Earthing Calculation

Earthing Calculation

document.xlsThe Ig value was taken 50 kA instead of calculated value.The same is changed.

Revision:1Dated:2000-08-31

Calculations are based on IEEE Vol 80,1986 version.

1 Parameters for design purposes:Average soil resistivity at 0.5 m level ROW1 Ohm meter 39.29

1.1 Soil resistivity(taken for calculation) ROW Ohm meter 50.01.2 Soil resistivity due to concrete ROWS Ohm meter 5001.3 Fault current(for conductor size design) I KA 50.00 1.4 Fault duration for sizing TF1 sec 1.001.5 Fault duration for step and touch potentials TF2 sec 0.701.6 Depth of burial of grid conductors h metres 0.801.7 Thickness of concrete hs metres 0.201.8 Number of ground rod/pipe electrodes,3m long N nos 1501.9 Area of the earth mat AG sq.metres 640,000

1.10 Effective earth fault current kA 22.245(Refer enclosed annexure for this calculation)

2 Evaluation of conductor sizing

2.1 Factor K0 (1/ALPHAR)-TR216.41

2.2 TCAP is defined byTCAP = 4.184*SW*SH

3.752.3 Minimum area of cross section A = I*(SQRT(X1/X2))

where2.4 X1 = (TF1*ALPHAR*ROWE*10000)/TCAP

169.242.5 X2 = LN(1+(TM-TA)/(K0-TA))

1.312.6 Minimum area of cross section A sq.mm 568.432.7 Diameter of conductor mm 26.91

2.8 Temperature co-efficient of material ALPHAR 0.004232.9 Resistivity of earthing material ROWE micro-ohm/ 15.00

2.10 Specific heat of material SH cal/gram/C 0.1142.11 Density of material SW gram/cc 7.862.12 Reference temperature of material constant TR C 202.13 Max.allowable temperature for welded joints TM C 5002.14 Ambient temperature TA C 50

2.15 Corrosion allowanceAverage rate of corrosion is 0.12 mm per annum is considered for a period of 40 years. Total Corrosion expected =0.12*40 4.8 mm

2.16 Diameter of the conductor after corrosion= 31.71 mm.2.17 Diameter of the conductor chosen d 0.04 M

3 Permissible step and touch voltages3.1 E step = (1000+(6*CS(hs,K)*ROWS))*0.116/(SQRT TF2)3.2 E touch = (1000+(1.5*CS(hs,K)*ROWS))*0.116/(SQRT TF2)3.3 CS is a reduction factor depending upon the depth of gravel /concrete and reflection factor K.

CS is obtained from Figure 8 of IEEE. = 0.823.4 K = (ROW-ROWS)/(ROW+ROWS)

-0.818CS =1 for no gravel/ concrete.

Ig

North Chennai Thermal Power Project Stage II(2x525 MW)






ALTERNATIVELY,CS = 1-a*((1-s)/u) wherea=0.106 metre;s=(ROW/ROWS);U=(2hs+a)s = 0.1000u = 0.506a = 0.11CS = 0.811Choose a lower value of CS from the above.Hence CS=0.82

3.5 Permissible Estep volts 479.723.6 Permissible Etouch volts 223.91

4 Preliminary layout4.1 Length of the earth mat L metres 800.004.2 Breadth of the earth mat B metres 800.004.3 Assumed spacing for the conductors D metres 50.004.4 Total no of conductors parallel to length N1 nos (B/D)+1

174.5 Total no of conductors parallel to breadth N2 nos (L/D)+1

174.6 Total quantity of conductors laid LC metres (N1*L)+(N2*B)

27,200

4.7 LINT metres 8,1604.7 Total ground rods length LR metres 450 4.8 Effective grid conductor length L metres 35,878

5 Estimation of actual mesh and step voltages5.1 Emesh =5.2 E step =

5.3 KM = (1/(2*3.14))*(LN(A+B-C)+D*LN(E))

5.4 where A = D^2/(16*h*d)4,882.813

5.5 B = (D+2*h)^2/(8*D*d)166.4100

5.6 C = (h/4*d)5.000

5.7 D = (Kii/Kh)0.745

5.8 E = 8/(3.14*(2n-1))= 0.077

5.9 KM = (1/(2*3.14))*(LN(A+B-C)+D*LN(E))= 1.054

5.10 Kii=1 for grids with ground rods along the perimeterKii=1 for grids with ground rods at the grid cornersKii=1 for grids with ground rods along the perimeter and throughout the grid area

