SUBSTATION EARTHING 2

[jefferslot]

An industrial cement factory currently takes a supply at 415V 300KVA source (Utility). The existing Earth bonding at the site is substantial and the safety earth is about 0.3ohms.
the owner has now obtained a 1000KVA power transformer ratio 11000/433V with %Z of 4.75%. The utility will shortly be connecting the 11KV supply and has asked if an earthing study has been carried out because the new substation must be classed as HOT (voltage rise of up to 430V).
This is a small industrial plant and I am not sure an expensive detailed earthing analysis is necessary. Anyone had any similar situation?
with substantial bonding and such a low earth value what else would an earthing study reveal?

Jeff

 

[7anoter4]

In the previous case the transformer was located on Utility territory so a grounding conductor was provided
in order to return the short-circuit current to the transformer neutral point acting the protection. The Grounding Electrode connection
[of 0.3 ohm] was provided in order to reduce the touch potential. See NEC art 250.
In the second case no grounding conductor could be provided and the phase to Ground fault current return path will be
the grounding electrode connection. One has to calculate the short-circuit current and then the grounding conductor size to
withstand the short-circuit current up to protection fault clearing time and also the touch and step potential. See IEEE Std 80.

 

[pwrtran]

You should be careful of the 0.3 ohm!
If it was tested by isolating the existing station ground, which means all neutral coming in to the station and the secondary side neutral going out of the station are all disconnected from the station ground, then 0.3 ohm is a super good result. Assume the 11kV line-to-ground fault is 3000 Amp at the station, GPR<=1000 V.

 

[jefferslot]

Thanks pwrtran. 0.3ohm is valid the site is situated on land with a high water table and the grounding is superb, better than i have ever seen actually