LEENA ELECTRO MECHANICAL PVT LTD.

Design Details of Earthing Grid Of GRID Sub-Station (Igatpure TSS)

1 Given Datas
(a) Name of Sub-Station Igatpure TSS
(b) System Voltage (V) 132 KV
(C) Size of TSS
(i) Length = 50.820+7.550+24.300+37.220 119.89 Mtrs
(ii) Width = 74.120 max 74.12 Mtrs
(d) Area of TSS =(50.820x74.120)+(7.550x45.550)+(24.300x21.700)+(37.220x21.700) = 5445.665 Sqmt
(e) Soil Resistivity = 2 x 3.14 x A x R (ρ)
Where
(i) DISTACE BETWEEN CONSECUTIVE SPIKES (A) Mtrs
(ii) SOIL RESISTANCE (R) Ohm
Soil Resistivity = (ρ) 28.84 Ohm-Mtr
(f) Stone Resistivity (ρs) 3000 Ohm-Mtr
(g) Fault Current at Source Bus
Fault current on Primary side for 3 ph 40000 Amp
As per report given by M.S.E.T. Co.Ltd
(h) Duration of fault:
(i) To Determine of size of Earth Mat (t) 1 Sec
(ii) To Determine the Step &Touch Potential (ts) 0.5 Sec
(i) Main Earth Grid Material Mild Steel Rod
(j) Size of Earth Electrode:
(i) Dia 40 mm
(ii) Length 4 Mtrs
(k) Permissible Earth resistance of Earth Grid (R(Ω)) 0.5 Ohm
(l) Thickness of Crushed Rock (hs) 0.1 Mtr
(m) Distance between Earth Mat Conductor (D) 5 Mtrs
(n) Depth of earthing Grid Conductor (h) 0.6 Mtrs
(o) Corrective Protection Factor (For Ground Potential Rise) (Cp) 0.55
(p) Decrement factor for entire Duration (Df) 1
(q) Capacity of Power Transformer at TSS 50 MVA
(r) Corrective Protection Factor (From graph depending on k & hs) Cs 0.55
(s) Resistance of the main earthing Mat = 0.5 Ohm

2 Design Calulation:-
(A) Fault Current
Fault level at the incoming bus (i.e. 132kV side) for 3 PH 9150 MVA
Fault current on Primary side for 3 ph (Ig) 9150/(√3x132) KA
40 KA

As per report given by M.S.E.T. Co.Ltd

Faults current at H.T. Bus Bar considered for 3ph (Ig) 40000 Amp
Duration of fault: = 1 Sec
For determining the size of earthing mat
= 0.5 Sec
For determining the safe and mesh
potential

(B) Size of the earthing grid (mat) conductor

(a) Area of earthing mat (A) = 12.15 x Ig x√t/1000
Where
A = Cross- sectional area of earthing grid conductor in square millimetre
Ig = r.m.s. value of fault current (Short Circuit Current at H.T. Bus Bar) in Amperes.

t = Duration of fault current in Second

Cross-sectional area of earthing grid conductor or mat (A) = 12.15 x40000 x√1/1000
= 486 Sqmm
Or Say = 490 Sqmm
Taking of Corrosion Effect as 100% 490 x2 Sqmm
Therefore Size of Earthing Mat Conductor = 980 Sqmm
100x10 mm MS flat area is = 1000 Sqmm

Size of standard MS flat near to 980 Sqmm area is 100x10mm (1000 Sqmm)
There fore 100x10mm size MS flate is proposed for the earthing mat conductorHence to
use 100x10mm size MS flate for Earth Mat conductor
 or Equivalent Diameter of the Round Grid Conductor, d = 40 mm

(C) Minimum Length of earthing mat Conductor (Lmin)

Lmin = (Km x Kim x Soil resistivity x fault current x √t) =
(116+0.17 x Stone Resistivity)
Lmin = (Km x Kim x ρ x Ig x √ts) =
(116+0.17 x ρs)
where
D = Spacing between parallel conductors of the grid meters (same spacing in both = 5 Mtrs
direction
d = diameter of grid conductor mtrs. = 0.04 Mtrs
h = depth of grid mtrs. = 0.6 Mtrs
Calculations for Km x Kim
(i) Km= 1/2π{In ((D²/16 h.d) + (D+2h)²/8.D.d + h/4.d) + Kiim/Kh In 8/ π(2n-1)}
nA = Number of parallel conductors of grid in transverse direction = 16 Nos
nB= Number of parallel conductors of grid in longitudinal direction = 11 Nos
(ii) Kim = (0.656 + 0.172 x n) = 2.938
(iii) egde n = √nA x √nB = 13.266
(iv) Kii = 1 (For grids with earth electrodes along the perimeter, or for grids with earth = 1
electrods in the grid corners, as well as both along the perimeter and throughout the grid

Kii= 1/(2n) 2/n ( For grids without earth electrodes or grids with only a few earth
electrodes, none located in the corners or on the perimeter.

