DATA1.0SYSTEM DATADescriptionSymbolUnitValueRms value of symmetrical fault currentIkA=25Fault clearing time for conductor selectiontrsec=1Fault diversion factorSf=0.71.1SOIL DATASoil Resistivity (Actual Value)W-m=16.3Soil Resistivity (Design value for safety)rW-m=40Surface resistivityrsW-m=3000Thickness of Surface layerhsm=0.11.2GRID CONDUCTOR DATAThe Conductor Selected is 40 mm dia Mild steel RodMax allowable temperature of the conductorTmoc=500Ambient temperatureTaoc=50Reference temperature for material constantTroc=20Coefficient of resistivity at Tro C (Mild Steel)ar=0.0045Resistivity of ground conductorrrm W-cm=13.8Thermal capacity factorTCAPJ/cm3/oC=3.749Diameter of grid conductor in mdm=0.04Spacing of grid conductor ChosenDm=4.4Depth of burial of Grid conductorhm=0.6Duration of shocktsSec=0.51.3SWITCHYARD DATA(Refer Annexure-2 for Earthgrid layout) )Length of switchyard earthmatm=41Width of switchyard earthmatm=21Area of substationASq.m=861Grid conductor periphery lengthLpm=124Maximum distance along grid lengthLxm=41Maximum distance along grid widthLym=211.4EARTH ELECTRODE DATADiameter of Pipe/rod earth electrode in mDem=0.04Length of each rod conductor in mLrm=3No.of Earth electrodes (Earth station)NNos=73Total length of ground rodsLRm=234(Refer Annexure-1 for No. of Earth Electrodes)

