DATA1.0SYSTEM DATADescriptionSymbolUnitValueRms value of
symmetrical fault currentIkA=25Fault clearing time for conductor
selectiontrsec=1Fault diversion factorSf=0.71.1SOIL DATASoil
Resistivity (Actual Value)W-m=16.3Soil Resistivity (Design value
for safety)rW-m=40Surface resistivityrsW-m=3000Thickness of Surface
layerhsm=0.11.2GRID CONDUCTOR DATAThe Conductor Selected is 40 mm
dia Mild steel RodMax allowable temperature of the
conductorTmoc=500Ambient temperatureTaoc=50Reference temperature
for material constantTroc=20Coefficient of resistivity at Tro C
(Mild Steel)ar=0.0045Resistivity of ground conductorrrm
W-cm=13.8Thermal capacity factorTCAPJ/cm3/oC=3.749Diameter of grid
conductor in mdm=0.04Spacing of grid conductor ChosenDm=4.4Depth of
burial of Grid conductorhm=0.6Duration of
shocktsSec=0.51.3SWITCHYARD DATA(Refer Annexure-2 for Earthgrid
layout) )Length of switchyard earthmatm=41Width of switchyard
earthmatm=21Area of substationASq.m=861Grid conductor periphery
lengthLpm=124Maximum distance along grid lengthLxm=41Maximum
distance along grid widthLym=211.4EARTH ELECTRODE DATADiameter of
Pipe/rod earth electrode in mDem=0.04Length of each rod conductor
in mLrm=3No.of Earth electrodes (Earth station)NNos=73Total length
of ground rodsLRm=234(Refer Annexure-1 for No. of Earth Electrodes)
&R&"Arial,Bold"
CALCULATION1.0SYSTEM
DATA2.0METHODOLOGY:DescriptionSymbolUnitValueRms value of
symmetrical fault currentIkA-402.1The Earthing Design Calculation
is done as per the recommendations given in IEEE Std. 80,2000Fault
clearing time for conductor selectiontrsec-11.1SOIL DATA2.2The
Corrosion Allowance is Considered for Mild Steel as per the
recommendations given in CBIP Manual Publication No.223Soil
Resistivity (Average)rW-m-90(Refer Annexure 2 for Soil resistivity
data)2.3The Following are considered for determination of grounding
conductor lengthSurface resistivityrsW-m-3000Thickness of Surface
layerhsm-0.15hTotal Length of Earth mat1.2GRID CONDUCTOR
DATAhLength of Auxiliary Earth mats for IsolatorsThe Conductor
Selected is 40 mm dia Mild steel RodhLength of Earth ElectrodesMax
allowable temperature of the conductorTmoc-500Ambient
temperatureTaoc-503.0CALCULATION:Reference temperature for material
constantTroc-20Coefficient of resistivity at 0o C (Mild Steel)a0
(1/K0)-0.004233.1Selection of cross section of grid conductor
(A)Coefficient of resistivity at Tro C (Mild Steel)ar-0.0039As per
IEEE 80-2000Resistivity of ground conductorrr-15The Crosssection of
Conductor is Given byThermal capacity
factorTCAPJ/cm3/oC-3.749Diameter of grid conductor in
mdm-0.04A=mm2Spacing of grid conductor ChosenDm-10(assumed)Depth of
burial of Grid conductorhm-0.6Duration of
shocktsSec-1.025A=mm23.749/10000 ln 202.22+5001x0.0045x13.8 .