(v) Kh = √(1+h) = 1.2649

(i) Km= 1/2π{In ((D²/16 h.d) + (D+2h)²/8.D.d - h/4.d) + Kiim/Kh In 8/ π(2n-1)} =

Km =
(a) Km x Kim = 0.4067
(b) Soil Resistivity (ρ) = 28.84
(c) Faults current at H.T. Bus Bar ( Ig ) = 40000
(d) Time (ts)= 0.5 second (assumed maximum duration of shock) = 0.5 Sec
(e) Stone Resistivity ( ρs ) = 3000 Ohm-mtr
Lmin = (Km x Kim x ρ x Ig x √ts) = 529.96 Mtrs
(116+0.17 x ρs)

(D) Tolerable Touch Potential

E touch (tolerable) = [ 1000 + 1.5 Cs ( hs. K ) ρ s ] x 0.116/ √ts
Where
(i) Corrective factor Cs ( hs. K ) = 0.77
K = (ρ - ρs) / (ρ + ρs) = -0.94235
(ii) Thickness of crushed rock (hs) = 0.1 Mtrs
( iii) Resistivity of crushed rock (ρs) = 3000 Ohm-mtr
( iv) Duration of faults (ts) = 0.5 Sec
E touch (tolerable) = [ 1000 + 1.5 Cs ( hs. K ) ρ s ] x 0.116/ √ts = 570.07 Volt
or Say = 570 Volt

(E) Estimated Touch Potential

E touch (estimated ) = ρ x Km x Ki x Ig / Lmin
Where
(i) Soil Resistivity (ρ) = 28.84 Ohm-mtr
(ii) Km x Kim = 0.4067
(iii) Faults current at H.T. Bus Bar ( Ig ) = 40000 A
(iv) Lmin = 529.96 Mtrs
E touch (estimated ) = ( ρ x km x kim x Ig) / Lmin = 885.30 Volt

(i) The estimated e-touch 885.30 volt is more than tolerable e-touch 570 volt thus the length of earth mat is not adequate. The length of earthing mat
conductor shall be increased to:
Average Length of Rod = 6 Mtrs
Total Length L = L1+L2+L3 = 2723.56975 Mtrs
L1= LONGITUDINAL + TRANSVERSE = 2415.19 Mtrs
LONGITUDINAL (11*74.12)+(3*45.55)+(13*21.70) = 1234.07 Mtrs
TRANSVERSE (16*50.82)+(8*7.55)+(5*61.52) = 1181.12 Mtrs
 L2 = Overlap @ 15 cm = 60.37975 Mtrs
L3 = Earth Electrode (50 x 4) + (12 x 4) = 200 + 48 = 248 Mtrs
Etouch (estimated ) = (ρ x km x kim x Ig )/ Lmin = 172.26 Volt
Etouch (estimated ) < Etouch (tolerable )
Hence length of earthing mat conductor is adequate = 2723.56975 Mtrs

(F) Total length of Earth Conductor (L) = L1+L2+L3 = 2723.56975 Mtrs

L1= LONGITUDINAL + TRANSVERSE ( Earth Conductor) = 2415.19
L2 = Overlap @ 15 cm = 60.37975
L3 = Earth Electrode [(50 x 4) + (12 x 4)] = 200 + 48 = 248 Mtrs

Number of earth electrodes used in the earthing

Number of earth electrodes at the perimeter = 46 Nos.
Number of earth electrodes for 132kV Lightning Arrestor = 12 Nos.

Number of earth electrodes for Power Transformer earthing = 2 Nos.

Number of earth electrodes for Control room earthing = 2 Nos.

Total Earth Electrode = 62 Nos.

(G) Ground Resistance

(i) Rg =ρ/4 x (√π/√Ag) + ρ/L

Where
L = Total length of buried conductors in mtrs = 2723.56975 Mtrs
Ag = Area occupied by the earthing grid in mtrs sq = 5445.665 Sqmt
ρ = Resistivity of earth in ohm-mtr (Soil Resistivity) = 28.8400 Ohm-mtrs
Rg = Station ground resistance in Ohm
Rg =ρ/4 x (√π/√Ag) + ρ/L = 0.18 Ohms.
Ground resistance 0.18 Ohms which is less then 0.5 Ohm ( Hence Safe)

(H) Ground Potential Rise = Rg xIG

 Where
(i) Rg = Station ground resistance in Ohm = 0.18 Ohm
(ii) IG = Cp x Df xIg 22000 A
(iii) Cp = Corrective protection factor accounting for the relative increase of fault = 0.55
currents during the station life span; for a zero future system growth Cp=1
(iv)Df = Decrement factor for the entire duration of fault ( to allow for the effects of = 1
asymmetry of the fault current wave).
= 1.0 for fault current duration of 0.5 second or more

(v) Ig= r.m.s. value of symmetrical grid fault current in A = 40000

Ground Potential Rise = Rg x IG = 4041.84 V
So Earthing Design & Drawing is OK

(I) Tolerable Step Potential

E step (tolerable) = [ 1000 + 6 Cs ( hs. K ) ρ s ] x 0.116/ √ts
Where
(i) Corrective factor Cs ( hs. K ) = 0.55
( ii) Resistivity of crushed rock (ρs) = 3000 Ohm-mtr
( iii) Duration of faults = 0.5 Sec

E step (tolerable) = [ 1000 + 6 Cs ( hs. K ) ρ s ] x 0.116/ √ts = 1788.13 Volt

(J) Estimated Step Potential

E step (estimated) = ( ρ x Ks x Ki x Ig) / L
Where
(a) Soil Rssistivity (ρ) = 28.84 Ohm-mtr
(b) Faults current at H.T. Bus Bar ( Ig ) = 40000 Amp
(c) Total length of buried conductors in mtrs (L) = 2723.56975
(d) Ks =1/π [ 1/2h + 1/(D+h) + 1/D ( 1-0.5n-2 ) ] = 0.385757
Where
(i) h = 0.6 Mtrs
(ii) nA = 16 Nos.
(iii) nB = 11 Nos.
 (iv) n= nA since nA > nB = 16 Nos.
(v) D = 5 Mtrs
(e) Ki (0.656 + 0.172 x n) = 3.408
E step (estimated) = ( ρ x Ks x Ki x Ig) / L = 556.8 V
E step (estimated) < E step (tolreable)
Hence design of earthing mat is in order and safe