Earthing Yaher

LARSEN & TOUBRO LIMITEDECC Division - EDRC (Electrical)

PROJECT: 33/11kV S/S for Al Ain REF NO: G1409--RL7830J361A PAGE 02 OF 06PREPARED CHECKED APPROVED DATE

TITLE: EARTHING DESIGN CALCULATION FOR SKA AKS KR 21-02-2016

THIS DESIGN IS GENERALLY BASED ON IEEE 80 - 2000AREA OF CROSS SECTION

S = I x SQRT(t) / k

S = AREA OF CONDUCTOR IN Sq.mm

I = EARTH FAULT CURRENT IN AMPS

t = EARTH FAULT CLEARING TIME IN secs.

k = SQRT(( Qc x (B+20)/delta20) x ln ( 1+ (Qf-Qi)/(B+Qi)))

I = 31500 amps

t = 1 sec

Qc = 0.00345 J / C mm3 { Data for copper conductor }

B = 234.5 C

delta20 = 0.000020 Ohm.mm

Qi = 40 C

Qf = 700 C

k = SQRT( (0.00345x (234.5 + 20) /0.00002) x ln ( 1+ ((700-40 ) / (235 +40) ) )= 231.909

S = 31500 x SQRT (1.0) / 231.909= 135.83 Sq.mm

Corrosion allowance considered for the conductor in percentage = 10 %Minimum area of cross-section required = ### Sq.mmHence conductor size selected is, 300 Sq.mm (Which is the minimum size specified in the specification)

CONDUCTOR SIZE CHOSEN IS 50 x 6 Copper Flat

S/S -CONDUCTOR SIZING - YAHER

Qc = Volumetric Heat capacity of conductor ( J / oC mm3 )

B = Reciprocal of Temp.Coefft. of Resistivity ( oC )

delta20 = Electrical Resistivity of conductor at 20 oC ( Ohm.mm )

Qi = Initial Temperature of Conductor ( oC )

Qf = Final Temperature of Conductor ( oC )

LARSEN & TOUBRO LIMITED `ECC Division - EDRC (Electrical)



CALCULATIONS ARE BASED ON IEEE 80 - 2000.DESIGN DATASystem Voltage = 33 kV

System Grounding = SOLID

Soil Resistivity p = 11.85 Ohm.m

Top Layer resistivity ps = 2000 Ohm.m (Actual value of resistivity of crushed rockshall be used after measurements)

Max. Earth Fault Current (I) = 23625 AMPS (Current division factor 0.75 as per specification)

Conductor Size = 50 x 6 Copper Flat

Equivalent diameter d = 0.0195 m

Length of Substation L = 40 m Equivalent area for cond.laying = 57x46 - 38x27(Grid is extended by 0.5m on both sides)

Breadth of Substation B = 40 m Equivalent side = Sqrt (57x46 - 38x27) = 40m

No: of grids in X direction = 12

No: of grids in Y direction = 12

Equivalent Number of grids n = 12 i.e. SQRT(12 * 12)

Spacing of conductors D = 3.8 m

Total length of conductors Lc = #VALUE! m

Area of Grid Ag = #VALUE! sq.m

Depth of burial h = 0.5 m (As per specification)

Depth of Top Layer hs = 0.08 m

Total No of electrodes m = 15 Nos

Length of electrode l = 3 m

Total length of electrodes Lr = 45 m

Diameter of electrodes d1 = 20 mm

Duration of Fault Current ts = 1 sec (As per specification)

S/S - YAHER




Tolerable Touch Potential (BASED ON IEEE 80 - 2000 CHAPTER 8)

Etouch = ( 1000 + 1.5 * Cs * ps ) * 0.116 / SQRT(ts) For body weight of 50 kg

Cs = 1 - a * { ( 1- p/ps) / ( 2 * hs + a) } Refer page 41 IEEE 80 - 1986a = 0.106 m

1 - 0.106*{( 1 - 12 / 2000 ) / ( 2 * 0.08 + 0.106 ) }Cs = 0.6039

Etouch = (1000 + 1.5 * 0.6039 * 2000 ) * 0.116 / SQRT( 1.0 )= 326.14 Volts

Tolerable Step Potential (BASED ON IEEE 80 - 2000 CHAPTER 8)

Estep = ( 1000 + 6.0 *Cs * ps ) * 0.116 / SQRT(ts) For body weight of 50 kgEstep = (1000 + 6.0 * 0.6039 * 2000 ) * 0.116 / SQRT( 1.0 ) = 956.58 Volts

