Design Philosophy for Direct Stroke Lightning Protection
Direct strokes lightning can damage outdoor substation equipment and Busbar.
To protect the substation equipment and increase reliability of the outdoor substation / switchyard, substation design engineers should do the exercise on direct stroke lightning Protection.
This article gives detail idea about DSLP (Direct Stroke Lightning Protection) which is based on “High Voltage Engineering” by Prof. Razevig.
Location of LMs has been ascertained using Dr. Razevig’s method illustrated in the “High Voltage Engineering” book authored by Dr. D.V.Razevig.
Protection Zone of 33/11kV outdoor Substation
Protective zone of single lightning mast :
Let “ad” be the Lightning Mast (LM) of height “h“. Let us take a point “e” on ground surface at a distance of “0.75h” from axis of LM. Take point “c” on ground surface at a distance “1.5h” from axis of LM. Take a point f on axis of LM at a height “0.8h” from ground. Draw a lines “a” to “e” & “f” to “c“.
Let the straight lines cut at “b“.
The protective zone is under the line “ab” and “bc” all around the axis of LM.
It is clear that point “b” is situated at a height equal to “2/3h“.
Radius of protection at a level “hx<=2/3h” is:-
rx = 1.5*h*(1-(hx/0.8*h)) —————————(1)
Radius of protection at a height “hx>2/3h” is:-
rx = 0.75*h*(1-(hx/h))——————————-(2)
The Lightning masts of height greater than “30 meters” becomes ineffective.
For determining the radius of protective zone of a lightning mast of height “h>30 meters”.
The values obtained from “eq-1” and “eq-2” must be multiplied by the coefficient “p” where “p” can be given as:
p = 5.5/SQRT(h)————————————–(3)
So radius of protection in case of “h>30mtrs” becomes
for hx <= 2/3h
rx = 1.5*h*(1-(hx/0.8*h))*p————————(4)
for hx >= 2/3h
rx = 0.75*h*(1-(hx/h))*p—————————-(5)
Protective zone for two lightning masts :
The protective zone between two lightning masts have considerably greater dimensions than the sum of protective zones of two single lightning masts. If it is necessary to protect a point situated midway between the lightning masts at a height of h0, the distance between the lightning masts of height h must be.
a0 = 7*(h-ho)——————————————-(6)
a <= a0
ie, must not be greater than seven times the active height of the lightning masts.
Or, if the height of lightning masts and the distance between then are known, height of a point which lies mid-way between the lightning masts and will be protected from lightning discharge is found as
ho = h – (a/7)——————————————–(7)
If height of the lightning masts is greater than “30m”, above two equations become:
a0 = 7*(h-ho)*p—————————————–(8)
a <= a0
ho = h – a/(7*p)——————————————(9)
The internal part of the protective zone of two LMs in a plane passing through both the LMs is bounded by the arc of a circle which can be constructed on three points; two of them are tips of the LMs and the third one is situated midway between the LMs at a height equal to “h0“.
Radius of protection of fictitious LM is
For hx <= 2/3ho
rox = 1.5*ho*(1-(hx/0.8*ho))—————————(10)
For hx >= 2/3ho
rox = 0.75*ho*(1-(hx/ho))——————————(11)
For LMs of height more then “30mtrs” the equations become
For hx <= 2/3ho
rox = 1.5*ho*(1-(hx/0.8*ho))*p———————–(12)
For hx >= 2/3ho
rox = 0.75*ho*(1-(hx/ho))*p—————————(13)
Protective zone for two lightning masts of different heights :
Two lightning masts are of different heights of h1 and h2, separated by a distance of a.
Construction of protective zones of two lightning masts of different heights is shown below.
At first the protective zone of higher lightning mast is constructed. After that through the tip of the lightning mast of smaller height a horizontal line is drawn till it intersects the protective zone of the single lightning mast of greater height.
Considering that the tip of a certain fictitious lightning mast coincides with this point of intersection 3.
Protective zones of the lightning masts 2 and 3 of the same height h2 and situated at a distance a’, are constructed.
Protective zone for three lightning masts :
In this case the external part of the protective zone is determined in the same way as the protective zone of two LMs. Taking a pair LMs at a time and repeating the procedure around the periphery.
The internal part of the protective zone of a number of lightning masts is not constructed.
An object of height hx situated inside the triangle (or rectangle) formed by the lightning masts will be protected in case the diameter D of the circle passing through the tips of the lightning masts (or the diagonal of the rectangle at the corners of which the lightning masts are situated) is not Greater than 8 times active height of the lightning masts.
So, for h<=30mtrs
D <= 8*(h-hx)————————————————-(14)
and for h>30mtrs
D <= 8*(h-hx)*p———————————————-(15)
WHERE p = 5.5/SQRT(h)
Verification of diameter of circumscribing circle “Dc” :
For the triangle ABC circumscribing a circle of diameter “Dc“;
Where “R” is the radius of circumscribing circle.