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DESCRIPTION
EARTHMAT DESIGN CALCULATION
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Description Symbol Unit Value
1.0 SYSTEM DATA
Rms value of symmetrical fault current I kA = 31.5 (132kV Side)
Fault clearing time for conductor selection tr sec = 1 (132kV Side)
1.1 SOIL DATA
Soil Resistivity (Average) ρ Ω-m = 33.36
Surface resistivity ρs Ω-m = 3000
Thickness of Surface layer hs m = 0.1
1.2 GRID CONDUCTOR DATA
The Conductor Selected is 25 mm dia MS Rod
Max allowable temperature of the conductor Tmoc = 620
Ambient temperature Taoc = 50
Reference temperature for material constant Troc = 20
Coefficient of resistivity at Tro C (Mild Steel) αr = 0.00423
Resistivity of ground conductor ρr µ Ω-cm = 15
Thermal capacity factor TCAP J/cm3/oC = 3.749
Diameter of grid conductor in m d m = 0.025
Spacing of grid conductor Chosen D m = 9
Depth of burial of Grid conductor h m = 0.6
Duration of shock ts Sec = 0.5
1.3 SWITCHYARD DATA
(Refer Annexure-1 for Earthgrid layout )
Length of Rectangle -132/33 kV Side m = 73
Width of Rectangle -132/33 kV Side m = 101.5
Area of substation A Sq.m = 7409.5
Grid conductor periphery length Lp m = 349.0
Maximum distance along grid length Lx m = 73.00
EARTHMAT DESIGN
PROJECT: 132/33kV JANDAHA NEW SUBSTATION SHYAMA POWER (l) LTD
EARTHMAT DESIGN CALCULATION Page 2 of 11 Doc.No.:SPL-BSTS-JANDAHA-EARTHMAT.CAL-005
Maximum distance along grid width Ly m = 101.5
1.4 EARTH ELECTRODE DATA
Diameter of Pipe/rod earth electrode in m De m = 0.04
Length of each rod conductor in m Lr m = 3
No.of Earth electrodes N Nos = 54
Total length of ground rods LR m = 162
(Refer Annexure-1 for No. of Earth Electrodes)
2.0 METHODOLOGY:
2.1 The Earthing Design Calculation is done as per the recommendations given in IEEE Std. 80,2000
2.2Manual on Earthing.
2.3 The Following are considered for determination of grounding conductor lengthTotal length of EarthmatLength of Earth electrodes
The Corrosion Allowance is considered for Mild Steel as per the recommendations given in CBIP
PROJECT: 132/33kV JANDAHA NEW SUBSTATION SHYAMA POWER (l) LTD
EARTHMAT DESIGN CALCULATION Page 3 of 11 Doc.No.:SPL-BSTS-JANDAHA-EARTHMAT.CAL-005
3.0 CALCULATION:
3.1
The Cross-section of Conductor is given by
A = mm2
31.5 mm2
A = 3.749 x 10-4 x ln 216.41 + 6201 x 0.00423x 15 216.41 + 50
A = 383 Sq.mm
Where K0 = 1 / ar - tr
K0 = 216.41
Calculated Conductor Size =
As per CBIP Manual on earthing, a corrosion allowance of 15% is considered = 383 x 1.15
440 Sq.mm
Selection of cross section of grid conductor (A)
⎟⎟⎠
⎞⎜⎜⎝
⎛
+
+⎟⎟
⎠
⎞
⎜⎜
⎝
⎛ −
aToKmToK
lnr.ρr.αrt
4TCAP.10
I
For 383 Sq.mm, Size of Conductor Required =
=
Selected Conductor Size = 25mm dia MS Rod
3.2 Determination of grounding conductor length:
The Earth Mat spacing considered for safe Step & Touch Potential = 9 m x 9 m
Area of Substation earthing A = 7410 Sq.m
Grid conductor periphery length Lp = 349.0 m
Maximum distance along grid length Lx = 73.00 m
Maximum distance along grid width Ly = 101.5 m
Length of each ground rod Lr = 3 m
Total length of ground rods LR = 162 m (Refer Annexure - 1)
Spacing of grid conductor chosen D = 9.0 m
Hence safe.
