EARTHMAT DESIGN 132KV SUBSTATION EASY EXPLANATION

Design Parameters for EARTHMAT DESIGN 132 KV

Reference Standard IEEE – 80 – 2000

Resistivity Data for EARTHMAT DESIGN 132 KV SS

Soil Resistivity, ρ = 90 Ω m

Surface layer resistivity, ρ_s= 3000 Ω m

Surface layer thickness, h_s=0.10 m

Reflection factor between different resistivities

K = (ρ – ρ_s) /( ρ+ ρ_s)

= -0.941747573

Surface layer derating factor, Cs = 1-(0.09*(1-ρ/ρ_s))/(2*hs+0.09)

= 0.698965517

System Data for EARTHMAT DESIGN 132 KV SS

Decrement factor for determining IG, D_f = 1

Fault current division factor (split factor), S_f = 0.5

Maximum Fault current in the system, I_F = 31.5KA =31500 A

Maximum grid current that flows between ground grid and surrounding earth (including dc offset) IG = D_Fx S_F xI_F =1 x 0.5 x 31500 = 15750 A

Duration of fault current for sizing ground conductor tc = 1s

Duration of fault current for determining decrement factor, tf = 0.05s

Duration of shock for determining allowable body current, ts = 1s

Conductor Data for the EARTHMAT DESIGN 132KV SS

Specific heat of MS rod/ Flat, SH = 0.114 Kcal/kg

Specific weight of MS rod/ Flat, SW = 7.86 gm/cc

Therefore, TCAP = 4.186 * SH * SW = 3.750 J/(cm³ ·°C)

Co-efficient of linear expansion at 20 deg C of MS rod/ Flat, α_r =0.00423 /°C)

Resistivity of MS Rod/ Flat. Pr = 15 micro ohm-m

Reference temperature, Tr = 20° C

Maximum temperature, Tm = 620° C

Ambient temperature, Ta = 50° C

K_o = 1/α_o or (1/α_r) – T_r in °C = 1/.00423 – 20 = 216.4

Therefore, Area of the Conductor, A = $[\frac{10^{-3}}{\sqrt{\frac{TCAP \times 10^{-4}}{t_c \alpha_r \rho_r}} \ln\left(\frac{K_0 + T_m}{K_0 - T_a}\right)}]$ = 680.851 sqmm

the required dia is = $[ \sqrt{\frac{4A}{\pi}}]$ = 29.4 mm

Corrosion Allowance

(Reference IEEE Transaction of Power Apparatus & System Vol.PAS-98,No.6 Nov. 1979)

For First 12 Years = 0.1291 mm/yr.

For Next 12 Years = 0.0646 mm/yr.

Only negligible corrosion is found to take place in the initial earthmat design 132KV SS thereafter.

Total Corrosion (0.1291x 12 + 0.0646x 12 ) =2.32 mm in the diameter

Taking it as =2.5 mm in the diameter

So the conductor size is = 29.4+2.5 =31.9 mm

Considering the Sub-soil ground rod conductor of size D = 40 mm

And size of the grid flat conductor = 75 x 8 mm

Grid Data for earthmat design 132KV SS

Total area enclosed by ground grid, A = 8256 m²

Spacing between parallel conductors, D = 5 m

Depth of ground grid conductors, h = 0.6 m

Reference depth of grid, h0 = 1 m

Total no of parallel conductors in x direction = 96/5 = 19.2

Total no of parallel conductors in Y direction = 86/5 = 17.2

Maximum length of grid conductors in X direction, Lx = 96 m

Maximum length of grid conductors in Y direction, Ly = 86 m

Maximum distance between any two points on the grid, Dm = 96 m

Total length of grid conductor , Lc = 3322.4 m

Peripheral length of grid, Lp = 2*(86+96) = 364 m

L_C= 19.2 * 86 +17.2 * 96 =3302.4 m

Length of ground rod at each location, Lr = 3 m

Number of rods placed in area A, nR = 226 Nos.

Total length of ground rods, LR = nR*Lr = 678 m

Total effective length of grounding system conductor, including grid and ground rods, LT= Lc+LR = 3302.4+678 = 3980.4 m

Calculation of effective length of Lc + LR for mesh voltage

Effective length of Lc + LR for mesh voltage for grids with ground rods in the corners, as well as along the perimeter and throughout the grid, the effective burried length is $[L_M = L_C + \left[1.55 + 1.22 \left(\frac{L_r}{\sqrt{L_x^2 + L_y^2}}\right)\right] L_R]$

$[L_M = 3302.4 + \left[1.55 + 1.22 \left(\frac{3}{\sqrt{96^2 + 86^2}}\right)\right] 678]$

= 4372.56

Calculation of effective length of Lc + LR for step voltage

Effective length of Lc + LR for step voltage

LS = 0.75*Lc+0.85*LR = 0.75*3302.4+0.85*678 = 3053.1m

Geometry of earthmat design of 132KV SS = Rectangular

Geometric factor na = 2*Lc/Lp = 2* 3302.4/364 = 18.15

nb = $[\sqrt{\frac{L_p}{4\sqrt{A}}}]$ = $[\sqrt{\frac{364}{4\sqrt{8256}}}]$ =1.001

nc = $\left[\frac{L_x L_y}{A}\right]^{\frac{0.7A}{L_x L_y}}$

= 1, since rectangular

nd = $\frac{D_m}{\sqrt{L_x^2 + L_y^2}}$ =1( because of square, rectangular, l shaped)

Geometric factor composed of factors na, nb, nc and nd

n = na*nb*nc*nd

= 18.15

Factors in the calculation of step and touch voltages of earthmat design 132KV SS

Corrective weighting factor that emphasizes the effects of grid depth, simplified method

Kh = $[\sqrt{1 + \frac{h}{h_0}}]$

= $\sqrt{1 + \frac{0.6}{1}}$

= 1.26

Correction factor for grid geometry, simplified method

Ki = 0.644+0.148*n

= 0.644+0.148*18.15

= 3.330

Corrective weighting factor that adjusts for the effects of inner conductors on the corner mesh, for earthmat design 132KV SS

For, grid with ground rods along the perimeter, or for grids with ground rods in the grid corners as well as both along the perimeter and throughout the grid area.

Kii = 1

Spacing factor for step voltage, simplified method

Ks = $\frac{1}{\pi}\left[\frac{1}{2}h + \frac{1}{D+h} + \frac{1}{D} \left(1 - 0.5^{(n-2)}\right)\right]$

= $\frac{1}{\pi} \left[ \frac{1}{2 \times 0.6} + \frac{1}{5 + 0.6} + \frac{1}{5} \left( 1 - 0.5^{(18.25 - 2)} \right) \right]$

= 0.322

Geometrical factor for mesh voltage simplified method

Km = $\frac{1}{2\pi} \left[ \frac{\ln D^2}{16hd} + \frac{(D + 2h)^2}{8Dd} - \frac{h}{4d} \right] + \frac{K_{ii}}{K_h} \ln\left(\frac{8}{\pi(2n - 1)}\right)$

= $\frac{1}{2\pi} \left[ \frac{\ln(5^2)}{16 \times 0.6 \times 0.040} + \frac{(5 + 2 \times 0.6)^2}{8 \times 5 \times 0.040} - \frac{0.6}{4 \times 0.040} \right] + \frac{1}{1.26} \ln\left(\frac{8}{3.14 \times (2 \times 18.15 - 1)}\right)$