Earth Mat Calculation

EARTHMAT DESIGN CALCULATION FOR 33/11KV NEW SUB-STATION AT MANBAZAR (PKG-III) IN PURULIA DISTRICT OF WEST BENGAL Description 1.0

Symbol

Unit

I tr

kA sec

= =

25

ρ ρs

Ω-m

=

35.89

Surface Resistivity

Ω-m

=

2000

Thickness of surface layer

hs

m

=

0.2

Tm Ta Tr

°C °C °C

= = =

620

=

0.00423

=

0.0039

SYSTEM DATA RMS Value of symmetrical fault current Fault clearing time for conductor selection

1.1

1.2

SOIL DATA Soil Resistivity (Average)

Coefficient of resistivity at 0° C (Mild Steel) Coefficient of resistivrty at Tr°C (Mild Steel)

(As per soil resistivity Report)

αo(1/Ko) αr

50 20

ρr

µ Ω-cm

=

15

TCAP

J/cm³/°C

=

3.749

Equivalent Diameter of grid conductor in m

d

m

=

0.02

Spacing of grid conductor Chosen

D

m

=

4

Depth of burial of Grid conductor

h

m

=

0.6

Duration of shock

ts

Sec

=

1.0

(Refer Annexture-II for Earthgrid layout) Length of Rectangle

m

=

48

Width of Rectangle

m

=

16

A Lp Lx Ly

Sq.m

=

768.0

m

=

128

m

=

48

m

=

16

De Lr N

m

= = = =

0.02

Resistivity of ground conductor Thermal capacity factor

SWITCHYARD DATA

Area of substation Grid conductor periphery length Maximum distance along grid length Maximum distance along grid width 1.4

3

Grid Conductor Data The Conductor Selected is 50x6 mm MS Flat Max allowable temperature of the conductor Ambient temperature Reference temperature for material constant

1.3

Value

EARTH ELECTRODE DATA Diameter of Pipe/rod earth electrode in m Length of each rod conductor in m No.of Earth electrodes Total length of ground rods

□(64

m Nos m

3 100 300

(Refer Annexture-I 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

The Average soil resistivity value is taken from the soil test report.

2.3

Design of Earthing Mat for High Voltage Substation - CBIP Publication No.223 (1992)

2.4

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

3.1

Determination of grounding conductor length: The Earth Mat spacing considered for safe Step & Touch Potential Area of Substation A Sq.M Grid conductor periphery length Lp m Maximum distance along grid length Lx m Maximum distance along grrd width Ly m Length of each ground rod Lr m Total length of ground rods LR m Spacing of grid conductor chosen D m

=

4m x 4m

= = = = = =

768.0

=

4

128 48 16 3 300

Estimation of Grid Conductor Length Rectangle-1 (16 m x 48 m) The number of conductors parallel to the breadth

=

The number of conductors parallel to the length

=

5

Length of Conductor for Rectangle

=

128

=

448

Lc

=

448

LT

=

Lc + LR

=

748

=

1

kA

=

25

Ohms

= = =

1 4 0.602

The total length of the grid conductor along the Length & Width in the switchyard Grid conductor Combined Length (Lc) used in the switchyard Effective length of the buried Conductor in m 3.2

13

Calculation of Tolerable Touch voltage (E t) & Step voltage (Es) The Surface Layer Derating Factor

0.09 x Cs =

Cs is given by

1 - 𝜌/

12hs + 0.09

Tolerable Touch Voltage

Cs =

0.8196

Et is given by

((1000+1.5Cs x ρs) x 0.116)/(√t_s Et50 = ) Tolerable Step Voltage

Et50 =

401.229 V

Es50 =

1256.915 V

Rg =

0.602 Ω

Es is given by

((1000+6Cs x ρs) x 0.116)/(√t_s ) Es50 =

3.3

Determinatoin of Grid resistance (Rg)

Rg = ρ

1/Lt+1/(√20.A)1+1/(1

Since the Grid resistance is Rg is Less than 1Ω, the Design is Safe. 3.5

Determination of Maximum Grid Current ( IG )

IG = Df.Sf.If Where

Df - Decrement Factor for Tf Sf - Current Division Factor If - Symmetrical ground fault current No. of 33kV Transmission Line No. of 11kV Distribution Line

Rg - Station Ground Impedence Sf = 0.5 (Refer Curve C-1 of IEEE-80-2000) Ig = 1 x 0.5 x 25 =

12.5 kA

Shock duration time ts=1 sec. 3.5

Determination of Ground Potential Rise (GPR)

GPR = IG x Rg =

7519.87 V

Calculation of Attainable Mesh Voltage (Et) & Step Voltage (Es)

3.6 3.6.1

Determination of the effective no. of Parallel Conductor (n)

n = na x nb x nc x nd Where

na = 2𝐋𝐜/ nb =

=

7.000

=

1.075

=

0.0009115 1.000

√(□(6 □(64&〖"("

nc =

nd = 1 (For square, rectangle, & L- Shaped grids.)

3.6.2

Determination of

Ki :

3.6.3

Determination of

Km :

□

nd =

1

n=

7.522

Ki =

Km =

1.757 -1.34988313

□(64&1/2π)[ln(□(64&D²/16hd

4.771

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 throughout the grid area)

Kh =

Kii =

□(64&

= Where

1.265

h0 = 1m (Grid reference depth) Km =

3.6.4

1

Determination of

0.5445

Ks:

□(64&1/𝜋

Ks =

=

0.41230

Determination of the effective buried length of conductor ( Lm for Touch voltage)

3.6.5

For grids with earth electrodes

Lm =

Lc+(1.55+1.22(□(64&𝐿𝑟 =

934.701

Determination of the effective buried Length of conductor ( Ls for Step Voltage):

3.6.6

Ls = 0.75 □(+ 0.85 L

for grids with or without ground rods

Ls = Attainable Mesh Voltage

Em = Attainable Step voltage

591

Em is given by

□(64&(ρ x Em =

347.745 V

Es =

549.979 V

Em= Et50= Es= Es50=

347.745 V 401.229 V 549.979 V 1256.915 V

Es is given by Es =

□(64&(ρ x

So now,

4.0

Conclusion: a)

Em vs. Et50 The calculated Mesh Voltage is Lower than the tolerable touch voltage.

b)

Es vs. Es50 The calculated step voltage is lower than the tolerable Step Voltage.

c) Grid Resistance Rg The calculated Grid Resistance is less than 1 Ohm. The calculated Mesh Voltage The calculated is Lower than step thevoltage tolerable The calculated istouch lowervoltage. than Gridthe Resistance tolerableisStep lessVoltage. than 1 Ohm.

Hence the Design is Safe. 1

1

1

3

3

Annexture-I Estimation of Number of Earth Electrodes for 33/11kV New Sub-station at Manbazar-II in Purulia District of West Bengal (Pkg-III) No. of Earth Electrodes Sl.No. Description 1 2 3 4

P.Transformer 33kV L.A 33KV H.G FUSE 33KV C.T

5

33KV VCB

6

33KV P.I

7

ISOLATOR (33KV + 11KV) = 10+10

8

GANTRY TOWER

9

CONTROL ROOM

8 12 4 12 4 20 20 10 10

10 11 12 13 Total No. of Earth Electrode=

100

Length of Each Electrode (m) = Total Length of Earth Electrodes (m) =

3 300