5.11 Kh = SQRT(1+h/ha) where ha=1 metre.= 1.342

5.12 Number of parallel conductors n = SQRT(N1*N2)17

5.13 KI,the irregularity factor = 0.656+0.172*n3.580

5.14 Hence E mesh = KM*KI*ROW*I*1000/L116.939 volts R1

PERMISSIBLE VALUE 223.91 volts

Interconnections to the grid(30% of calculated quantity)

(ROW*KM*KI*Ig*1000)/L(ROW*KS*KI*Ig*1000)/L







5.16 KS = (1/3.14)*(A+B+C(1-E))WHERE

5.17 A = 1/(2*h)0.625

5.18 B = (1/(D+h))0.020

5.19 C = 1/D0.020

5.20 E = 0.5^(n-2)0.0000305

5.21 KS = 0.21175.22 Hence E step

23.494 VoltsPERMISSIBLE VALUE 479.72 Volts

6 Minimum conductor length requirementTo achieve the above conditions,the following condition has to be satisfied.

6.1 L =18,737 metres

7 Evaluation of grid resistance7.1 RG ohm ROW*(F+G*(1+(1/J)))

where7.2 F = (1/L)

0.000037.3 G = 1/(SQRT (20*A))

0.000287.4 J = 1+(h*(SQRT (20/A)))

1.0047.5 Hence RG = 0.029 ohm

8 Grid potential riseTotal grid potential rise IR Volts I*RG

651.38 volts R1

The mat will be made to suit the above requirement.As shown in the enclosed diagram.

KS*KI*ROW*Ig*1000/L

(KM*KI*ROW*Ig*1000)/Permissible Emesh

Hence from the above calculation the conductors provided meet the Step and touch potential criteria and also the other requirement.

The mesh width will be varied or close mat will be made where Electrical equipments are more and other areas the same will be adjusted.




(1000+(6*CS(hs,K)*ROWS))*0.116/(SQRT TF2)(1000+(1.5*CS(hs,K)*ROWS))*0.116/(SQRT TF2)






*1000)/Permissible Emesh


1 Parameters for design purposes:1.1 Soil resistivity ROW Ohm meter1.2 Soil resistivity due to concrete ROWS Ohm meter1.3 Fault current I KA1.4 Fault duration for sizing TF1 sec1.5 Fault duration for step and touch potentials TF2 sec1.6 Depth of burial of grid conductors h metres1.7 Thickness of concrete hs metres1.8 Number of ground rod/pipe electrodes,3m long N nos1.9 Area of the earth mat AG sq.metres

1.10 Effective earth fault current Ia kA2 Evaluation of conductor sizing

2.1 Factor K0

2.2 TCAP is defined byTCAP =

2.3 Minimum area of cross section A =where

2.4 X1 =

2.5 X2 =

2.6 Minimum area of cross section A sq.mm2.7 Diameter of conductor mm

2.8 Temperature co-efficient of material ALPHAR2.9 Resistivity of earthing material ROWE micro-ohm/

2.10 Specific heat of material SH cal/gram/C2.11 Density of material SW gram/cc2.12 Reference temperature of material constant TR C2.13 Max.allowable temperature for welded joints TM C2.14 Ambient temperature TA C

2.15 Corrosion allowanceAverage rate of corrosion of steel is 61 mils for the first 12 years and 50% of that value 30.5 mils for the next 12 years.After this period,it is assumed that no corrosion takes place.(Reference IEEE Transactions of PAS-Volume 98.

Total corrosion=(61+30.5)2 =183 mils ie 4.64mm.2.16 Diameter of the conductor after corrosion= 31.55 mm.2.17 Diameter of the conductor chosen d 0.04 M

3 Permissible step and touch voltages3.1 E step =3.2 E touch =3.3 CS is a reduction factor depending upon the depth of gravel /concrete and reflection factor K.

CS is obtained from Figure 8 of IEEE. =3.4 K =

CS =1 for no gravel/ concrete.

ALTERNATIVELY,CS =a=0.106 metre;s=(ROW/ROWS);U=(2hs+a)s =u =a =CS =Choose a lower value of CS from the above.Hence CS=0.82

3.5 Permissible Estep volts3.6 Permissible Etouch volts

4 Preliminary layout4.1 Length of the earth mat L metres4.2 Breadth of the earth mat B metres4.3 Assumed spacing for the conductors D metres4.4 Total no of conductors parallel to length N1 nos

4.5 Total no of conductors parallel to breadth N2 nos

4.6 Total quantity of conductors laid LC metres

4.7 Interconnections to the grid LINT metres4.7 Total ground rods length LR metres4.8 Effective grid conductor length L metres


5.3 KM =

5.4 where A =

5.5 B =

5.6 C =

5.7 D =

5.8 E ==

5.9 KM ==


5.11 Kh ==

5.12 Number of parallel conductors n =

5.13 KI,the irregularity factor =

5.14 Hence E mesh =

PERMISSIBLE VALUE5.15 KI,the irregularity factor =

5.16 KS =WHERE

5.17 A =

5.18 B =

5.19 C =

5.20 E =

5.21 KS =5.22 Hence E step

PERMISSIBLE VALUE6 Minimum conductor length requirement

To achieve the above conditions,the following condition has to be satisfied.