23.7 mm
14.34 440×
PROJECT: 132/33kV JANDAHA NEW SUBSTATION SHYAMA POWER (l) LTD
EARTHMAT DESIGN CALCULATION Page 4 of 11 Doc.No.:SPL-BSTS-JANDAHA-EARTHMAT.CAL-005
Estimation of Grid Conductor Length
Rectangle (73m x 101.5m)
The number of conductors parallel to the length = 12
The number of conductors parallel to the width = 9
Length of Conductor for Rectangle
Grid conductor Combined Length (Lc) used in the switchyard LC = 1790 m
Effective length of the buried Conductor in m LT = LC + LR
= 1952 m
3.3 Determination of Maximum Grid current (IG)
IG = Df.Sf.If
Where
Df - Decrement factor for Tf = 1Sf - Current division factor = 0.73If - Symmetrical Ground fault current = 31.5 kA
No of 132kV Transmission line shield wires = 4 (As per Layout)
No of 33kV Transmission line shield wires = 0
The total length of the grid conductor along the Length & Width in the switchyard
1790 m=
No of 33kV Transmission line shield wires = 0
Sf = 0.73 (Refer Table C-13 of IEEE-80-2000)
Maximum Grid current
IG = 1 x 0.73 x 31.5= 23.00 kA
Shock duration time ts = 0.5 sec
3.4 Calculation of Tolerable Touch voltage(Et) & Step voltage(Es)
Cs =
Cs = 1 - 0.09(1- (33.36/3000))
2 x 0.1 + 0.09
Cs = 0.693
0.09sh 2sρρ-1 0.09
-1 +
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
∴
PROJECT: 132/33kV JANDAHA NEW SUBSTATION SHYAMA POWER (l) LTD
EARTHMAT DESIGN CALCULATION Page 5 of 11 Doc.No.:SPL-BSTS-JANDAHA-EARTHMAT.CAL-005
Tolerable Touch Voltage Et is given by
Et50 =
= (1000+1.5 x 0.693 x 3000) x 0.116 0.5
Et50 = 676 V
Tolerable Step Voltage Es is given by
Es50 =
= (1000+6 x 0.693 x 3000) x 0.116
0.5
Es50 = 2210 V
3.5 Determination of Grid resistance (Rg)
Rg =
Rg = 33.36 1 + 1 1 + 1
( )S
SS
t116.0C61000 ×ρ×+
( )S
SS
tC 116.05.11000 ××+ ρ
Ω
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛
+
++ρ
A20h1
11A.20
1L1
T
g
1952 20 x 7410 1+0.6 20 Ohm 7410
Rg = 0.188 Ohms
Since the Grid resistance Rg is Less than 1Ω, the design is safe.
3.6 Determination of Ground Potential Rise (GPR)
GPR = IG x Rg
= 23 x 0.188= 4324 V
3.7 Calculation of Attainable Mesh voltage(Et) & Step voltage(Es)
3.7.1 Determination of the effective No.of Parallel Conductor (n)
n = na x nb x nc x nd
Where
na =
= 2 x (1789.5 / 349)
= 10.26
PLCL
2
PROJECT: 132/33kV JANDAHA NEW SUBSTATION SHYAMA POWER (l) LTD
EARTHMAT DESIGN CALCULATION Page 6 of 11 Doc.No.:SPL-BSTS-JANDAHA-EARTHMAT.CAL-005
nb =
= 349 4 x 7410
= 1.01
nc =
= 73 x 101.5 ( 0.7 x 7410)/(73 x101.5)
7410
= 1
nd = 1 (For Square,Rectangular & L-shaped grids)
nd = 1
n = 10
3.7.2 Determination of Ki:
Ki =
= 0.644 + (0.148 x 10.00)
⎟⎟⎠
⎞⎜⎜⎝
⎛
A4PL
YLXL0.7A
AYL
XL×
⎟⎟⎠
⎞⎜⎜⎝
⎛×
( )n0.1480.644 ×+
∴
( )
= 2.18
3.7.3 Determination of Km:
Km =
WhereKii = 1 (For a grid with earth electrodes along the perimeter or for grids with electrodes in
grid corners as well as both along the perimeter and througout the grid area)
Kh =
= 1.265
Where ho=1m (Grid reference depth)
Km = 1 ln 92 + (9 + 2 x 0.6)2 - 0.6 + 1 ln 8
2x 3.14 16 x 0.6 x 0.04 8 x 9 x 0.04 4 x 0.04 1.265 3.14 x (2 x (18.00-1))
Km = 0.692
⎟⎠⎞
⎜⎝⎛ +
hoh1
∴
( )( ) ⎥
⎥⎦
⎤
⎢⎢⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛−π
+⎟⎟⎠
⎞⎜⎜⎝
⎛−
++
π 1n28ln.
