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 ProcedureRisk 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°.
8
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 Model1963 - 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 ( ).
Allowable stroke current ( ).
Used to calculate striking distance, . This determines the spheres radius.
ZS
ISS
15
Rolling Sphere Method
Surge Impedance
RhR
VE
ZhR
hr
CC
C
SC
ln
ln ln
2 0
60 2 2
0
!
"#$
%&' =
=!
"#$
%&!!"#
$%&
=
=
RrC Corona radius
radius of the conduuctorAverage height of conductorBIL
hVEC
=
=
0 == Limiting corona gradiant, 1500 kVm
16
Rolling Sphere Method
Stroke Current
I Z Z
I Z
SS S
SS
= ( ) = ( )
= ( )
1 1
2
2 2
0 94 1 1
2
. .
. .
BIL BIL
CFO== ( )2 068. CFO
ZS
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.
S kIS kI
k
m
f
=
=
=
826 25
1
0 65
0 65
.
..
Ground or Wirres Lightning Mastsk = 1 2.
18
Rolling Sphere Method
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Rolling Sphere Method
BUT WAIT!!!!!
What if the actual stoke current is greater than calculated?
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20
Rolling Sphere Method
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.
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