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CALCULATI ONS ARE BASED ON I EEE Std 80- 2000
1.0.0 PURPOSE AND SCOPE
2.0.0 DESIGN INPUT
2.1.0 SYSTEM PARAMETERS
2.1.1 System Voltage = 33/11 kV
2.1.2 System Grounding = Solid
2.1.3 Maximum Fault Current I
f = 25000 A (As per OPTCL spec)
2.1.4 Duration of Fault Current t
s = 1 s (As per OPTCL spec)
2.2.0 ENVIRONMENTAL PARAMETERS
2.2.1 Soil Resistivity r =
2.2.2 Top Layer resistivity (Crushed rock) r
s
=
2.2.3 Length of switchyard Lx = 14 m (As per Layout)
2.2.4 Breadth of Switchyard Bx = 14 m (As per Layout)
2.3.0 OTHER DETAILS
2.3.1 Conductor diameter d = 0.031 m (31mm dia MS Rod)
~=
2.3.2 Conductor Area = (75 x 10 mm MS Flat)
2.3.3 Depth of burial (minimum) h = 0.6 m (As per OPTCL spec)
2.3.4 Depth of Top Layer h
s = 0.1 m (As per OPTCL spec)
2.3.5 No of electrodes
Transformer = 2 (2# Neutral)
Lightning Arrester = 3 (3 No. x 1)
PT/CVT = 3 (3 No. x 1)
Tower with Peak = 3 (3 No. x 1)
Outer Fence = 15 (4 No. x 1)
2.3.5 No of electrodes n
R = 26
2.3.6 Length of electrode L
R = 2 m
2.3.7 Diameter of electrode 2b = 25 mm
SWITCHGEAR AND CONTROL PVT. LTD.
PROJECT
The purpose of the document is to design the Earthmat for the 33/11kV AIS Substation. The important aspect is that the design should achieve
"Tolerable Touch and Step potential " with the Earth grid resistance less than 1 ohm.
33/11kV Substation Earthing Calculation (Infocity) TITLE
Substation Package-S1 for 33/11kV Infocity Susbstation (New) under OPTCL.
3000 ohm.m
750 Sq.mm
(As per soil resistivity
report)
(As per OPTCL spec)
9.00 ohm.m
3 of 9
PREPARED CHECKED APPROVED DATE
DJS DJS DJS 15/07/2014
SWITCHGEAR AND CONTROL PVT. LTD.
PROJECT
33/11kV Substation Earthing Calculation (Infocity) TITLE
Substation Package-S1 for 33/11kV Infocity Susbstation (New) under OPTCL.
3.0.0 CALCULATIONS
3.1.0 CALCULATIONS - PRELIMINARY
L
x
3.1.1 Area of Switchyard A
g = Lx*Bx
= 14 x 14
B
x
=
3.1.2 Spacing of Conductors D
s = 2.0 m
3.1.3 No: of grids in X direction N
x = (Lx/D
s
)+1
= 014/2+1
= 8
3.1.4 No: of grids in Y direction N
y = (Bx/D
s
)+1
= 014/2+1
= 8
3.1.5 Total length of conductors L
c = Nx * Bx + Ny * Lx
= 224
3.1.6 Total length of electrodes L
r = 26 x 2
= 52 m
3.1.7 Peripheral length of Grid L
p = 2 x (Lx + Bx)
= 2 x ( 14 + 14 )
= 56 m
3.1.8 Maximum distance between two points on grid D
m = (Lx
2
+ Bx
2
)
= ( 14 + 14 )
= 20 m
3.1.5 Geometric factor n
n
a = 2.L
c
/L
p (Eqn. 85 in page 93)
= 2 x 224 / 56
= 8.0
196 Sq.m
4 of 9
PREPARED CHECKED APPROVED DATE
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SWITCHGEAR AND CONTROL PVT. LTD.
