PROJECT: 6MW SOLAR PHOTOVOLTAIC POWER PLANT CLIENT : ARHYAMA SOLAR POWER PRIVATE LIMITED EPC CONTRACTOR : CIRUS SOLAR PRIVATE LIMITED CONTRACTOR : PnU ENGINEERING PVT LTD, HYDERABAD DOCUMENT TITLE : EARTHING LAYOUT 33KV EARTHING CALCULATIONS 22/02/2013 0 EARTHING CALCULATIONS AJAY RRK RRK DATE Rev : NO DESCRIPTION DESIGNED CHECKED APPROVED DOCUMENT NO : PU-13-02-007 Sheet 1 of 1 0 DESIGN BASIS 1) Resistivity data has been taken from the available report .6 m Ground Grid Area = 148.6 M Sheet 1 of 1 .6 M (Fig-1) Switch yard Length of the area = 14 m Width of the area = 10. Vishnupuram b) IEEE80-2000 c) CBIP Manual on substation publication number : 223 1.4 m² Assuming conductor spacing D 3 No. 14 M EQUIVALENT AREA 10. of conductors parallel to length side( nl) = 5 No.BLOCK1 : 33KV Switchyard Reference Documents a) Electrical resistivity test report at plant site for 132kv Switch Yard. of conductors parallel to width side ( ns) = 6 m Therefore buried length in grid area Lc = (nl x length)+(ns x width) Total buried length LC = 133. 015758 mm2 900 mm2 L*B 75 X 12 GI FLAT Sheet 1 of 1 .37 of IEEE Std 80-2000. Temperature.mm 420. tc = 1 sec (v) Max.93 J/cm3/oC (xi) K0 = (1/αr) -Tr = 292. αr (ix) o 0. Tm = 620 0 C (vi) Ambient Temperature.Subject : EARTHING DESIGN CALCULATION FOR 33 KV SUBSTATION 1) IEEE 80-2000 2) CBIP Manual on substation publication number : 223 1.of resistivity at reference = Temperature.5 o C Conductor Size :As per Eq. Ta = 50 0 C (vii) Reference temp for material constant. conductor size is given by tc αr ρr x 104 TCAP A = If Ko + Tm ln Ko + Ta Substituting the input data. Allowable Temperature. we get Conductor size. TCAP = 3.Tr = 20 0 C (viii) Thermal Co-eff. If = 25 kA (iv) Fault Duration.1 µΩ/cm3 (x) Thermal Capacity Factor.0) Earthing conductor sizing for main earth gridInput data :(i) Conductor material = GI (ii) System Voltage = 33 kV (iii) Fault Current. ρr = 20. A By considering 30% allowance A A A Hence the CONDUCTOR size selected.0032 1/ C Resistivity of Ground Conductor at ref. 323 = = = = sq. 6 metre (iv.1) Length of Earth grid = 14 metre (iv.t. tc(as per Specification) = 1 sec (iv.75 0 2. Kii = 1 (xiii) Reference Depth of grid.75 0 = 2.r. hs = 0.2 2. LM = 184. of ground electrodes LA Generator Transformers Switchyard corners Towers (viii.2) length of each rod in m Lr (viii. ρs = 3000 Ω-metre (xi) Spacing for Earthing Mat .1) Total no.1) Length of one earth mat conductor Lx = 14 metre (vii.mm (vi) Depth of burial w. LR Effective length of buried conductorLT = LR + Lc = length in M no.2) Width of Earth grid = 10.4) (viii.6 metre (vii.75 metre = 157 metre (viii. ground level.2) Width of one earth mat conductor Ly = 10.4 sq.6 metre (vii. LS = 119.3) Area of Earth grid. If = 25 KA (iii) Fault Duration.4) Effective Buried Length for step voltage.75 3 2.15 metre (xvii) Fault Current division factor (Sf) = 