DSLP Calculation : 69 kV Switchyard
By Rolling Sphere Method, as per IEEE-std 998-1996
Nomenclature used in the calculations
S Sphere Radius
H Mast Height
A Bus Height
W&C Horizontal distance from origin of sphere (OOS) to bus
T Maximum separation from Mast to Bus for protection
Y Minimum Phase to steel clearance
Z Horizontal distance between OOS and Line drawn between two Masts
L Half the separation between two Masts
X Maximum separation between two Masts
D Elevation difference between Mast and Bus
E Elevation difference between Mast and OOS
J Horizontal distance between Mast and OOS
K Diagonal distance between Masts when four masts support the sphere
P Distance between Masts when four masts support the sphere
Q Distance between Masts when three masts support the sphere
The procedure for masts is as follws
a) Calculate the surge impedance, Zs (see Annex C.)
b) Calculate the critical stroke current, Is from Eq 5-2A
c) Calculate the strike distance, S (which will become the sphere radius) from Eq 5-1 B.
Calculate T as shown by the calculations that follows. T is the maximum horizontal
distance from the mast that an object at a height, A, is protected from a direct
stroke. A circle with radius, T, is the area of protection afforded by a single mast for
an object at height, A
d)
Calculate X, the maximum separation of two masts to prevent a side stroke. (It may help to
visualize a sphere resting on the ground that is then rolled over to just touch the two masts.
The bus is arranged so that it also just toches the surface of the sphere. By studying the various
views of the figure, it can be seen that this determines the maximum separation to prevent
side strokes.)
e)
f) Calculate P, the maximum separation of masts to prevent a vertical stroke.
g) Calculate Q, the maximum separation of three masts to prevent a vertical stroke.
With this information masts can be spotted in the substation; arcs can be drawn around them
and adjustments can be made for an optimal layout.
h)
69 kV Substation example -Protection by Mast
Zs = 300.00 ohm
BIL = 350.00 kV
Is = 2.2x BIL/ Zs = 2.57 kA Eq 5-2A
k = 1.20 for Mast
S= 26.25. k. Is0.65 = 58.13 feet Eq 5-1B
1 feet = 0.3048 Mtrs 17.72 Mtr Mtr = 0.3048 x feet
H = 18.29 Mtr Assumed
1Mtrs 3.28083 feet = 60.00 feet
A1 = 5.79 Mtr Bus height
19.00 feet
A2 = 4.27 Mtr Bus height
14.00 feet
C1= Sqrt (S2- (S-A1)2 = 13.10 Mtr
42.99 feet feet= 3.28083* Mtrs
C2= Sqrt (S2- (S-A2)2 = 11.53 Mtr
37.84 feet
Also C= S-T
T1 = S-C1 = 15.14 feet
= 4.62 Mtr
T2 = S-C2 = 20.29 feet
6.19 Mtr
These values are the maximum separation between the mast and protected bus for the two bus heights A.
Maximum distance between two masts for side stroke
W1= Sqrt (S2- (S-A1)2 = 42.99 feet
W2= Sqrt (S2- (S-A2)2 37.84 feet
Y = 3.00 feet
0.91 Mtrs
Z1=W1-Y = 39.99 feet
Z2=W2-Y = 34.84 feet
L1= Sqrt (S2- Z12 = 42.19 feet
L2= Sqrt (S - Z2
2 2
= 46.53 feet
X1= 2L1 = 84.38 feet
X2=2*L2 = 93.07 feet
These values are the maximum separation of two masts for protection of buses at two heights A.
Maximum distance between masts for vertical stroke sphere supported by four masts
D must be less than or equal to H-A for protection at height A.
D1=H-A1 = 41.00 feet
D2=H-A2 = 46.00 feet
E1= S-D1 = 17.13 feet
E2= S-D2 = 12.13 feet
J1= Sqrt (S2- E12 = 55.55 feet
J2= Sqrt (S2- E22 = 56.85 feet
K1=2J1 = 111.10 feet
K2=2J2 = 113.70 feet
P1=K1/1.414 = 78.57 feet
P2=K2/1.414 = 80.41 feet
These values are the maximum spacing of four masts for protection of buses at two heights A.
r the two bus heights A.
two heights A.
o heights A.
