EARTHMAT DESIGN
Description Symbol Unit Value
1.0 SYSTEM DATA
Rms value of symmetrical fault current I kA = 40 Fault clearing time for conductor selection tr sec = 1 1.1 SOIL DATA
Soil Resistivity (Average) ρ Ω-m = 75.6109
(Refer Annexure 2 for Soil resistivity data)
Surface resistivity ρs Ω-m = 3000
Thickness of Surface layer hs m = 0.1
1.2 GRID CONDUCTOR DATA
The Conductor Selected is 40 mm dia Mild steel Rod
Max allowable temperature of the conductor Τm oc = 620
Ambient temperature Τa
o
c = 50
Reference temperature for material constant Τr
o
c = 20
Coefficient of resistivity at 0o C (Mild Steel) α0 (1/Κ0) = 0.00423 Coefficient of resistivity at Tro C (Mild Steel) α
r = 0.0039
Resistivity of ground conductor ρr μ Ω-cm = 15
Thermal capacity factor TCAP J/cm3/οC = 3.749
Diameter of grid conductor in m d m = 0.04
Spacing of grid conductor Chosen D m = 18
Depth of burial of Grid conductor h m = 0.6
Duration of shock ts Sec = 1.0
1.3 SWITCHYARD DATA
(Refer Annexure-1 for Earthgrid layout )
Length of Rectangle - A m = 298
Length of Rectangle - B m = 324
Length of Rectangle - C m = 415
Width of Rectangle - A m = 50
Width of Rectangle - B m = 90
Width of Rectangle - C m = 208
Width of Rectangle - D m = 47.5
Area of substation A Sq.m = 135558
Grid conductor periphery length Lp m = 1621
Maximum distance along grid length Lx m = 415
Maximum distance along grid width Ly m = 348
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 = 265
Total length of ground rods LR m = 795
(Refer Annexure-3 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 2.2
2.3 The Following are considered for determination of grounding conductor length Total length of Earthmat
Length of Earth electrodes
Length of Auxiliary Earth mat for isolators
3.0 CALCULATION:
3.1 Selection of cross section of grid conductor (A)
The Cross-section of Conductor is given by A = mm2 40 mm2 A = 3.749 x 10-4 x ln 236.4 + 620 1 x 0.0039 x 15 236.4 + 50 A = 467 Sq.mm Where K0 = 1 / α0 = 236.4
For 467 Sq.mm, Size of Conductor Required = =
Corrosion allowance:
For First 12 years the corrosion = For Next 12 years the corrosion =
Total corrosion =
Corrosion Allowance in mm = 91.5 x 0.0254 = 2.32 mm
Diameter of MS Rod Required with corrosion allow = 24.4 + 2.32 + 2.32
= 29.04 mm
Selected Conductor Size =
3.2 Determination of grounding conductor length:
The Earth Mat spacing considered for safe Step & Touch Potentia= 18 m x 18 m
Area of Substation A = 135558 Sq.m
Grid conductor periphery length Lp = 1621 m
Maximum distance along grid length Lx = 415 m Maximum distance along grid width Ly = 348.0 m
Length of each ground rod Lr = 3 m
Total length of ground rods LR = 795 m
Spacing of grid conductor chosen D = 18 m
Maximum distance between any two point on the grid Dm = 574 m 91.5 mils 24.4 mm 61 mils 30.5 mils 40 mm dia MS Rod ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + + ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − a T o K m T o K ln r .ρ r .α r t 4 TCAP.10 I 14 . 3 4 467×
Estimation of Grid Conductor Length Rectangle - A (298m x 50m)
The number of conductors parallel to the breadth = 18 The number of conductors parallel to the length = 4 Length of Conductor for Rectangle -A = 2092 m
Rectangle -B (324m x 90m)
The number of conductors parallel to the breadth = 19
= 6
Length of Conductor for Rectangle - B = 3654 m
Rectangle -C (415m x 208m)
The number of conductors parallel to the breadth = 24 The number of conductors parallel to the length = 13 Length of Conductor for Rectangle - C = 10387 m
Rectangle -D (109m x 47.5m)
The number of conductors parallel to the breadth = 7 The number of conductors parallel to the length = 4 Length of Conductor for Rectangle - D = 769 m
= 16902 m
Length of Aux. earth mats for single earth switch 7848 m Length of Aux. earth mats for double earth switch 864 m Length of Aux. earth mats without earth switch 270 m Total length of auxilary earthmat required 8982 m LC = 16902+8982
= 25884 m
Effective length of the buried Conductor in m LT = LC + LR = 25884 + 795
= 26679 m
Ig = DfxSfxIf
where
Df = Decrement factor = 1 1
Sf = Current division factor
If = Symmetrical ground fault current
Sf =
` where
Determination of Maximum Grid Current:
Grid conductor Combined Length (Lc) used in the switchyard
The total length of the grid conductor along the Length & Width in the switchyard
The number of conductors parallel to the length
⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + Rg Zeq Zeq
Zeq - Equivalent impedence of transmission line overhead shieldwires No of 400kV Transmission line shield wires = 12
No of 220kV Transmission line shield wires = 7
Zeq - 0.098+j0.047 (Refer page no. 151, Table C-1 of IEEE-80-2000) Rg - Station ground impedence
0.098+j0.047 0.098+j0.047+0.1 0.108 0.108+0.1 Ig = 1x0.52x40 = 20.8 = 20.8kA
3.3 Calculation of Tolerable Touch voltage(Et) & Step voltage(Es) without gravel
The Surface Layer Derating factor Cs is given by
Cs = 1 (Refer cl.8.3 of IEEE 80-2000)
Tolerable Touch Voltage Et is given by
Et50 = =
Et50 = 129 V
Tolerable Step Voltage Es is given by
Es50 =
=
Es50 = 169 V
3.4 Determination of Grid resistance (Rg)
Rg =
Rg =
Rg = 0.090 Ohms
0.52 Sf =
(As per Layout)
= = ( ) S S S t 116 . 0 C 6 1000+ ×ρ × ( ) S S S t 116 . 0 C 5 . 1 1000+ ×ρ × Ω ⎥ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜⎜ ⎜ ⎜ ⎜ ⎝ ⎛ + + + A h A LT 1 20 1 1 . 20 1 1 ρ Ω ⎥ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜⎜ ⎜ ⎜ ⎜ ⎝ ⎛ + + + 133558 20 6 . 0 1 1 1 133558 20 1 26679 1 61 . 75 x
(
)
1 116 . 0 6 . 75 1 6 1000+ × × ×(
)
1 116 . 0 6 . 75 1 5 . 1 1000+ × × ×3.5 Determination of Ground Potential Rise (GPR)
GPR= Ig x Rg = 20800 x 0.09 = 1872 V
3.6 Calculation of Attainable Mesh voltage(Et) & Step voltage(Es)
3.6.1 Determination of the effective No.of Parallel Conductor (n) n = na x nb x nc x nd Where na = = 2 x (25884/ 1621) = 31.94 nb = = 1621 4 x 135558 = 1.05 nc = = = 415 x 348 ( 0.7 x 135558)/(415x348) 135558 = 1.04 nd = Dm √Lx2+Ly2 = 574 √4152+3482 nd = 1.05 n = 37.00 3.6.2 Determination of Ki: Ki = = 0.644 + (0.148 x 37) = 6.12 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ A P L 4 Y L X L 0.7A AY L X L ⎟⎟ × ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ×
(
0.148 n)
0.644 + × P L C L 2 ∴3.6.3 Determination of Km:
Km =
Where
Kii = 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 = Km = 0.661 3.6.4 Determination of Ks: Ks = = Ks = 0.3
3.6.5 Determination of the effective buried Length of conductor (Lm for Touch Voltage)
For grids with Earth electrodes
Lm =
=
Lm = 27122 m
3.6.6 Determination of the effective buried Length of conductor (Ls for Step Voltage):
Ls = 0.75Lc + 0.85LR for grids with or without ground rods = (0.75 x 25884) + ( 0.85 x 795)
Ls = 20089 m
Attainable Mesh voltage Em is given by
⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + ho h 1 ∴ ⎥⎦ ⎤ ⎢⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − − + + + π D 1 0.5n 2 1 h D 1 h 2 1 1 R L y L x L r L . . C L × ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + + + 2 2 22 1 55 1
(
)
(
)
⎥⎥⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − + ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − + + 1 2 8 ln . 4 8 2 16 ln 2 1 2 2 n K K d h Dd h D hd D h ii π π ⎥⎦ ⎤ ⎢⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛− − + + + 1 0.5(37 2) 18 1 6 . 0 18 1 6 . 0 2 1 14 . 3 1 X 795 2 348 2 415 3 22 1 55 1 25884 × ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + + + . .(
)
(
)
(
)
⎥⎥⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − + + 1 37 2 14 . 3 8 ln . 265 . 1 1 04 . 0 4 6 . 0 04 . 0 18 8 6 . 0 2 18 04 . 0 6 . 0 16 18 ln 14 . 3 2 1 2 2 x x x x x x x x(
)
(
)
(
)
⎥⎥⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − + + 1 37 2 14 . 3 8 ln . 265 . 1 1 04 . 0 4 6 . 0 04 . 0 18 8 6 . 0 2 18 04 . 0 6 . 0 16 18 ln 14 . 3 2 1 2 2 x x x x x x x xEm =
= ( 75.61 x 0.661x 6.12x 20.8*1000) /27122
Em = 234 V
Attainable Step voltage Es is given by
Es =
= ( 75.61 x 0.3 x 6.12 x 20.8*1000)/20089
Es = 143 V
4.0 CONCLUSION:
a) Es vs Es50
The Calculated Step Voltage of 143 V is lower than the tolerable Step Voltage 169 V (Without gravel)
b) Grid Resistance Rg
The Calculated Grid Resistance of 0.09 Ohms is less than 1 Ohm
Hence the design is safe without gravel and gravel need not be provided in future areas.
m L g l i K m K × × × ρ s L G l i K s K ρ× × ×
Corner Mesh Potential, Vm = GPR*Value of corner mesh potential percentage 100 and GPR = I*Rg Hence GPR = 20800*0.094 = 1955 Volts GRID AREA = 135558 m2 MESH AREA = 18X18 m2
therefore no. of meshes on each side = 135558/(18x18) = 418.3889
no of meshes on each side = sqrt(418.38) = 20.45
Refer graph (fig B1) eclosed the value of corner mesh potential is 12% for 18x18 m2 and 20 nos of meshes on each side.
Corner mesh potential = 1955*12
100
= 234.6
= 234.6V < 480V
