Earthing design calculations for solar power plant subsbations

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