6.1 L =

7 Evaluation of grid resistance7.1 RG ohm

where7.2 F =

7.3 G =

7.4 J =

7.5 Hence RG =

8 Grid potential riseTotal grid potential rise IR Volts

45.0500

50.00 1.000.700.800.20

1010,000

23.40

(1/ALPHAR)-TR216.41

4.184*SW*SH3.75

I*(SQRT(X1/X2))

(TF1*ALPHAR*ROWE*10000)/TCAP169.24

LN(1+(TM-TA)/(K0-TA))1.31

568.4326.91

0.0042315.000.114

7.8620

50050

Average rate of corrosion of steel is 61 mils for the first 12 years and 50% of that value 30.5 mils for the next 12 years.After this period,it is assumed that no corrosion takes place.


CS is a reduction factor depending upon the depth of gravel /concrete and reflection factor K.0.82

(ROW-ROWS)/(ROW+ROWS)-0.835

1-a*((1-s)/u) where

0.09000.506

0.110.809

479.72223.91

100.00100.0010.00

(B/D)+111

(L/D)+111

(N1*L)+(N2*B)2,200

66030.002,895

(ROW*KM*KI*I*1000)/L(ROW*KS*KI*I*1000)/L

(1/(2*3.14))*(LN(A+B-C)+D*LN(E))

D^2/(16*h*d)195.313

(D+2*h)^2/(8*D*d)42.0500

(h/4*d)5.000

(Kii/Kh)0.745

8/(3.14*(2n-1))0.121

(1/(2*3.14))*(LN(A+B-C)+D*LN(E))0.617

Kii=1 for grids with ground rods along the perimeter and throughout the grid areaSQRT(1+h/ha) where ha=1 metre.

1.342

SQRT(N1*N2)11

0.656+0.172*n2.548

KM*KI*ROW*I*1000/L1,222.491 volts

223.910.656+0.172*n

2.548(1/3.14)*(A+B+C(1-E))

1/(2*h)0.625

(1/(D+h))0.093

1/D0.100

0.5^(n-2)0.0019531

0.2603KS*KI*ROW*I*1000/L

515.599479.72

(KM*KI*ROW*I*1000)/Permissible Emesh7,395.77 metres

ROW*(F+G*(1+(1/J)))

(1/L)0.00035

1/(SQRT (20*A))0.00224

1+(h*(SQRT (20/A)))1.0360.213 ohm

I*RG10,665.86 volts


1 Parameters for design purposes:1.1 Soil resistivity ROW Ohm meter1.2 Soil resistivity due to concrete ROWS Ohm meter1.3 Fault current I KA1.4 Fault duration for sizing TF1 sec1.5 Fault duration for step and touch potentials TF2 sec1.6 Depth of burial of grid conductors h metres1.7 Thickness of concrete hs metres1.8 Number of ground rod/pipe electrodes,3m long N nos1.9 Area of the earth mat AG sq.metres

1.10 Effective earth fault current Ia kA2 Evaluation of conductor sizing

2.1 Factor K0

2.2 TCAP is defined byTCAP =

2.3 Minimum area of cross section A =where

2.4 X1 =

2.5 X2 =

2.6 Minimum area of cross section A sq.mm2.7 Diameter of conductor mm

2.8 Temperature co-efficient of material ALPHAR2.9 Resistivity of earthing material ROWE micro-ohm/

2.10 Specific heat of material SH cal/gram/C2.11 Density of material SW gram/cc2.12 Reference temperature of material constant TR C2.13 Max.allowable temperature for welded joints TM C2.14 Ambient temperature TA C



3 Permissible step and touch voltages3.1 E step =3.2 E touch =3.3 CS is a reduction factor depending upon the depth of gravel /concrete and reflection factor K.

CS is obtained from Figure 8 of IEEE. =3.4 K =

CS =1 for no gravel/ concrete.