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CALCULATION1.0SYSTEM DATA2.0METHODOLOGY:DescriptionSymbolUnitValueRms value of symmetrical fault currentIkA-402.1The Earthing Design Calculation is done as per the recommendations given in IEEE Std. 80,2000Fault clearing time for conductor selectiontrsec-11.1SOIL DATA2.2The Corrosion Allowance is Considered for Mild Steel as per the recommendations given in CBIP Manual Publication No.223Soil Resistivity (Average)rW-m-90(Refer Annexure 2 for Soil resistivity data)2.3The Following are considered for determination of grounding conductor lengthSurface resistivityrsW-m-3000Thickness of Surface layerhsm-0.15hTotal Length of Earth mat1.2GRID CONDUCTOR DATAhLength of Auxiliary Earth mats for IsolatorsThe Conductor Selected is 40 mm dia Mild steel RodhLength of Earth ElectrodesMax allowable temperature of the conductorTmoc-500Ambient temperatureTaoc-503.0CALCULATION:Reference temperature for material constantTroc-20Coefficient of resistivity at 0o C (Mild Steel)a0 (1/K0)-0.004233.1Selection of cross section of grid conductor (A)Coefficient of resistivity at Tro C (Mild Steel)ar-0.0039As per IEEE 80-2000Resistivity of ground conductorrr-15The Crosssection of Conductor is Given byThermal capacity factorTCAPJ/cm3/oC-3.749Diameter of grid conductor in mdm-0.04A=mm2Spacing of grid conductor ChosenDm-10(assumed)Depth of burial of Grid conductorhm-0.6Duration of shocktsSec-1.025A=mm23.749/10000 ln 202.22+5001x0.0045x13.8 . 202.22+50A=318. Sqmm1.3SWITCHYARD DATA(Refer Annexure-1 for Earthgrid layout) )WhereLength of Rectangle - 1 (400 kV Side)m-256K0=1 / ar - TrLength of Rectangle - 2 (220 kV Side)m-260=202.22Length of Rectangle - 3 (Control Room Side)m-1530Width of Rectangle - 1 (400 kV Side)m-82As per CBIP "Manual on substation" publication number 223, corrosion allowance isrecommended as per following ruleIn case of conductor to be laid in soil having soil resistivity greater than 100W-m- No allowanceIn case of conductor to be laid in soil having soil resistivity from 25 to 100W-m- 15% allowanceIn case of conductor to be laid in soil having soil resistivity lower tha 25W-m-30% allowanceconsidering soil resistivity for conductor sizing as 40 W-mA mm2=1.15 x A=1.15 x 317.97=366. SqmmFor 366 Sq.mm, Size of the Conductor Required is=366 x 4Width of Rectangle - 2 (220 kV Side)m-1323.14Width of Rectangle - 3 (Control Room Side)m-93.5=21.6 mmArea of substationASq.m-69617.5Selected Conductor Size is=40 mm dia MS RodHence safe.3.2Determination of grounding conductor length:The Earth Mat spacing considered for safe Step & Touch Potential=4.4 m x 4.4 mArea of SubstationA=861. SqmGrid conductor periphery lengthLp=124 mMaximum distance along grid lengthLx=41 mMaximum distance along grid widthLy=21.0 mLength of each ground rodLr=3 mTotal length of ground rodsLR=234 mSpacing of grid conductor chosenD=4.4Estimation of Grid Conductor LengthRectangle - (41m x 21m)The number of conductors parallel to the breadth=6The number of conductors parallel to the length=11Length of Conductor for Rectangle=477 mGrid conductor Combined Length (Lc) used in the switchyardLC=477 mEffective length of the buried Conductor in mLT=LC + LR=711 m3.3Determination of Maximum Grid current (IG)IG=Df.Sf.IfWhereDf-Decrement factor for Tf=1Sf-Current division factor=1If-Symmetrical Ground fault current=25 kAMaximum Grid currentIG=1 x 1 x 25=25 kAShock duration time ts = 1 sects=sec0.53.3Calculation of Tolerable Touch voltage(Et) & Step voltage(Es)k=From graph in IEEE - 80 corresponding to k = -0.983 and hs = 0.15Cs=k=-0.9740.770The Surface Layer Derating factor Cs is given byCs=Cs=Cs=0.694Tolerable Touch Voltage Et is given byEt50==Et50=676 VTolerable Step Voltage Es is given byEs50==Es50=2213 V3.4Determination of Grid resistance (Rg)Rg=Rg=Rg=0.690 OhmsSince the Grid resistance Rg is Less than 1W,the design is safe.3.5Maximum Grid current (IG):IG=sf x I=0.7 x 25=17.50 kA3.6Calculation of Attainable Mesh voltage(Et) & Step voltage(Es)3.6.1Determination of the effective No.of Parallel Conductor (n)n=na x nb x nc x ndWherena== 2 x (477 / 124)=7.69nb== (124 /(4 861))=1.03nc==For Square and Rectangular gridsnc=1=1nd=1 (For Square,Rectangular & L-shaped grids)nd=1n=7.9213.6.2Determination of Ki:Ki==0.644 + (0.148 x 7.921)=1.823.6.3Determination of Km:Km=WhereKii=1Kii=1(For grid with ground rods along the perimeter, or for grids with ground rods in the gridcorner, as well as both along the perimeter and throughout the grid area)Kh==1+(0.6/1)=1.265Where ho=1m (Grid reference depth)Km=Km=0.453.6.4Determination of Ks:Ks==Ks=0.43.6.5Determination of the effective buried Length of conductor (Lm for Touch Voltage)For grids with Earth electrodesLm==Lm=858 m3.6.6Determination of the effective buried Length of conductor (Ls for Step Voltage):Ls=0.75Lc + 0.85LR for grids with ground rods=(0.75 x 477) + (0.85 x 234)Ls=557 mAttainable Mesh voltage Em is given byEm== (40 X 0.45 X 1.82 X 17.5 X 1000) / 858.29Em=668 VAttainable Step voltage Es is given byEs==(40 X 0.4 X 1.82 X 17.5 X 1000) / 557Es=915 V4.0CONCLUSION:a)Em vs Et50The Calculated Mesh Voltage of 668 V is lower than the tolerable Touch Voltage 676 V.Hence the design is safe.b)Es vs Es50The Calculated Step Voltage of 915 V is lower than the tolerable Step Voltage 2213 V.Hence the design is safe.c)Grid Resistance RgThe Calculated Grid Resistance of 0.69 Ohms is less than 1 OhmsHENCE THE DESIGN IS SAFE

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Annex-1ANNEXURE - 1Estimation of No. of Earth ElectrodesS.NODESCRIPTIONNO. OF EARTH ELECTRODE1.1LA401.2CT&PT181.3ISMB POLE , T/F,ISO WITH E/S15Total No. of Equipment Earth Electrodes73Total length for 3 m long earth electrodes.219Grid corners & along periphery of 1 m long rod15NO. OF EARTH ELECTRODES220 kv1line 1-4MBGRIDLA12CVT6CORNERS0CVT12COMMONLM4TOWER92461343400 kVBALL-2BALL-1ICT1ICT2MBLA37106CVT3636REAC242TRAFO615178652TOWER27CORNERS887130Total Length of Earth Electrodes234

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MBD000FD35E.unknown

MBD00119F60.unknown

MBD001536CA.unknown

MBD001690B3.unknown

MBD0016D007.unknown

MBD001586CD.unknown

MBD00133500.unknown

MBD0015126B.unknown

MBD00121EFC.unknown

MBD00120EF9.unknown

MBD001011D9.unknown

MBD00113774.unknown

MBD0010038E.unknown

MBD0004FB2F.unknown

MBD0006EEAF.unknown

MBD000ACB4C.unknown

MBD000B1AC1.unknown

MBD0009CCD4.unknown

MBD000A116F.unknown

MBD00096E1C.unknown

MBD00054103.unknown

MBD0004D77D.unknown

MBD0004F1B9.unknown

MBD00048FBD.unknown