Earth Mat Calculations

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EARTH MAT DESIGN CALCULATION FOR 33 KV S/STN

for M/s Sukhbir Agro Energy Ltd, Shahjahanpur, U.P.

1. The Average Soil Resistivity measured as : 86.31 Ohm mtr. 2. Design Calculation are based on IEEE80-2000.

3. Maximum Symmetrical Fault current : 31.5 kA

4. Duration of Fault :

For Earth Mat Design for HV S/Stn. Is to be considered as 1 Sec.

For the purpose of determining the Safe Step & Mesh Potential a duration of .5 Sec is considered.

5. Maximum GRID Current

DF = 1.05 (Decrement factor for entire duration) SF = 0.8 split factor

Ig = 31.5 KA (Single Line to Ground Fault)

IG = Df x Sf X Ig = 31.5 x 1.05 x 0.8 = 26.46 KA.

6. Calculations of Conductors Size of Sub-Soil Ground

Area : A = IG 1 √ (TC x 10-4_{) Logn (KO+Tm)} (TCxαRxPR) (KO+Ta) = 26.46 x 1 √ (3.28 x 10-4_{) Logn (605+1500)} (3x0.0016x15.9) (605+40) = 370.35 sq.mm

Recommended Size of GI Flat = 400 Sq.mm = 50 x 8 mm2 Dia = 0.02257 mt

Dy = Spacing between // Conductors on shorter side = 1 mt. Dx = Spacing between // Conductors on Longest side = 1 mt.

Nx = No. of sub-soil ground conductors on shorter side = 12 Ny = No. of sub-soil ground conductors on longest side = 33

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Control Room:

Nx = 6+1= 7 Ny = 6+1= 7

N= √ (Nx x Ny )= √ (7 x 7) = 7

Area of Switchyard (31.8 x 10.4) : 330.72 sq. Mtr. Area fo Control Room (6 x 6 ) : 36 sq. Mtr.

Total Area : 366.72 sq. Mtr.

Avg. Spacing = 1 mt Dep. Of Burial = 0.6 mt

Equivalent Dia of Strip = Πd2_{/A = 400mm}2_{= 0.02257 mt} Dia = 0.02257 mt

7. Calculation for Resistance of Earth Mesh Rmesh =

[

√

{

_√

}]

= 86.31 ( 1___+ 1_____ ( 1 + 1 ) 1045.3 √20x366.72 1+0.6 √20/ 366.72 = 1.974 Ω

Hence, the resistance is more than 1 Ω, The DESIGN IS NOT SAFE. More earth rods to be provided to bring down resistance to less than 1 ohm.

Resistance of Grounding Rod

R = ρ [ logn (8L/d)-1] d= 0.04 mtr 2πL

= 86.31 [ logn (8x 3/0.040)-1] 2π x 3

= 24.724 Ω

8. Actual Mesh Potential

E mesh actual =( Pa x Ig x Km x Ki) Lt

Km = 1(logn (D²) + (D+2h) ² - h) + Ki/kh logn ( 8 ) 2π (16hd 8Dd 4d) π (2n-1)

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Km1 = 1(logn (D²) + (D+2h) ² - h) D= 1 mtr, N= 19.89 2π (16hd 8Dd 4d) Client = 1 (logn [ (1²) + (1+2x0.6) ² - 0.6 ) 2π (16x0.6x0.02257) 8x1x0.02257 4x0.02257) + 1__ logn (8/ π(2x19.89 – 1) 1.264 = 0.168 Ki = .656+.172*19.89= 4.07

E mesh actual =( Pa x Ig x Km x Ki) Lt = 86.31 x 26.46 x .168 x 4.07 1045.3 = 1493.42 Volts 9. Emesh Tolerable = 165+ 0.25 x 3000 = 1294 Volts 0.707

10. Actual Touch Potential = (Ig x km x k2 x P)/ L

= 86.31 x 26.46 x .168 x 4.07 1045.3 = 1493.42 Volts

E Touch tolerable = (1000 + 1.5Cs x 3000) x 0.157/

√ts

= (1000 + 1.5 x 0.873 x 3000) x 0.116/ 0.707

= 808.63 V

Cs= 1-0.106 [ 1-(136.7/3000)/2x0.6+0.106]

= 0.873

E touch Actual = 1493.42 V, Hence design is safe.

11. Attainable Step Voltage (Actual Step Potential)

Es = Ks x Ki x P x I Lt

Ks = 1 (1 + 1 + 1 ( 1- 0.5) n-2 π ( 2h D+h D

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Ks = 1 (1 + 1 + 1 ( 1- 0.5) 19.89-2 3.142 ( 2x.6 2+.6 2

= .3183(.833+.384+.9999) Ks = .7056

Ki = 4.07

Actual STEP Potential (Estep) = 86.31X26452X0.7056X4.07 1045.3

= 6272.3 Volts

Tolerable E step = (1000 + 6Cs x 3000) x 0.116/

√ts

Cs= 1-0.106 [ 1-(136.7/3000)/2x0.6+0.106]

= 0.873

E touch tol. = (1000 + 6(0.873) x 3000) x 0.116/

√0.707

) = 2742.32 V

Since the Actual Step Potential is not less than tolerable Step Potential, hence design is not safe.

