure. e uncertainty, complexity, and cost of performing a detailed analysis of a shielding
system has historically resulted in simple rules of thumb being utilized in the design of lower voltage
facilities. Extra high voltage (EHV) facilities, with their critical and more costly equipment compo-
nents, usually justify a more sophisticated study to establish the risk vs. cost benet.
Because of the above factors, it is suggested that a four-step approach be utilized in the design of a
protection system:
1. Evaluate the importance and value of the facility being protected.
2. Investigate the severity and frequency of thunderstorms in the area of the substation facility and
the exposure of the substation.
3. Select an appropriate design method consistent with the above evaluation and then lay out an
appropriate system of protection.
4. Evaluate the eectiveness and cost of the resulting design.
e following paragraphs and references will assist the engineer in performing these steps.
12.2  Lightning Parameters
12.2.1  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. e principal ones
are as follows:
S I e= + .
/ .
2 30 1 1975
1 6 8
( ) ( )Darveniza et al
(12.1)
S I=
.
10 198 7 1993
0 65
Anderson ( ); ( )IEEE
(12.2)
S I= .
/
9 4 1974
2 3
Whitehead ( )
(12.3)
S I=
.
8 1985
0 65
IEEE( )
(12.4)
S I= . .
.
3 3 1981
0 78
Suzuki et al ( )
(12.5)
where
S is the strike distance in meters
I is the return stroke current in kiloamperes
It may be disconcerting to note that the above equations vary by as much as a factor of 2:1. However,
lightning investigators now tend to favor the shorter strike distances given by Equation 12.4. Anderson, for
example, who adopted Equation 12.2 in the 1975 edition of the Transmission Line Reference Book (1987),

Get Electric Power Substations Engineering, 3rd Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.