0.3) Effective Buried Length for mesh voltage.D = 3 (xii) Corrective weighting factor.75 2 2.3) Total ground rod length. of electrodes 2.39 metre (ix) Soil Resistivityconsidered for design.3) Additional length of conductor considered (for interconnecting earthing system) Grid conductor combined length.02 metre (viii. ρ = 10 Ω-metre (x) Surface Resistivity.1) CALCULATIONS FOR TOLERABLE TOUCH VOLTAGE (Etouch ) AND STEP VOLTAGE( Estep) Input Data : (i) System Voltage. Lc 10 metre 144 metre (vii. h0 = 1 metre (xvi) Thickness of Concrete . A = 148.metre (v) Conductor Size for main grid = 900 sq. Vs = 33 kV (ii) System fault Current.7 metre Sheet 1 of 5 . h = 0.75 meter = 13. 09 [(1-ρ/ρs)/(2hs + 0.2.116/ √tS ] Tolerable Etouch50= ( From eq.08 m 2 148.53 of IEEE Std 80-2000.4 m 1.32of IEEE Std 80-2000 ) Where. Tolerable Etouch50 = 732.54 of IEEE Std 80-2000 A Where.3) Calculation of Grid Resistance.2Rm Where.5CS ρS) 0. R1 and group of ground rods.0. ( From eq.29 of IEEE Std 80-2000) Substituting the values. Tolerable Estep50 2437.13 Coefficient from fig 25(b) of IEEE Std 80-2000 4.05*(Lx/Ly) + 1. R1 = k1 Lc 2 Lc ρ R ln ΠLc .76 Volts = 2.5 ( from clause 16.2h for conductors buried at depth h in m Diameter of conductor in m Area covered by conductors in m2 Coefficient from fig 25(a) of IEEE Std 80-2000 k2 = 10 144 ohm-m m 0.09)] Cs = 0. Cs = Reduction factor for derating the nominal value of surface layer resistivity.116/ √tS ] = (from eq.3341 Ω Sheet 2 of 5 .27 of IEEE Std 80-2000 ) Cs = 1 .48 Volts b) Tolerable Estep50 [(1000 + 6CS ρS) 0.40 Coefficient K1 and k2 of schwarz's fromula IEEE Std 80-2000 ) R1 = 0.05*(Lx/Ly) + 4.33 k2= -0.k2 + a ' From eq. ρ Lc = = Soil Resistivity in Ω-m Total length of all connected grid conductors in m a' 2a = = A k1 = = a.06 k1= -0.3000 m 0. Rg : As per Eq.Rm2 The total system resistance.2 of IEEE Std 80-2000 ) Substituting the above values. R1 = Ground resistance of grid conductors in Ω R2 = Ground resistance of of all ground rods in Ω Rm = Mutual ground resistance between the group of grid conductors. R1 R2 . Rg = R1 + R2 . R2 in Ω Ground resistance of the grid.2.2) a) Calculation of Tolerable Etouch & Estep Voltage : Tolerable Etouch50 : From IEEE Std 80-2000 [(1000 + 1.7700 tS = Duration of shock current in seconds = 0. k2 + 1 + π Lc Lr A Rm = 0. Lr Lx Ly = Length of each ground rod = = Maximum length of grid conductor in x-direction = Maximum length of grid conductor in y-direction = = 20 m 14 10. R2 From eq. for Grid rods in the periphery.3403 Ω 2.4) Calculation of Maximum Ground Potential Rise : Max. Rm From eq.56 of IEEE Std 80-2000 ρ Rm k1 Lc 2 Lc = ln . IG = grid current If x Sf = IG = = IG x RG Volts 17.91 of IEEE Std 80-2000 L2x + L2y Where. Emesh = [ ρ Km Ki IG / LM ] Lr LM = Lc + 1.55 of IEEE Std 80-2000 ρ R2 4 LR = ln 2 k1 .71 Volts Since.163 Ω Mutual ground resistance between the grid and