PROJECT
33/11kV Substation Earthing Calculation (Infocity) TITLE
Substation Package-S1 for 33/11kV Infocity Susbstation (New) under OPTCL.
n
b = 1.0 (Eqn. 86 in page 93)
= 1.0
n
c = 1 (Eqn. 87 in page 93)
= 1.0
n
d = 1 (Eqn. 88 in page 93)
= 1.0
n = n
a
. n
b
. n
c
. n
d (Eqn. 84 in page 93)
= 8.0 x 1.0 x 1.0 x 1.0
= 8.0
3.2.0 TOLERABLE TOUCH POTENTIAL
E
touch = ( 1000 + 1.5 C
s
r
s
) * 0.116 / t
s (Eqn. 32 in page 27)
C
s = 1 - 0.09 ( 1- r/r
s
) / ( 2 h
s
+ 0.09) (Eqn. 27 in page 23)
= 1-0.09*(1-009/3000)/(2*0.10+0.09)
= 0.6906
E
touch = ( 1000 + 1.5 C
s
r
s
) * 0.116 / t
s
= (1000 + 1.5 * 0.6906 * 3000 ) * 0.116 / 1.0
= 476.49 V
3.3.0 TOLERABLE STEP POTENTIAL
E
step = ( 1000 + 6.0 *C
s
* r
s
) * 0.116 / t
s (Eqn. 30 in page 27)
= (1000 + 6.0 * 0.6906 * 3000 ) * 0.116 / 1.0
= 1557.94 V
5 of 9
PREPARED CHECKED APPROVED DATE
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SWITCHGEAR AND CONTROL PVT. LTD.
PROJECT
33/11kV Substation Earthing Calculation (Infocity) TITLE
Substation Package-S1 for 33/11kV Infocity Susbstation (New) under OPTCL.
3.4.0 TOUCH POTENTIAL ATTAINABLE
E
mesh = r K
m
K
i
I / L
m (Eqn. 80 in page 91)
L
m = L
c
+ L
r (Eqn.90 in page 94)
= 224+52
= 276 m
K
m = 1 . ln D
s
2
+ (D
s
+ 2h )
2
- h + K
ii
ln 8
2p 16 hd 8 D
s
d 4d K
h
p (2n-1)
(Eqn. 81 in page 93)
1.00 for grids with ground rods in perimeter / all along the grid
K
ii =
1/(2n)
2/n
for grids with no ground round rods or grids with only a few ground rods
Here we consider grids with ground rods in perimeter
h
o = 1.00 m ( reference depth of grid)
K
h = (1+h/h
o
) (Eqn. 83 in page 93)
= ( 1 + 0.6 / 1.0 )
= 1.2649
K
m = 1 Ln 2.0 + (2.0 + 2*0.60 ) -0.60 +1 8
2 8*2.0*0.03 4*0.03 1.26 9 (2*8.0-1)
= 0.3147
K
i = 0.644 + 0.148 * n (Eqn 89 in page 94)
= 0.644 + 0.148 *8.0
= 1.8280
E
mesh = r K
m
K
i
I / L
m (Eqn 80 in page 91)
= 009 * 0.3147 * 1.8280 * 25000 / 276.00
= V
16*0.6*0.03
468.91
6 of 9
PREPARED CHECKED APPROVED DATE
DJS DJS DJS 15/07/2014
SWITCHGEAR AND CONTROL PVT. LTD.
PROJECT
33/11kV Substation Earthing Calculation (Infocity) TITLE
Substation Package-S1 for 33/11kV Infocity Susbstation (New) under OPTCL.