Mesh Potential Attainable (BASED ON IEEE 80 - 2000 CHAPTER 16)

Emesh = ( p * Km * Ki * I ) / ( Lc + 1.15 * Lr )

Km =

Kii = 1.00 for grids with ground rods in perimeter

Kh = SQRT ( 1+ h/ho)ho = 1.00 m ( reference depth of grid)Kh = SQRT( 1 + 0.500 / 1.0 ) = 1.2247

Km = 1/ (2 * 22/7) * { ln [ 3.8^2/ (16 * 0.500 * 0.02 ) + (3.8 + 2 * 0.500 ) ^2 /( 8 * 3.8 * 0.02) - 0.500/ ( 4 * 0.02) ] + 1.00 / 1.2247 * ln [ 8 / (22/7 * (2 * 12.00 - 1 ) ) ] }

= 0.4819Ki = 0.644 + 0.148 * n = 0.644 + 0.148 *12

= 2.4200Emesh = #VALUE!

= #VALUE! VoltsStep Potential Attainable (BASED ON IEEE 80 - 2000 CHAPTER 16)

Estep = ( p * Ks * Ki * I ) / ( Lc + 1.15xLr )

Ks =

n = 12 (Higher of the no.of grids in X & Y direction)Ks = ( 1 / 22/7 ) * { 1 / (2 * 0.500 ) + 1 / ( 3.8 + 0.500 ) + 1 / 3.8 * ( 1 - 0.5^(12 - 2 ) }

= 0.4758 Estep = #VALUE!

= #VALUE! Volts

S/S - YAHER

1/ (2 * ) * { ln[ D^2 /(16 * h * d)+ (D + 2h ) ^2 / (8* D * d) - h / (4 * d) ] + Kii / Kh ln[8 / ( * (2 * n-1) ) ] }

1 / * { 1 / (2 * h) + 1 / (D + h) + 1 / D * (1- 0.5^(n-2) ) }




Comparison of Tolerable Etouch & Estep and Attainable Emesh & Estep

Tolerable Etouch = 326.14 Volts

Tolerable Estep = 956.58 Volts

Attainable Emesh = #VALUE! Volts

Attainable Estep = #VALUE! Volts

Since Attainable Emesh is less than Tolerable Etouch andAttainable Estep is less than Tolerable Estep, the design is safe.

S/S - YAHER




CACULATION OF GROUNDING RESISTANCE

BASED ON IEEE 80-2000 CHAPTER 14

RESISTANCE OF GROUND RODS

Re =

K1 = 1.37 Refer Fig 25A page 67 IEEE 80 - 2000Re = 12 / (2 * 15 * 22/7 * 3 ) * [ ln (8 * 3 / 20 / 1000 )) - 1 + 2 * 1.37 *

#VALUE!= #VALUE! Ohms

RESISTANCE OF GROUND GRIDRs =

K1 = 1.37

K2 = 5.7 Refer Fig 25B page 67 IEEE 80 - 2000h' = SQRT(d * h)

= SQRT(0.02 * 0.50) = 0.0988 Rs = #VALUE!


MUTUAL RESISTANCE BETWEEN GROUND GRID AND GROUND RODSRse =

K1 = 1.37

K2 = 5.7Rse = #VALUE!


COMBINED RESISTANCE OF GROUND GRID AND GROUND RODS

Rg = (Re * Rs - Rse^2 ) / ( Re + Rs - 2 * Rse)Rg = #VALUE!= #VALUE! Ohms

CONCLUSIONHence the design adopted meets the specification requirement with respect to i) STEP & TOUCH POTENTIALii) COMBINED RESISTANCE OF GRID & GROUND RODS < 1.0 OHM

NOTEThe conductors shall be uniformly distributed in area of 57m x 46m (Extended on both sides by 0.5M) leaving the space for building area of 38m x 27m.i.e. Earth mat conductor as per above calculation shall be laid in s/s around the building adopting the calculatedspacing.

S/S - YAHER

p / (2 * m * * l ) * [ln (8 * l / d) - 1 + 2 * K1 * l / SQRT(A) * (SQRT(m)-1)^2 ]

p / ( * Lc) * [ ln (2 * Lc / h') + K1 *( Lc / SQRT(A)) - K2 ]

p /( * Lc) * [ln (2 * Lc / l) + K1 * (Lc / SQRT(A)) - K2 + 1]

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