12. E-STEP at Fencing

E Step actual = (Pa x Ig x Ks x Ki) Lt

Ks = 0.7056 Ki = 4.07

= 0.7056 x 4.07 x 86.31 x 26.452 = 6222.58 Volts 1045.3

Estep Fencing Tolerable = 165 + Pa

/

√ts = 165+86.31/.707 = 355.459 volts

Since E-Step fencing Actual is more than E-Step fencing tolerable, then fencing shall be earthed separately with 4 Nos. Earth Rods at each corner of fencing.

13. Sustained Earth Fault current Ip= (9+.0135XCsXPs)Lt/Km x Ki x P

=(9+.0135X.873x3000x1045.3 )/(0.168*4.07*86.31) = 785.65 Amp

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The Relay clearing ground fault shall be set to a minimum Pick up current below the sustained earth fault current.

14. Equipment Earthing

Each equipment will be provided double earth with 50 x 8mm GI Strip.

This will be connected to Main Earth Mat.

200 mt Extra M.S. Flat 50 x 8 mm to be added for risers.

15. Length of conductor

20% margin for corrosion :

31.8 x 12 + 10.4 x 33 = 724.8 x 1.2 mtr = 869.76 sq. mtr Control Room : = (42 + 42) x 1.2 = 100.8 sq. mtr

Total Area = 970.56 sq. mtr

Length of Rods : 25 x 3 = 75

Total Length : 970.5 + 75 = 1045.3 mtr

16. Grid Potential Rise (GPR)

26.452 x 1.974 = 52.216 kV

GPR is more than 5 KV & hence the design is not safe. Hence more earth rods will be provided to bring down GPR less than 5 kV.

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Annexure-A

INPUT PARAMETERS

1. Symmetrical fault current = 31500Amp = 31.5 KA 2. Fault current diversion factor = 0.8

3. Duration of fault current for Short current rating = 3 Sec

4. Duration of shock current for determining allowable body current = 0.5sec 5. Surface gravel layer resistivity = 3000 Ω m

6. Soil resistivity = 86.31 Ωm 7. Surface layer thickness = 100mm

8. Depth of Burial of earth material = 0.6 mtrs 9. Max length in X direction = 31.8 mts 10. Max length in Y direction = 10.4 mts 11. No. of long conductors = 12 nos 12. No. of short conductors = 33 nos 13. Equivalent Dia of conductors = 0.02257 14. X Section of earth mat = 50 x 8 mm

15. Dia of 3 mtr. Long ground rods = 40 mm 16. Average Spacing = 1 mt.

17. Area of S/W Yard (31.8 * 10.4) + 36 = 366.72 sq.mtr. 18. Total length of strip = 1045.3 mts

19. Resistance of grounding rod = 25.739 Ω 20. Resistance of grid = 1.974 Ω

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AMPACITY – CALCULATIONS

1. Type of conductors = Mild Steel 2. Ref Temp for material constants = 20°C

3. Thermal coefficient of resistivity αr = 0.0016/°C 4. Fusing temp of conductor = 1510°C

5. Ambient Temp = 40°C

6. Resistivity of conductor at Ref temp = 15.9 uΩ m

7. Thermal capacity / Unit volume Tcap = 3.283 J/Cm3 °C 8. Ko = 605

9. Time duration for sizing of Earth Mat conductor = 3sec 10. Current sizing of Earth Mat Conductor = 26, 452 Amp. 11. Fault Level = 31.5 kA

12. Soil Resistivity = 86.31 Ωm

13. GPR = 52.216 kV

14. Resistance of grid = 1.974 Ω 15. Av. Spacing = 1mtr.

16. Extra Rods for Earthing of Fencing 3mtr x 40mm = 6 Nos.

17. Total Rods = 25 + 6 = 31 Nos.

18. Size of Main Electrodes = 3 mtr. X 40 mm dia 19. Emesh (Actual) = 1493.42 V

20. (Tolerable) = 1294 V 21. Estep (Actual) = 6272.3 V 22. (Tolerable) = 2742.32 V

23. Estep at Fencing (Actual) = 6222.58 V

24. (Tolerable) = 3554.59 V

25. Sustained Earth Fault Current = 785.65 Amp. 26. Fault Level = 31,500 A

27. Duration of shock for determining allowable body current = 0.5 sec 28. Max. Grid current = 26.452 kA