rolling sphere method for lightning...

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


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


Requires:

Surge impedance ( ).

Allowable stroke current ( ).

Used to calculate striking distance, . This determines the spheres radius.

ZS

ISS

15


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


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


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


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BUT WAIT!!!!!

What if the actual stoke current is greater than calculated?

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20


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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rolling sphere method for lightning...

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