Calculation sheet Station Grounding Grid
Rev. 1
i
CONTENTS 1.0 GENERAL 1

2.0 REFERENCES 1

3.0 CALCULATION 1
3.1 General 1
3.2 Conductor Size Calculation 1
3.3 Check of touch voltage and step voltage 2
3.4 Initial Design Parameters 3
3.5 Grid Resistance 3
3.6 Maximum Grid Current 4
3.7 Ground Potential Rise 4
3.8 Calculation of Mesh & Step Voltages 4
3.9 Verification on suitability of conductor length (L) 6

4.0 CONCLUSION 6
Calculation sheet Station Grounding Grid

1.0 GENERAL

This calculation is prepared to design the station earthing with IEEE Standard 80 to achieve a
low resistance earth path and to ensure safe touch and step voltages for the 240MW Coal Fired
Power Plant.
Area A = 150×200 + 50×100 = 35,000 m
23.1.4 Depth of ground grid conductors in m: h = 0.5 m

3.1.5 Ground connection method : Thermal welding type 3.2 Conductor Size Calculation

200
m

100
m
150
m

250
m

Calculation sheet Station Grounding Grid


⎦
⎥
αρ
ln

(From IEEE 80, Eq. 33, page 67)

where

A
mm
2
= conductor cross section in mm
2
I = rms max. Ground fault current in kA, : 20.000kA
T
m
= maximum allowable temperature in °C (250°C).
T
a
= ambient temperature in °C (31°C)
T
r
= reference temperature for material constants in °C (20°C)
α
0
= thermal coefficient of resistivity at 0 °C
α
r
= thermal coefficient of resistivity at reference temperature T
⎥
⎦
⎤
⎢
⎣
⎡
⎟
⎠
⎞
⎜
⎝
⎛
+
−
+
×××
⋅=
31234
31250
1ln
422.3
107241.100393.01
20
4
2
mm
A
≈ 114.7 mm


where

ρ
s
= surface material resistivity in Ω-m, 2094.8
ρ = soil resistivity in Ω-m, 23.91 3.3.1 Touch Voltage Criterion

Assuming the person’s weight to be at least 50 kg, the tolerable step and touch voltages shall
be as follows: ()()
[
]
VtKhCE
sssstouch
7.365116.0,5.11000
50
=+=
ρ
(From IEEE 80, Eq. 26, page 46)
Calculation sheet Station Grounding Grid C
s

ρ
s
the resistivity of the surface material in Ω-m, 2094.8

t
s
duration of shock current in s, 1.0 3.4 Initial Design Parameters

3.4.1 Spacing

a) Horizontal Spacing: 20 m
b) Vertical Spacing: 20 m

3.4.2 Total length of buried conductor (L)

Main Conductors : ( 250(m) × 6(EA) ) + ( 200(m) × 3(EA) ) + ( 150(m) × 11(EA) ) + ( 100(m)
× 3(EA) = 4,050 [m]

The total buried length is 4,050 m.

3.4.3 Burial depth of ground grid conductors in m (h): 0.5 [m] 3.5 Grid Resistance

⎥
⎥

1
1
20
11
ρ
(From IEEE 80, Eq. 40, page 82)

where
R
g
= station ground resistance in Ω
ρ = average earth resistivity in Ω-m
A = the area occupied by the ground grid in m
2

L = total buried length of conductors in m
h = depth of the grid in m, 0.5

Calculation sheet Station Grounding Grid

][063.0
35000
20
5.01
1
1
3500020
1
4050
1

requirements. (from IEEE 80, Section 12.1, page 81) 3.6 Maximum Ground Grid Current
ICDI
ISI
Gpfg
gff
=
=
(from IEEE 80, Eq. 53&54, page 91)

where
I
G
= maximum ground grid current in A
D
f
= decrement factor; 1.0
S
f
= current division factor relating the magnitude of fault current to that of its portion
flowing between the grounding grid and surrounding earth, 0.5
I
f
= rms value of symmetrical fault current in A, 40kA
C

VRIGRP
gG
1260063.020000 =×=×= exceeds 365.7 [V], determined in Section 3.3.1 as the safe value of E
touch50
. Therefore, further
design evaluations are necessary. The calculation which accounts for geometrical factors, a
corrective factor, the soil resistivity, and the average current density per unit of buried
conductor can be found hereinafter in Section 3.8. 3.8 Calculation of Mesh & Step Voltages

3.8.1 Mesh Voltage (E
m
)

E
m
= ρK
m
K
i
I
G
/L (from IEEE 80, Eq. 66, page 113)

a) K

+
−
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
1
216
2
84
8
21
2
2
ππ
ln ln
()

(from IEEE 80, Eq. 68, page 113)

where 5744.0
)2(
1

nnn
Number of parallel conductors in one direction 0142.0=d
Diameter of grid conductors in m

Thus, we get (
)
1488.1
122.112
8
ln
2251
5744.0
0142.04
5.0
0142.0208
5.0220
014205.016
20
ln
2
1
2
2
=
b) K
i
, Corrected factor for grid geometry, simplified method

K
i
= 0.656 + 0.172×n =0.656 + 0.172× 11.22 = 2.5867 (from IEEE 80, Eq. 69, page 114) c) Mesh Voltage, E
m E
m
= ρK
m
K
i
I
G
/L = 23.91×1.1488× 2.5867 × 20000 / 4050 = 350.9 < 365.7[V] (E
touch50
) 3.8.2 Step Voltage (E
s

⎥
−
11
2
11
105
2
π
.
(from IEEE 80, Eq. 74, page 115)

Calculation sheet Station Grounding Grid

where 20=D Spacing between parallel conductors in m 14)14,9max(),max(
=
==
BA
nnn
Number of parallel conductors in one direction

h=0.5 Depth of ground grid conductors in m

Thus, we get


, Corrected factor for grid geometry, simplified method

K
i
= 0.656 + 0.172n =0.656 + 0.172×14 =3.064 (from IEEE 80, Eq. 69, page 114)

c) Step Voltage, E
s E
s
= ρK
s
K
i
I
G
/L=23.91 × 0.3498 × 3.064 × 20000 / 4050 = 126.5 < 1114.7[V] (E
step50
) 3.9 Verification on suitability of conductor length (L)

The suitability of conductor length buried can be verified with the following equation. ()()
][886,3

*E
step50

Area Size Length resistance *E
m
*E
s

Chile
35000 120
20×20
4050 0.063 365.7[V] 1114.7[V]
TPP [m
2
] mm
2

[m×m]
[m]
[Ω]
350.9[V] 126.5[V]

Note :
1. *E
touch50
: Tolerable Touch potential Voltage
2. *E
step50
: Tolerable Step potential Voltage
3. *E