ALTERNATIVELY,CS =a=0.106 metre;s=(ROW/ROWS);U=(2hs+a)s =u =a =CS =Choose a lower value of CS from the above.Hence CS=0.82

3.5 Permissible Estep volts3.6 Permissible Etouch volts

4 Preliminary layout4.1 Length of the earth mat L metres4.2 Breadth of the earth mat B metres4.3 Assumed spacing for the conductors D metres4.4 Total no of conductors parallel to length N1 nos

4.5 Total no of conductors parallel to breadth N2 nos

4.6 Total quantity of conductors laid LC metres

4.7 Interconnections to the grid LINT metres4.7 Total ground rods length LR metres4.8 Effective grid conductor length L metres


5.3 KM =

5.4 where A =

5.5 B =

5.6 C =

5.7 D =

5.8 E ==

5.9 KM ==


5.11 Kh ==

5.12 Number of parallel conductors n =

5.13 KI,the irregularity factor =

5.14 Hence E mesh =

PERMISSIBLE VALUE5.15 KI,the irregularity factor =

5.16 KS =WHERE

5.17 A =

5.18 B =

5.19 C =

5.20 E =

5.21 KS =5.22 Hence E step

PERMISSIBLE VALUE6 Minimum conductor length requirement

To achieve the above conditions,the following condition has to be satisfied.

6.1 L =

7 Evaluation of grid resistance7.1 RG ohm

where7.2 F =

7.3 G =

7.4 J =

7.5 Hence RG =

8 Grid potential riseTotal grid potential rise IR Volts

73.0500

50.00 1.000.700.800.20

10200,000

23.40

(1/ALPHAR)-TR216.41

4.184*SW*SH3.75

I*(SQRT(X1/X2))

(TF1*ALPHAR*ROWE*10000)/TCAP169.24

LN(1+(TM-TA)/(K0-TA))1.31

568.4326.91

0.0042315.000.114

7.8620

50050

Average rate of corrosion of steel is 61 mils for the first 12 years and 50% of that value 30.5 mils for the next 12 years.After this period,it is assumed that no corrosion takes place.


CS is a reduction factor depending upon the depth of gravel /concrete and reflection factor K.0.82

(ROW-ROWS)/(ROW+ROWS)-0.745

1-a*((1-s)/u) where

0.14600.506

0.110.821

479.72223.91

500.00400.0021.00

(B/D)+120

(L/D)+125

(N1*L)+(N2*B)19,948

5,98430.00

25,966

(ROW*KM*KI*I*1000)/L(ROW*KS*KI*I*1000)/L

(1/(2*3.14))*(LN(A+B-C)+D*LN(E))

D^2/(16*h*d)861.328

(D+2*h)^2/(8*D*d)76.0060

(h/4*d)5.000

(Kii/Kh)0.745

8/(3.14*(2n-1))0.058

(1/(2*3.14))*(LN(A+B-C)+D*LN(E))0.752

Kii=1 for grids with ground rods along the perimeter and throughout the grid areaSQRT(1+h/ha) where ha=1 metre.

1.342

SQRT(N1*N2)22

0.656+0.172*n4.492

KM*KI*ROW*I*1000/L474.659 volts

223.910.656+0.172*n

4.492(1/3.14)*(A+B+C(1-E))

1/(2*h)0.625

(1/(D+h))0.046

1/D0.048

0.5^(n-2)0.0000008

0.2288KS*KI*ROW*I*1000/L

144.479479.72

(KM*KI*ROW*I*1000)/Permissible Emesh25,760.69 metres

ROW*(F+G*(1+(1/J)))

(1/L)0.00004

1/(SQRT (20*A))0.00050

1+(h*(SQRT (20/A)))1.0080.076 ohm

I*RG3,776.08 volts

ABB-PE EARTHING DESIGN FOR NEYVELI IN POWER BLOCK AREA document.xls

Page 18


1 Parameters for design purposes:1.1 Soil resistivity ROW Ohm meter 731.2 Soil resistivity due to concrete ROWS Ohm meter 5001.3 Fault current I KA 50.00 1.4 Fault duration for sizing TF1 sec 1.001.5 Fault duration for step and touch potentials TF2 sec 0.701.6 Depth of burial of grid conductors h metres 0.801.7 Thickness of concrete hs metres 0.201.8 Number of ground rod/pipe electrodes,3m long N nos 501.9 Area of the earth mat AG sq.metres 120,000

