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
BLOCK1
:
33KV Switchyard
Reference Documents a) Electrical resistivity test report at plant site for 132kv Switch Yard, Vishnupuram b) IEEE80-2000 c) CBIP Manual on substation publication number : 223
1.0 DESIGN BASIS 1) Resistivity data has been taken from the available report .
14 M
EQUIVALENT AREA
10.6 M (Fig-1) Switch yard
Length of the area
=
14
m
Width of the area
=
10.6
m
Ground Grid Area
=
148.4
m²
Assuming conductor spacing D
3
No. of conductors parallel to length side( nl)
=
5
No. 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 =
133.6 M
Sheet 1 of 1
Subject : EARTHING DESIGN CALCULATION FOR 33 KV SUBSTATION
1)
IEEE 80-2000
2)
CBIP Manual on substation publication number : 223
1.0)
Earthing conductor sizing for main earth gridInput data :(i)
Conductor material
=
GI
(ii)
System Voltage
=
33
kV
(iii)
Fault Current, If
=
25
kA
(iv)
Fault Duration, tc
=
1
sec
(v)
Max. Allowable Temperature, Tm
=
620
0
C
(vi)
Ambient Temperature, Ta
=
50
0
C
(vii)
Reference temp for material constant,Tr
=
20
0
C
(viii)
Thermal Co-eff.of resistivity at reference =
Temperature, αr
(ix)
o 0.0032 1/ C
Resistivity of Ground Conductor at ref. Temperature, ρr
=
20.1
µΩ/cm3
(x)
Thermal Capacity Factor, TCAP
=
3.93
J/cm3/oC
(xi)
K0 = (1/αr) -Tr
=
292.5
o
C
Conductor Size :As per Eq.37 of IEEE Std 80-2000, conductor size is given by tc αr ρr x 104
TCAP A
=
If
Ko + Tm ln Ko + Ta
Substituting the input data, we get Conductor size, A By considering 30% allowance A A A Hence the CONDUCTOR size selected,
323
= = = =
sq.mm
420.015758 mm2 900 mm2 L*B
75
X
12
GI FLAT
Sheet 1 of 1
2 2.1)
CALCULATIONS FOR TOLERABLE TOUCH VOLTAGE (Etouch ) AND STEP VOLTAGE( Estep) Input Data :
(i)
System Voltage, Vs
=
33
kV
(ii)
System fault Current, If
=
25
KA
(iii)
Fault Duration, tc(as per Specification)
=
1
sec
(iv.1)
Length of Earth grid
=
14
metre
(iv.2)
Width of Earth grid
=
10.6
metre
(iv.3)
Area of Earth grid, A
=
148.4
sq.metre
(v)
Conductor Size for main grid
=
900
sq.mm
(vi)
Depth of burial w.r.t. ground level, h
=
0.6
metre
(vii.1)
Length of one earth mat conductor Lx
=
14
metre
(vii.2)
Width of one earth mat conductor Ly
=
10.6
metre
(vii.3)
Additional length of conductor considered (for interconnecting earthing system) Grid conductor combined length, Lc
10
metre
144
metre
(vii.4)
(viii.1) Total no. of ground electrodes LA Generator Transformers Switchyard corners Towers (viii.2) length of each rod in m Lr (viii.3) Total ground rod length, LR Effective length of buried conductorLT = LR + Lc
=
length in M no. of electrodes 2.75 3 2.75 0 2.75 2 2.75 0 = 2.75 meter = 13.75 metre =
157
metre
(viii.3) Effective Buried Length for mesh voltage, LM
=
184.02
metre
(viii.4) Effective Buried Length for step voltage, LS
=
119.39
metre
(ix)
Soil Resistivityconsidered for design, ρ
=
10
Ω-metre
(x)
Surface Resistivity, ρs
=
3000
Ω-metre
(xi)
Spacing for Earthing Mat ,D
=
3
(xii)
Corrective weighting factor, Kii
=
1
(xiii)
Reference Depth of grid, h0
=
1
metre
(xvi)
Thickness of Concrete , hs
=
0.15
metre
(xvii)
Fault Current division factor (Sf)
=
0.7
metre
Sheet 1 of 5
2.2) a)
Calculation of Tolerable Etouch & Estep Voltage : Tolerable Etouch50 : From IEEE Std 80-2000 [(1000 + 1.5CS ρS) 0.116/ √tS ] Tolerable Etouch50= ( From eq.32of IEEE Std 80-2000 ) Where, Cs = Reduction factor for derating the nominal value of surface layer resistivity. ( From eq.27 of IEEE Std 80-2000 )
Cs = 1 - 0.09 [(1-ρ/ρs)/(2hs + 0.09)]
Cs
= 0.7700 tS = Duration of shock current in seconds = 0.5 ( from clause 16.2.2 of IEEE Std 80-2000 ) Substituting the above values, Tolerable Etouch50 = 732.48 Volts
b)
Tolerable Estep50
[(1000 + 6CS ρS) 0.116/ √tS ]
=
(from eq.29 of IEEE Std 80-2000)
Substituting the values, Tolerable Estep50
2437.76 Volts
=
2.3) Calculation of Grid Resistance, Rg : As per Eq.53 of IEEE Std 80-2000, R1 R2 - Rm2 The total system resistance, Rg