3.5.0 STEP POTENTIAL ATTAINABLE
E
step = r K
s
K
i
I / L
s (Eqn 92 in page 94)
L
s = 0.75 L
c
+ 0.85 L
r (Eqn 93 in page 94)
= 0.75 *224.00+ 0.85* 52.00
=
n = 8
K
s = 1* 1 + 1 + 1 (1- 0.5
(n-2)
) (Eqn. 94 in page 94)
p 2h D
s
+ h D
s
= ( 1 / 3.14 ) * { 1 / (2 * 0.6 ) + 1 / ( 2.0 + 0.6 ) + 1 / 2.0 * ( 1 - 0.5^(8 - 2 ) }
= 0.5444
E
step = r K
s
K
i
I / L
s
= ( 9.00 * 0.5444 * 1.8280 * 25000 ) / 212.2
= 1055.10 V
4.0.0 CALCULATION OF GROUNDING RESISTANCE
4.1.0 RESISTANCE OF GROUND RODS
R
e = r ln (4 L
R
/b ) - 1 + 2 K
1
L
R
(n
R
-1)
2
2p n
R
L
R Ag (Eqn. 55 in page 66)
K
1 = 1.41-0.04x for H = 0 (Eqn. 55 in page 66)
1.20-0.05x for H = 1/10 A
g
1.13-0.05x for H = 1/6 A
g (Fig 25(a) in page 67)
H = 0.6 m
K
1 = 1.41-0.04x
= 1.41-0.04(L/B)
= 1.41-0.04*14.00/14.00
= 1.3700
R
e = 9.00 / (2 * 26 * 3.14 * 2.0 ) * [ ln (4 * 2.0 / 0.0309) - 1 + 2 * 1.370 *
(2 / sqrt(196)) * (sqrt (26)-1)^2 ]
= 0.3258 ohm
212.2 m
7 of 9
PREPARED CHECKED APPROVED DATE
DJS DJS DJS 15/07/2014
SWITCHGEAR AND CONTROL PVT. LTD.
PROJECT
33/11kV Substation Earthing Calculation (Infocity) TITLE
Substation Package-S1 for 33/11kV Infocity Susbstation (New) under OPTCL.
4.2.0 RESISTANCE OF GROUND GRID
R
s = r ln (2 L
c
/ h') + K
1
L
c
- K
2
(Eqn. 54 in page 66)
p L
c
A
g
K
2 = 5.50+0.15x for H = 0
4.68+0.10x for H = 1/10 A
g
4.40-0.05x for H = 1/6 A
g (Fig 25(b) in page 67)
K
2 = 4.68+0.10x
= 4.68+0.10(L/B)
= 4.68+0.10*14.00/14.00
= 4.78
h' = (d . h) (Page 66)
= (0.03 * 0.6)
= 0.136 m
R
s = ( 9.00 / (3.14 * 224)) * [ ln ( 2 * 224 / 0.1362 ) + 1.370 *
( 224 / sqrt (196.00 )) - 4.780 ]
= 0.3227 Ohms
4.3.0 MUTUAL RESISTANCE BETWEEN GROUND GRID AND GROUND RODS
R
se = r ln (2 L
c
/ L
r
) + K
1
L
c
- K
2
+1
p L
c
A
g (Eqn 56 in page 66)
= 9.00 / (3.14 * 224) * [ln (2 * 224 / 2.0 ) + 1.370 *
( 224.00 / sqrt (196.00 )) - 4.780 + 1]
= 0.3011 ohms
4.4.0 COMBINED RESISTANCE OF GROUND GRID AND GROUND RODS
R
g = R
e
. R
s
- R
se
2
(Eqn. 53 in page 66)
R
e
+ R
s
- 2R
se
= 0.3258 * 0.3227 - 0.3011
0.3258 * 0.3227 - 2 * 0.3011
= 0.3126 Ohm
THIS IS LESS THAN THE RECOMMENDED VALUE OF LESS THAN 1.0 OHM
8 of 9
PREPARED CHECKED APPROVED DATE
DJS DJS DJS 15/07/2014
SWITCHGEAR AND CONTROL PVT. LTD.
PROJECT
33/11kV Substation Earthing Calculation (Infocity) TITLE
Substation Package-S1 for 33/11kV Infocity Susbstation (New) under OPTCL.
5.0.0 RESULTS
5.1.1 Permissible Spacing of conductors D
s = 2 m
5.1.2 Total length of mainmat conductors L
c = 224 m
5.1.3 Attainable step potential = 1055.10 V
5.1.4 Attainable touch potential = 468.91 V
5.1.5 Tolerable step potential = 1557.94 V
5.1.6 Tolerable touch potential = 476.49 V
Thus Attainable Emesh is less than Tolerable Etouch and
Attainable Estep is less than Tolerable Estep
5.1.7 Combined resistance of Grid & ground rods = 0.3126 ohm < 1.0 ohm
6 CONCLUSION
Permissible Spacing of conductors for the present layout is 2m. Hence, Earthmat layout shall be prepared with spacing
lesser or equal to 2m.
