Spl Bsts Jandaha Earthmat.cal 005

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 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



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×



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 +

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

∴



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



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



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 ×××



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





Epppl-

Line

Epppl-

Line

Epppl-

Text Box

0.73

Epppl-

Text Box

0.188

epppl dell

Oval

Spl Bsts Jandaha Earthmat.cal 005

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