KK
d4h
Dd8h2D
hd16Dln
21
h
ii22
PROJECT: 132/33kV JANDAHA NEW SUBSTATION SHYAMA POWER (l) LTD
EARTHMAT DESIGN CALCULATION Page 7 of 11 Doc.No.:SPL-BSTS-JANDAHA-EARTHMAT.CAL-005
3.7.4 Determination of Ks:
Ks =
= 1 1 + 1 + 1 1- 0.5(11-2)
3.14 2 x 0.6 9 +0.6 9
Ks = 0.334
3.7.5 Determination of the effective buried Length of conductor (Lm for Touch Voltage)
For grids with Earth electrodes
Lm =
= 1790 + 1.55 + 1.22 3 x 162 732 + 101.52
Lm = 2045 m
3.7.6 Determination of the effective buried Length of conductor (Ls for Step Voltage):
Ls = 0.75Lc + 0.85LR for grids with or without ground rods
⎥⎦⎤
⎢⎣⎡
⎟⎠⎞⎜
⎝⎛ −−+
++
π2n5.01
D1
hD1
h211
RL
yLxL
rL..CL ×
⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜
⎝
⎛
⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜
⎝
⎛
⎟⎠⎞⎜
⎝⎛ +
++22
221551
= (0.75 x 1790) + ( 0.85 x 162)
Ls = 1480 m
Attainable Mesh voltage Em is given by
Em =
= ( 33.36 x 0.692 x 2.18 x 23000) / 2045
Em = 566 V
Attainable Step voltage Es is given by
Es =
= ( 33.36x 0.334 x 2.18 x 23000) / 1480
Es = 377 V
4.0 CONCLUSION:a) Em vs Et50
The Calculated Mesh Voltage of 566 V is lower than the tolerable Touch Voltage 676 V.b) Es vs Es50
The Calculated Step Voltage of 377 V is lower than the tolerable Step Voltage 2210 V.
c) Grid Resistance Rg
The Calculated Grid Resistance of 0.188 Ohms is less than 1 OhmHence the design is safe.
mLGliKmK ×××ρ
sLGliKsKρ ×××
PROJECT: 132/33kV JANDAHA NEW SUBSTATION SHYAMA POWER (l) LTD
EARTHMAT DESIGN CALCULATION Page 8 of 11 Doc.No.:SPL-BSTS-JANDAHA-EARTHMAT.CAL-005
Estimation of No. of Earth Electrodes for 132/33kV Jandaha New S/S
S.NO DESCRIPTION NO. OF EARTH ELECTRODES
1 132kV SWITCHYARD
1.1 SA 12
1.2 CVT 9
1.3 TOWERS WITH PEAK 4
1.4 TRANSFORMER 8
2 33kV SWITCHYARD
2.1 SA 6
2.2 TOWERS WITH PEAK 9
3 LIGHTNING MASTS 2
4 GRID CORNERS 4
54
Length of Each Electrode 3 m
Total Length of Earth Electrodes 162 m
Total No. of Earth Electrodes
ANNEXURE - 1
PROJECT: 132/33kV JANDAHA NEW SUBSTATION SHYAMA POWER (l) LTD
EARTHMAT DESIGN CALCULATION Page 9 of 11 Doc.No.:SPL-BSTS-JANDAHA-EARTHMAT.CAL-005
PROJECT: 132/33kV JANDAHA NEW SUBSTATION SHYAMA POWER (l) LTD
EARTHMAT DESIGN CALCULATION Page 10 of 11 Doc.No.:SPL-BSTS-JANDAHA-EARTHMAT.CAL-005
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