the rod bed.06 0. Lr 2b nR = = = length of each rod in m diameter of rod in m number of rods placed in area A 2.5) Calculation of Attainable Emesh Voltage : From Eq. Lr ( nR .22 Lr From eq. Ground potential above remote earth.55+1. GPR = 5954.3071 Ω Subtituting the above values.75 0. Rg = 0. GPR Where.Ground resistance of the rod bed.5 KA Substituting the values. Ground Potential Rise. GPR > Estep50 (as per Figure 33 of IEEE Std 80-2000).75 k1 = Coefficient from fig 25(a) of IEEE Std 80-2000 R2 = m m 1.80 of IEEE Std 80-2000.08 5 LR = Total length of ground rods 13.6 m m Sheet 3 of 5 .calculate Attainable Emesh and Attainable Estep voltages 2.1 )2 -1 + b 2π nRLR A Where. 84 nb = 1.0339 metre ( From eq.85 to 88 of IEEE Std 80-2000 na (2 Lc)/Lp = nb = 4 A 0.84 of IEEE Std 80-2000) Lp From eq. d Kh = 1 + h / h0 Kii h - 1. Km AttainableEmesh Voltage Sheet 4 of 5 .6 m Rounded of to nearest digit 0. na = 5.005 nc = 1 nd = 1.5 Ki = corrective Factor. 8 ln 4d Equ.87 = 6 As per Eq.23 Volts From eq. of Parallel Paths. The corrective Factor.83 of IEEE Std 80-2000 ) Kh = No. LM 184.IG = grid current = 17.4251 = 611.89 of IEEE 80-2000.00 n = 5.89of IEEE Std 80-2000 Substituting the values. the Spacing Factor for Mesh Voltage. n = Where. Diameter of Earth conductor.26 na nb nc nd (From eq.51 = 13. D2 1 Km = (D + 2h)2 ln + 16hd 2π 8Dd + π(2n-1) Kh = 0.7 A Lx Ly Lx Ly nc Dm nd = I L2x + L2y A Where. Ki = = 49. Lp = Dm = Peripheral length of the grid Maximum distance between any two points on the grid Substituting the above values.2 m = 17.02 = m m As per Eq.75 LR = Total length of ground rods Subtituting the above values. = 1.644 + ( 0.148 x n ) = 1.51 = 0.81 of IEEE Std 80-2000. 47 Volts CONCLUSION : Tolerable Etouch = 732.92 of IEEE Std 80-2000.6) Calculation of Attainable Estep voltage : From Eq.47 Volts SAFE 0. Hence the design value of conductor size and length of conductor are adiquate and safe.3403 ohms SAFE Grid Resistance = SAFE 1) From the above results.2. Sheet 5 of 5 .76 Volts Attainable Estep = 1003.85 LR From eq.0.5n-2) + D+h 2h From eq. 1 1 Ks = 1 + Π 1 (1 .48 Volts Attainable Emesh = 611.92 of IEEE Std 80-2000 As per Eq.644 + ( 0.94of IEEE Std 80-2000 D No. Attainable Estep 3 = 1003.4527 As per Eq. Ki = = = 6 6 0.3875 m Substituting the values.51 Ks = 0.75 Lc + 0.23 Volts Tolerable Estep = 2437.it can be seen that the Attainable Emesh and Voltages are less 2) 3) than the Tolerable Etouch. Ls = Lc = 0.89 of IEEE 80-2000. the Spacing Factor for Step Voltage.148 x n ) Ki = 1.93of IEEE Std 80-2000 Total length of all connected grid conductors in m LR = Total length of ground rods Ls = 119. of Parallel paths. The corrective Factor. Attainable Estep = [ ρ Ks Ki IG / LS ] From eq.93 of IEEE Std 80-2000.94 of IEEE Std 80-2000. n n As per Eq. The grid resistance is less than 1 ohms.
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