= R1 + R2 - 2Rm
Where, 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 and group of ground rods, R2 in Ω
Ground resistance of the grid, R1
=
k1 Lc
2 Lc
ρ
R
ln ΠLc
- k2
+ a
'
From eq.54 of IEEE Std 80-2000
A
Where, ρ
Lc
= =
Soil Resistivity in Ω-m Total length of all connected grid conductors in m
a' 2a
= =
A k1
= =
a.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
0.3000 m 0.08 m 2 148.4 m 1.06
k1= -0.05*(Lx/Ly) + 1.13
Coefficient from fig 25(b) of IEEE Std 80-2000
4.33
k2= -0.05*(Lx/Ly) + 4.40
Coefficient K1 and k2 of schwarz's fromula IEEE Std 80-2000 ) R1
=
0.3341 Ω
Sheet 2 of 5
Ground resistance of the rod bed, R2 From eq.55 of IEEE Std 80-2000 ρ
R2
4 LR
=
ln
2 k1 . Lr ( nR - 1 )2
-1 + b
2π nRLR
A
Where, Lr 2b nR
= = =
length of each rod in m diameter of rod in m number of rods placed in area A
2.75 0.08 5
LR
=
Total length of ground rods
13.75
k1
=
Coefficient from fig 25(a) of IEEE Std 80-2000
R2
=
m m
1.06
0.163 Ω
Mutual ground resistance between the grid and the rod bed, Rm From eq.56 of IEEE Std 80-2000 ρ
Rm
k1 Lc
2 Lc
=
ln
- k2 + 1
+
π Lc
Lr
A
Rm = 0.3071 Ω Subtituting the above values, Rg = 0.3403 Ω 2.4)
Calculation of Maximum Ground Potential Rise : Max. Ground potential above remote earth, GPR Where, IG = grid current If x Sf =
IG
=
=
IG x RG
Volts
17.5 KA
Substituting the values, Ground Potential Rise, GPR
=
5954.71 Volts
Since, GPR > Estep50 (as per Figure 33 of IEEE Std 80-2000),calculate Attainable Emesh and Attainable Estep voltages 2.5)
Calculation of Attainable Emesh Voltage : From Eq.80 of IEEE Std 80-2000, for Grid rods in the periphery, Emesh = [ ρ Km Ki IG / LM ]
Lr LM = Lc + 1.55+1.22
Lr
From eq.91 of IEEE Std 80-2000
L2x + L2y Where, 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 10.6
m m
Sheet 3 of 5
IG
=
grid current
=
17.5
Ki
=
corrective Factor,
=
1.51
=
13.75
LR = Total length of ground rods Subtituting the above values, LM
184.02
=
m
m
As per Eq.81 of IEEE Std 80-2000, the Spacing Factor for Mesh Voltage, D2
1 Km =
(D + 2h)2
ln
+ 16hd
2π
8Dd
+
π(2n-1)
Kh
=
0.0339 metre
( From eq.83 of IEEE Std 80-2000 )
Kh
=
No. of Parallel Paths, n
=
Where,
8 ln
4d
Equ. Diameter of Earth conductor, d Kh = 1 + h / h0
Kii
h -
1.26 na nb nc nd (From eq.84 of IEEE Std 80-2000) Lp
From eq.85 to 88 of IEEE Std 80-2000 na (2 Lc)/Lp =
nb
= 4 A
0.7 A Lx Ly
Lx Ly nc
Dm nd
=
I L2x + L2y
A Where, Lp
=
Dm
=
Peripheral length of the grid
Maximum distance between any two points on the grid Substituting the above values, na
=
5.84
nb
=
1.005
nc
=
1
nd
=
1.00
n
=
5.87
=
6
As per Eq.89 of IEEE 80-2000, The corrective Factor, Ki
=
=
49.2
m
=
17.6
m
Rounded of to nearest digit
0.644 + ( 0.148 x n )
=
1.51
=
0.4251
=
611.23 Volts
From eq.89of IEEE Std 80-2000
Substituting the values, Km AttainableEmesh Voltage
Sheet 4 of 5
2.6)
Calculation of Attainable Estep voltage : From Eq.92 of IEEE Std 80-2000, Attainable Estep = [ ρ Ks Ki IG / LS ]
From eq.92 of IEEE Std 80-2000
As per Eq.94 of IEEE Std 80-2000, the Spacing Factor for Step Voltage, 1
1
Ks =
1 +
Π
1 (1 - 0.5n-2)
+ D+h
2h
From eq.94of IEEE Std 80-2000
D
No. of Parallel paths, n n As per Eq.89 of IEEE 80-2000, The corrective Factor, Ki
= = =
6 6 0.644 + ( 0.148 x n )
Ki
=
1.51
Ks
=
0.4527
As per Eq.93 of IEEE Std 80-2000, Ls
=
Lc
=
0.75 Lc + 0.85 LR
From eq.93of IEEE Std 80-2000 Total length of all connected grid conductors in m
LR
=
Total length of ground rods
Ls
=
119.3875 m
Substituting the values, Attainable Estep 3
=
1003.47 Volts
CONCLUSION : Tolerable Etouch
=
732.48 Volts
Attainable Emesh
=
611.23 Volts
Tolerable Estep
=
2437.76 Volts
Attainable Estep
=
1003.47 Volts
SAFE
0.3403 ohms
SAFE
Grid Resistance =
SAFE
1)
From the above results,it can be seen that the Attainable Emesh and Voltages are less
2) 3)
than the Tolerable Etouch. The grid resistance is less than 1 ohms. Hence the design value of conductor size and length of conductor are adiquate and safe.
Sheet 5 of 5
