Earthing IEEE80 Final(1)

7/27/2019 Earthing IEEE80 Final(1)

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DESIGN PROCEDURE BLOCK DIAGRAM

FIELD DATA

A,r

CONDUCTOR SIZEts,d,Io

TOUCH & STEPCRITERIA

Etouch70, Estep70

INITIAL DESIGND,n,LT,h

GRID RESISTANCERg,LT

GRID CURRENTIG

GPR < Etouch

MESH & STEPVOLTAGES

Em,Es,Km,Ks,Ki,Kii

Es<Estep

Em<Etouch

DETAILDESIGN

MODIFY DESIGND,n,LT,LR

YES

YES

YES

NO

NO

NO



EARTHING CALCULATION

A GENERAL DESIGN DATA

1 Soil Resistivity, : 80 Ohm-M

2 Gravel Resistivity, : 2500 Ohm-M

3 Symmetrical Short Circuit Current, Iefs : 25000 A

4 Duration of Earth Fault Current, ts : 1 Sec

5 Maximum Allowable Conductor Temp. : 700 ° C

6 Design Ambient Temperature : 40 ° C

7 Thickness of Crushed Gravel, : 0.102 mtr.

8 Depth of Earth Grid, h : 0.5 mtr.

9 Reference depth of the Grid, h o 1 mtr.

10 Frequency 60 Hz

STANDARDS USED

IEEE Gude for Safety in AC Substation

Grounding IEEE - 80 2000

B SIZE OF EARTHING CONDUCTOR :

Eqn.: 40 Page : 43

IEEE Std. 80 - 2000

Where

Material Proposed

= Resistivity of Conductor Material 0.00381 Ohm - M

= Thermal co-efficient of resistivity at reference temperature 1.78

Tm = Max. allowable temperature in °C 700 °C

Ta = Ambient temperature in °C 40 °C

Ko = 1/a0 or 1/ar - Tr in °C 242

=

a

m

r r c T K

T K

t

TCAP

I Amm

0

0

4

2

ln10

r a

r r

r a

s r

r

hs




Iefs = rms current in Ka 25 KA

t c = Duration of Current in s 1 Sec.

TCAP = thermal capacity per unit volume from Table 1 3.42 J/(cm³°C)

Amm² = Conductor cross section in mm² 101.37 mm²

= 200.10 kcmil

Amm² = 101.37 mm²

THE SIZE OF CONDUCTOR SELECTED = 11.4 mm

Diameter of the Grid Conductor ,d = 0.01136 mtr.

Rounded of = 0.01 mtr.

Eqn. 21, Page 21, IEEE 80 2000

Eqn. 27, Page 23, IEEE 80 2000

= Reflection factor between different material resistivities

= Resistivity of the earth beneath the surface material in W.m

= Surface material resistivity in W.m

= Thickness of the surface material in m

= Surface layer derating factor

= -0.94

= 0.70

Eqn. 30, Page 27, IEEE 80 2000

Where

= Step Voltage for body weight of 70 kg

= 1814.15 Volts

TOUCH & STEP CRITERIA

kcmil A

s

s K r r

r r

=

K

r

s r

K

09.02

109.0

=hs

C s

s

r

r

sC

hs

sC

70 step E

70 step E

s

s s stept

C E 157.0

)61000(70 r =






E MAXIMUM GRID CURRENT

Where

= Maximum grid current in A 25000 A

X/R= 10

Ta= 0.0265258

= Decrement factor for the entire duration of fault, given in s 1.0131761

= 25329.40268 A

F GROUND POTENTIAL RISE

= 6639.6 V

The safety to personnel is specified by IEEE 80, which requires to limit the development of

electrical potential to dangerous value during earth fault current.

The regulation stipulates the following parameters to be within the permissible limit

a) Step Voltage (Foot to Foot Contact)

b) Touch Voltage(Hand to Foot Contact)

A Mesh Voltage

Eqn. 80, Page 91,

IEEE 80, 2000

= Corrective factor for current irregu-larity

Where

CALCULATION FOR ACTUAL DERIVED STEP & MESH VOLTAGE

VERIFICATION FOR HUMAN SAFETY

R

Ly x

r C

imG

Ll L

L L

K K I Design Emesh

=

2222.155.1

)(r

Ki

g I

g f G I D I =

f D

G I

g G R I GPR =

GPR

n K i 148.0644.0 =

d cba nnnnn =




= 26.6

= 1 for square grids = 1

= 1 for square and rectangular grids = 1

= 1 for square, rectangular and L-shaped gri = 1

= 26.60

= 4.58

= Spacing factor for Mesh Voltage Eqn. 68 Page 113 IEEE 80

Eqn. 81, Page 93

IEEE 80, 2000

Where

= Corrective wieghting factor that adjusts the effect of inner conductors on the corner mesh

= 0.81

= 1.00 With Rods

= Corrective weighting factor that empasising the grid depth

=

Where

= Reference depth of grid = 1

= Depth of the ground grid conductor = 0.5

= 1.22

= 0.50

= 511.43 Volts

Km

=

12

8ln

48

2

16

ln

2

122

n Kh

Kii

d

h

Dd

h D

hd

D Km

Kii

Kh

ho

h1

ho

h

Kh

Km

)( Design Emesh

n

ii

n

K 2

)2(

1

=

Kii

P

T a

Ln =

an

bn

c

n

d n

n

Ki

Kii




Calculated Mesh Voltage is Lower than the Tolerable Touch Voltage.HENCE SAFE

B Step Voltage

Voltage developed for step as per the earthing system proposed during full Earth fault current

Eqn. 92, Page 94

IEEE 80, 2000

Where

= Spacing factor for Step voltage

Eqn. 94, Page 94, IEEE 80, 2000

= 0.440

= 4.5812736

= 619.78 Volts

= 620 Volts

Calculated Step Voltage is Lower than the Tolerable Step Voltage.HENCE SAFE

RC

G

L L

I Ki Ks Design Estep

=

85.075.0)(

r

Ks

=

Dh Dh Ks

n 25.011

2

11

Ks

Ki

)( Design Estep

)( Design Estep

n K i 148.0644.0 =






















SUMMARY

A EARTH GRID CONDUCTOR

Type of Conductor

Size of Conductor 200 MCM

Length of Conductor 8109 mtr

Depth of Conductor 0.5 m below GL

B GROUND RODS

Total Length of Ground Rods 600 mtr

Length of Individual Ground Rods 12 mtr

No. of Ground Rods 50 No.

C HUMAN SAFETY

Mesh Voltage511

620

Volt

1814

571.29

GPR Volt 6639.6

Permissible Value

Volt

opper, commercial hard - draw

Step Voltage

UNIT Designed Value





rr

20°C(mW.cm)

TCAP

Thermal

Capacity

[J/(cm³.°C]

1.72 1 3.42

1.78 2 3.42

4.4 3 3.85

5.86 4 3.85

8.62 5 3.85

2.86 6 2.56

3.22 7 2.6

3.28 8 2.6

8.48 9 3.58

15.9 10 3.28

17.5 11 4.44

20.1 12 3.9372 13 4.03

Documents

Earthing IEEE80 Final(1)