Earthing of Steelwork in HV SUBSTATION 1

[ROMM]

HI,
Currently designing a 110/11kV indoor GIS substation and the main earthgrid conductor has been calculated at 240sqmm. I am trying to find standards or information on how to size the earthing cables for earthing of gates, steel beams, columns, louvres etc located both inside and outside of the substation.

 

[cuky2000]

The minimum conductor cross section are based in the fusing current and/or in the maximum allowable temperature that cause annealing the conductor, avoid hazardous conditions or minimize other site specific risks. This calculation procedure is similar for both ANSI/IEEE and IEC standards. Simplified practical formula for sizing the ground conductor are as follow:

A[sub]kcmil[/sub]= I[sub]f[/sub] K[sub]f[/sub]( t [sub]c[/sub])[sup]1/2[/sup] or A[sub]mm2[/sub]~ 0.5 I[sub]f[/sub] K[sub]f[/sub]( t [sub]c[/sub])[sup]1/2[/sup]
[sub]Where:
A= Area of conductor: (A[sub]kcmil[/sub] in kcmil & A[sub]mm2[/sub] in mm[sup]2[/sup])
I[sub]f[/sub] = Fault current in kA rms (injected current in the earth)
t [sub]c[/sub] = Duration of fault current in seconds (typ. Backup relay time).
K[sub]f[/sub] = Constant for the material from table or determine at various values of fusing temperature conductors (~1084 oC for most Cu conductors) or limited conductor temperature (250 oC for Cu commercial hard-drawn) considering ambient temperature typically 40 oC [/sub]

Per IEC Std for bare Cu : Kf =1/[226(ln(1+(Ti –Tf)/(234.5+Ti)

[sup]Where:
Ti =Initial Temp in oC (max. Ambient Temp)
Tf= Permitted final temperature as follow:
Tf[sub]1[/sub]=500 oC: for visible conductor in location that are not generally accessible.
Tf[sub]2[/sub]=200 oC: for non-visible conductors in location that are generally accessible.
Tf[sub]3[/sub] =150 oC: Where hazards are greater (non-visible in location with increased fire risk or for earthing conductor laid together with PVC cables) [/sup][/color blue]

NOTES:
a- the standard eng practice consider normally two different conductor path in most structure.
b- Most steel structures connected to the ground via anchor bolts carry significant part of the part of the ground current; however, in conventional engineering calculation this is ignored and indirectly is considered as a safety margin.
c- For additional information, see IEC Std 60621-2, IEEE Std 80 and article 9.6 in the enclose publication based in IEEE.

http://www.usda.gov/rus/electric/pubs/1724e300/1724e300.pdf

 

[mc5w]

Common practice here in the U.S. is to use 4/0 American Wire Gauge copper wire to form a grid for the substation. This wire size is based on several factors:

1. The wire size has 19 strands which means that crimping and exothermic welding are more likely to have ALL of the wire strands conducting.

2. Fault currents spread out into the soil in such a way that larger sizes are a moot point - several 4/0 wires in parallel in say a 10 foot by 20 foot grid works.

3. 4/0 copper has reasonable ability to handle lightning current.