202.22+50A=318. Sqmm1.3SWITCHYARD DATA(Refer Annexure-1 for
Earthgrid layout) )WhereLength of Rectangle - 1 (400 kV
Side)m-256K0=1 / ar - TrLength of Rectangle - 2 (220 kV
Side)m-260=202.22Length of Rectangle - 3 (Control Room
Side)m-1530Width of Rectangle - 1 (400 kV Side)m-82As per CBIP
"Manual on substation" publication number 223, corrosion allowance
isrecommended as per following ruleIn case of conductor to be laid
in soil having soil resistivity greater than 100W-m- No allowanceIn
case of conductor to be laid in soil having soil resistivity from
25 to 100W-m- 15% allowanceIn case of conductor to be laid in soil
having soil resistivity lower tha 25W-m-30% allowanceconsidering
soil resistivity for conductor sizing as 40 W-mA mm2=1.15 x A=1.15
x 317.97=366. SqmmFor 366 Sq.mm, Size of the Conductor Required
is=366 x 4Width of Rectangle - 2 (220 kV Side)m-1323.14Width of
Rectangle - 3 (Control Room Side)m-93.5=21.6 mmArea of
substationASq.m-69617.5Selected Conductor Size is=40 mm dia MS
RodHence safe.3.2Determination of grounding conductor length:The
Earth Mat spacing considered for safe Step & Touch
Potential=4.4 m x 4.4 mArea of SubstationA=861. SqmGrid conductor
periphery lengthLp=124 mMaximum distance along grid lengthLx=41
mMaximum distance along grid widthLy=21.0 mLength of each ground
rodLr=3 mTotal length of ground rodsLR=234 mSpacing of grid
conductor chosenD=4.4Estimation of Grid Conductor LengthRectangle -
(41m x 21m)The number of conductors parallel to the breadth=6The
number of conductors parallel to the length=11Length of Conductor
for Rectangle=477 mGrid conductor Combined Length (Lc) used in the
switchyardLC=477 mEffective length of the buried Conductor in
mLT=LC + LR=711 m3.3Determination of Maximum Grid current
(IG)IG=Df.Sf.IfWhereDf-Decrement factor for Tf=1Sf-Current division
factor=1If-Symmetrical Ground fault current=25 kAMaximum Grid
currentIG=1 x 1 x 25=25 kAShock duration time ts = 1
sects=sec0.53.3Calculation of Tolerable Touch voltage(Et) &
Step voltage(Es)k=From graph in IEEE - 80 corresponding to k =
-0.983 and hs = 0.15Cs=k=-0.9740.770The Surface Layer Derating
factor Cs is given byCs=Cs=Cs=0.694Tolerable Touch Voltage Et is
given byEt50==Et50=676 VTolerable Step Voltage Es is given
byEs50==Es50=2213 V3.4Determination of Grid resistance
(Rg)Rg=Rg=Rg=0.690 OhmsSince the Grid resistance Rg is Less than
1W,the design is safe.3.5Maximum Grid current (IG):IG=sf x I=0.7 x
25=17.50 kA3.6Calculation of Attainable Mesh voltage(Et) & Step
voltage(Es)3.6.1Determination of the effective No.of Parallel
Conductor (n)n=na x nb x nc x ndWherena== 2 x (477 / 124)=7.69nb==
(124 /(4 861))=1.03nc==For Square and Rectangular gridsnc=1=1nd=1
(For Square,Rectangular & L-shaped
grids)nd=1n=7.9213.6.2Determination of Ki:Ki==0.644 + (0.148 x
7.921)=1.823.6.3Determination of Km:Km=WhereKii=1Kii=1(For grid
with ground rods along the perimeter, or for grids with ground rods
in the gridcorner, as well as both along the perimeter and
throughout the grid area)Kh==1+(0.6/1)=1.265Where ho=1m (Grid
reference depth)Km=Km=0.453.6.4Determination of
Ks:Ks==Ks=0.43.6.5Determination of the effective buried Length of
conductor (Lm for Touch Voltage)For grids with Earth
electrodesLm==Lm=858 m3.6.6Determination of the effective buried
Length of conductor (Ls for Step Voltage):Ls=0.75Lc + 0.85LR for
grids with ground rods=(0.75 x 477) + (0.85 x 234)Ls=557
mAttainable Mesh voltage Em is given byEm== (40 X 0.45 X 1.82 X
17.5 X 1000) / 858.29Em=668 VAttainable Step voltage Es is given
byEs==(40 X 0.4 X 1.82 X 17.5 X 1000) / 557Es=915
V4.0CONCLUSION:a)Em vs Et50The Calculated Mesh Voltage of 668 V is
lower than the tolerable Touch Voltage 676 V.Hence the design is
safe.b)Es vs Es50The Calculated Step Voltage of 915 V is lower than
the tolerable Step Voltage 2213 V.Hence the design is safe.c)Grid
Resistance RgThe Calculated Grid Resistance of 0.69 Ohms is less
than 1 OhmsHENCE THE DESIGN IS SAFE
&L&"Times New Roman,Bold"&11
x 120x153
3
``
1.55 + 1.22
(672+782 )
``
515+3998+
(67x(78/5226)((0.7x5226) / (67x78))
Annex-1ANNEXURE - 1Estimation of No. of Earth
ElectrodesS.NODESCRIPTIONNO. OF EARTH
ELECTRODE1.1LA401.2CT&PT181.3ISMB POLE , T/F,ISO WITH
E/S15Total No. of Equipment Earth Electrodes73Total length for 3 m
long earth electrodes.219Grid corners & along periphery of 1 m
long rod15NO. OF EARTH ELECTRODES220 kv1line
1-4MBGRIDLA12CVT6CORNERS0CVT12COMMONLM4TOWER92461343400
kVBALL-2BALL-1ICT1ICT2MBLA37106CVT3636REAC242TRAFO615178652TOWER27CORNERS887130Total
Length of Earth Electrodes234
(
)
S
S
S
t
116
.
