C.
The Network with the Lightning Transient
The lightning strikes the transmission line of the case study
network, as shown in Figure 7. The symbol of the lightning in
ATPDraw is shown in figure 6.
Figure 6.
The symbol of the lightning in ATPDraw
In ATPDraw the lightning is a surge function and its
parameters are shown in Figure 8.
Data: Amp = in [A], represents the peak value of surge.
n= Factor influencing the rate of rise of the function.
Increased n increases the maximum steepness.
Tsta & Tsto = Starting and ending time in [sec.].
The waveshape of the lightning current in ATPDraw is
shown in Figure 9.
Figure 7.
The network with lightning
Figure 8.
The lightning parameters
Proceedings of PMAPS 2012, Istanbul, Turkey, June 1014, 2012
400
Figure 9.
The waveshape of the lightning current
The voltage wave travelling along the line hit by lightning,
for instance, phase A, induces voltage waves on the
neighboring phases also. When the coupling factor between
phase A and phase B is K
ab,
then the voltage to the ground
induced on phase B is:
Vb = KabVa
The voltage on phase B and phase A in the case study is
shown in Figure 10 at X005.
Figure 10.
The overvoltage on the node X005
Kab
=
Vb
/
Va
from that
Kab
= 0.429
At 0.06 ms, the overvoltage is 17.313MV in phase A and
7.4275MV in phase B at same time can be calculated.
D.
Shielding Designs
There are two methods of shielding designs. One method is
based upon the work of Whitehead (Fig.11) and the concepts
discussed above. This figure gives the critical mean shielding
angle based upon the conductor height
hp
, (
hp
=
y
in Fig 11)
and conductor to ground wire spacing c. These quantities are
normalized in per unit of striking distance
rs
. The field results
of Pathfinder Project correlate well with this method.
The second method is a simple geometric construction for
placing the shield wires based upon the electromagnetic model
of shielding. The procedure can be discussed with reference to
figure. 12.
First, the critical shielding current should be found. It is
generally increased by 10 percent. Based upon this critical
current, the striking distance
rs
is known. To locate the center
C
for describing an arc with radius equal to the striking
distance
rs
, the ordinate is given by
rsg
and the abscissa is:
= 
+
Figure 11.
Critical shielding angle in terms of
normalized geometry
Note that for factor
Ksg=
1
Rsg= rs
. Draw an arc with
center C and radius
rs
passing through the outermost
conductor. It may cut the vertical line through the middle
conductor at same point S. A ground shield wire placed at this
point will protect all threephase conductors. Analytically the
height of the shield wire is
=

A qualification applies that the minimum clearance
required between the phase conductor and shield wire should
be observed. If this clearance is
Sps
and
hs
is less than
Sps
,
then make
Sps
=
hs
.
The arc thus drawn in Fig 13 may not cross the vertical
through the middle conductor. Two shield wires are then
required. Draw another arc with
P
3 as center and
Sps
as the
radius
(
Fig 13). The intersection of these two arcs gives the
location of the shield wire.