Rolling Spheres Method for
Lightning Protection
EPOW 6860
Surge Phenomena
Fall 2007
Joe Crispino
1
Design Problems
The unpredictable, probabilistic nature of lightning.
Lack of data due to infrequencies of lightning strikes in
switchyards.
Complexity in analyzing system in detail ($$$).
No known practical method of providing 100%
shielding.
2
Design Procedure
Risk Assessment
Evaluate the importance and value of the facility.
Consequences of a direct lightning strike.
Location
Frequency and severity of thunderstorms in area.
Exposure due to surrounding area.
Method of protection (surge arrestors, shielding).
Evaluate the effectiveness and cost of design.
3
Design Methods
Empirical Design Methods (Classical)
Assume that the shielding device (wire or mast) can
intercept all the lightning strokes arriving over the
subject area if the shielding device maintains a certain
geometrical relation (separation and differential height)
to the protected object.
4
Design Methods
Electrogoemetric Design Methods (EGM)
Attractive effect of the shielding device is a function
of the amplitude of the current of the lightning stroke.
Less intense strokes get by.
More intense strokes get intercepted.
Only allow strokes that will not cause flashover
or damage to protected object.
5
Design Methods
Empirical Design Methods (Classical) - 69kV & below
Fixed Angles Method (32.5%)
Empirical Curve Method (12.6%)
Electrogeometric Methods (EGM) - 345kV & above
Rolling Sphere Method (16.3%)
Mousa’s Software Subshield (21.1%)
6
Fixed Angle Method
“Rule of thumb” method.
Uses vertical angles to determine:
Total number of protection devices.
Position
Height
7
Fixed Angle Method
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Independent of
voltage, BIL, surge
impedance, stroke
magnitude, GFD,
insulation flashover,
etc.
α is commonly 45°.
β is usually 30°-45°.
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Fixed Angle Method
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9
Electrogeometric Model
1950’s - First 345kV transmission line.
Protection utilized empirical methods.
Outages due to lightning were much higher than
expected.
Led to extensive amount of research.
E. R. Whitehead - EGM
10
Electrogeometric Model
1963 - Young, et al. - EGM
1973 - Whitehead & Gilman
Most significant research.
Only for transmission lines.
1976 - Mousa - Subshield program
Integrated substations into EGM.
1977 - Lee - Rolling Sphere
11
Rolling Sphere Method
Developed by Ralph H. Lee in 1977 for shielding
buildings and industrial plants.
Extended by J.T. Orrell for use in substation design.
Builds on basic principles and theories from Whitehead.
12
Rolling Sphere Method
Use an imaginary sphere of radius S over the surface of
a substation.
The sphere rolls up and over (and is supported by)
lightning masts, shield wires, substation fences, 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.
13
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14
Rolling Sphere Method
Requires:
Surge impedance ( Z S ).
Allowable stroke current ( I S ).
Used to calculate striking distance,
determines the spheres radius.
S. This
15
Rolling Sphere Method
! 2 h $ VC
RC ln # & '
=0
R
E
" C%
0
! 2h $ ! 2h $
Z S = 60 ln # & ln # &
"R % " r %
C
Surge Impedance
RC = Corona radius
r = radius of the condu
uctor
h = Average height of conductor
VC = BIL
E0 = Limiting corona gradiant, 1500 kV m
16
Rolling Sphere Method
Stroke Current
1.1( BIL ) 2.2 ( BIL )
IS =
=
ZS
ZS
2
0.94 ( CFO )1.1 2.068 ( CFO )
=
IS =
ZS
ZS
2
17
Rolling Sphere Method
Strike Distance - the
probability of the stroke
tip terminating on an
object S far away is
greater than the
probability of it striking
another object S+n
away.
Sm = 8 kI 0.65
S f = 26.25 kI
0.65
k = 1 Ground or Wirres
k = 1.2 Lightning Masts
18
Rolling Sphere Method
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Rolling Sphere Method
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BUT WAIT!!!!!
What if the actual
stoke current is
greater than
calculated?
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Rolling Sphere Method
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What if the stroke
current is less?
As long as the stroke
current was calculated
using the BIL of the
equipment, the
equipment will be
protected.
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24
References
IEEE Std. 998-1996. IEEE Guide for Direct Lightning
Stoke Shielding of Substations.
Greenwood, A. Electrical Transients in Power Systems.
Abdel-Salam, M., et al. High Voltage Engineering Theory and Practice.
Zipse, D. Lightning Protection Systems: Advantages
and Disadvantages. IEEE Transactions on Industry
Applications, Vol. 30, No. 5, September/October 1994.
25