1.10 Effective earth fault current Ia kA 23.402 Evaluation of conductor sizing

2.1 Factor K0 (1/ALPHAR)-TR216.41

2.2 TCAP is defined byTCAP = 4.184*SW*SH

3.752.3 Minimum area of cross section A = I*(SQRT(X1/X2))

where2.4 X1 = (TF1*ALPHAR*ROWE*10000)/TCAP

169.242.5 X2 = LN(1+(TM-TA)/(K0-TA))

1.312.6 Minimum area of cross section A sq.mm 568.432.7 Diameter of conductor mm 26.91

2.8 Temperature co-efficient of material ALPHAR 0.004232.9 Resistivity of earthing material ROWE micro-ohm/ 15.00

2.10 Specific heat of material SH cal/gram/C 0.1142.11 Density of material SW gram/cc 7.862.12 Reference temperature of material constant TR C 202.13 Max.allowable temperature for welded joints TM C 500


Page 19

2.14 Ambient temperature TA C 50



3 Permissible step and touch voltages3.1 E step = (1000+(6*CS(hs,K)*ROWS))*0.116/(SQRT TF2)3.2 E touch = (1000+(1.5*CS(hs,K)*ROWS))*0.116/(SQRT TF2)3.3 CS is a reduction factor depending upon the depth of gravel /concrete and reflection factor K.

CS is obtained from Figure 8 of IEEE. = 0.823.4 K = (ROW-ROWS)/(ROW+ROWS)

-0.745CS =1 for no gravel/ concrete.

ALTERNATIVELY,CS = 1-a*((1-s)/u) wherea=0.106 metre;s=(ROW/ROWS);U=(2hs+a)s = 0.1460u = 0.506a = 0.11CS = 0.821Choose a lower value of CS from the above.Hence CS=0.82

3.5 Permissible Estep volts 479.723.6 Permissible Etouch volts 223.91


Page 20

4 Preliminary layout4.1 Length of the earth mat L metres 400.004.2 Breadth of the earth mat B metres 300.004.3 Assumed spacing for the conductors D metres 13.004.4 Total no of conductors parallel to length N1 nos (B/D)+1

244.5 Total no of conductors parallel to breadth N2 nos (L/D)+1

324.6 Total quantity of conductors laid LC metres (N1*L)+(N2*B)

19,1624.7 Interconnections to the grid LINT metres 5,7484.7 Total ground rods length LR metres 150.004.8 Effective grid conductor length L metres 25,083

5 Estimation of actual mesh and step voltages5.1 Emesh = (ROW*KM*KI*I*1000)/L5.2 E step = (ROW*KS*KI*I*1000)/L

5.3 KM = (1/(2*3.14))*(LN(A+B-C)+D*LN(E))

5.4 where A = D^2/(16*h*d)330.078

5.5 B = (D+2*h)^2/(8*D*d)51.2404

5.6 C = (h/4*d)5.000

5.7 D = (Kii/Kh)0.745

5.8 E = 8/(3.14*(2n-1))= 0.047

5.9 KM = (1/(2*3.14))*(LN(A+B-C)+D*LN(E))


Page 21

= 0.5815.10 Kii=1 for grids with ground rods along the perimeter

Kii=1 for grids with ground rods at the grid cornersKii=1 for grids with ground rods along the perimeter and throughout the grid area

5.11 Kh = SQRT(1+h/ha) where ha=1 metre.= 1.342

5.12 Number of parallel conductors n = SQRT(N1*N2)28


5.14 Hence E mesh = KM*KI*ROW*I*1000/L457.814 volts

PERMISSIBLE VALUE 223.915.15 KI,the irregularity factor = 0.656+0.172*n

5.4135.16 KS = (1/3.14)*(A+B+C(1-E))

WHERE5.17 A = 1/(2*h)

0.6255.18 B = (1/(D+h))

0.0725.19 C = 1/D

0.0775.20 E = 0.5^(n-2)

0.0000000 5.21 KS = 0.24665.22 Hence E step KS*KI*ROW*I*1000/L

194.262PERMISSIBLE VALUE 479.72


Page 22

6 Minimum conductor length requirementTo achieve the above conditions,the following condition has to be satisfied.

6.1 L = (KM*KI*ROW*Ia*1000)/Permissible Emesh24,000.74 metres

7 Evaluation of grid resistance

7.1 RG ohm ROW*(F+G*(1+(1/J)))where

7.2 F = (1/L)0.00004

7.3 G = 1/(SQRT (20*A))0.00065

7.4 J = 1+(h*(SQRT (20/A)))1.010

7.5 Hence RG = 0.097 ohm

8 Grid potential riseTotal grid potential rise IR Volts I*RG

4,833.56 volts

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Earthing Calculation