Page 755 - Industrial Power Engineering and Applications Handbook

Grounding practices 221715
22.14.4 Estimating the value of ground
conductor length (15)
Having determined the safe touch voltage E, (equation
(22.8)) the maximum mesh voltage E,, (actual) (equation
(22.18)) should be equal to or less than this voltage.
Thus by equating these two we can estimate the likely
length L of the ground conductors. Considering this for
an average human body of 70 kg
E, (actual) 5 E,oo,
P Km K, I,, 0.157 hyard
or 5 (1.5 C, pI + 1000) x T
L \t
-
P Km K, I,, 1, t
or L > (22.19)
(ISC, p, + 1000) x 0.157
This is the parameter that will help to decide the size and
type of the grounding grid and design of the mesh.
Since there are too many variables and parameters
related to a grounding station, the following practical
example illustrates a step-by-step procedure to design a
grounding station.

Example 22.9
To design a grounding grid, consider a power generating
station as shown in Figure 22.19, transmitting power at
400 kV through its own switchyard to another switchyard
remotely located.

Size of grounding conductor
Consider GI for grounding, and the same parameters of Section
22.14.1, leading to a minimum size of grounding conductor
as 80 Nmm', based on IS 3043. Fault level for a 400 kV
power station as in Table 13.10 is 40 kA (envisaging no
further rise in the fault level, IC = Ig)

grid
= 500 rnrn'
Figure 22.19 General layout of a power plant, station grounding
If we consider circular conductors then
grid, and switchyard grounding grid etc
= 500
4
or d = 25.24 rnrn
:. / - 3 x 0.433
For a power station, assuming a corrosion factor for a lifespan g' - (16 + 14.5) x 0.9
of 40 years as 0.12 mm/year then
(considering the lower side of Zp, to be on the safe side)
Corrosion depth = 2 x 4.8 = 9.6 rnm
= 4.73 kA
:. minimum d = 25.24 + 9.6
and /g2 = 40 - 4.73
= 34.84
= 35.27 kA as illustrated in Figure 22.20
say, 35 mm
where
/g, = fault current sharing by the power plant grounding
Sharing of ground fault current grid
/g2 = fault current sharing by the switchyard grounding
Rated current of the power plant grid (transmission system)
/g = total fault current, considered to be 40 kA for a
I,= 3x- 300 400 kV system as in Table 13.10.
13 x 400
For ease of understanding, the rest of the working is shown
= 3 x 0.433 kA in the form of Table 22.6.