0
C
5
.
1
1000
r
+
S
SS
t
116.0C5.11000
+
+
+
861
20
0.6
1
1
1
61
8
20
1
770
1
40
861
20
0.61
1
1
61820
1
770
1
40
W
+
+
+
A
h
A
L
T
20
1
1
1
.
20
1
1
r
A
h
A
L
T
20
1
1
1
.20
11
(
)
S
S
S
t
116
.
0
C
6
1000
r
+
S
SS
t
116.0C61000
(
)
(
)
-
p
+
-
+
+
p
1
n
2
8
ln
.
K
K
d
4
h
Dd
8
h
2
D
hd
16
D
ln
2
1
h
ii
2
2
1n2
8
ln.
K
K
d4
h
Dd8
h2D
hd16
D
ln
2
1
h
ii
2
2
(
)
.
5
.
0
116
.
0
3000
695
.
0
6
1000
+
.5.0
116.03000695.061000
(
)
(
)
-
+
-
+
1
8.559)
x
(2
x
3.14
8
.ln
1.265
1
0.04
x
4
1
0.04
x
5
x
8
1
x
5x2
0.04
x
1
x
16
5
ln
3.14
x
2
1
2
2
18.559) x (2 x 3.14
8
.ln
1.265
1
0.04 x 4
1
0.04 x 5 x 8
1 x 5x2
0.04 x 1 x 16
5
ln
3.14 x 2
1
2
2
+
+
-
a
T
o
K
m
T
o
K
ln
r
.
r
.
r
t
4
TCAP.10
I
a
T
o
K
m
T
o
K
ln
r
.
r
.
r
t
4
TCAP.10
I
-
-
+
+
+
2
559
.
8
5
.
0
1
5
1
1
5
1
1
2
1
14
.
3
1
2559.8
5.01
5
1
15
1
12
1
14.3
1
(
)
0.09
0.1
2
3000
50
-
1
0.09
-
1
+
0.090.1 2
3000
50
-1 0.09
-1
(
)
5
.
0
116
.
0
3000
695
.
0
5
.
1
1000
+
5.0
116.03000695.05.11000
m
L
G
l
i
K
m
K
m
L
G
l
i
K
m
K
s
L
G
l
i
K
s
K
s
L
G
l
i
K
s
K
(
)
n
0.148
0.644
+
n0.1480.644
-
-
+
+
+
p
2
n
5
.
0
1
D
1
h
D
1
h
2
1
1
2n
5.01
D
1
hD
1
h2
11
R
L
2
y
L
2
x
L
r
L
22
.
1
55
.
1
C
L
+
+
+
R
L
2
y
L
2
x
L
r
L
22.155.1
C
L
0.09
s
h
2
s
-
1
0.09
-
1
+
0.09
s
h 2
s
-1 0.09
-1
Y
L
X
L
0.7A
A
Y
L
X
L
Y
L
X
L
0.7A
A
Y
L
X
L
MBD000FD35E.unknown
MBD00119F60.unknown
MBD001536CA.unknown
MBD001690B3.unknown
MBD0016D007.unknown
MBD001586CD.unknown
MBD00133500.unknown
MBD0015126B.unknown
MBD00121EFC.unknown
MBD00120EF9.unknown
MBD001011D9.unknown
MBD00113774.unknown
MBD0010038E.unknown
MBD0004FB2F.unknown
MBD0006EEAF.unknown
MBD000ACB4C.unknown
MBD000B1AC1.unknown
MBD0009CCD4.unknown
MBD000A116F.unknown
MBD00096E1C.unknown
MBD00054103.unknown
MBD0004D77D.unknown
MBD0004F1B9.unknown
MBD00048FBD.unknown