GROUNDING OF NEUTRALS

[appunni]

Sir,
We have a substation with a 110/11kv , 10 MVA ynyn transformer and 11kv feeders from the l.v side of the transformer. In the same substation compound, we have a 11/0.44kv dyn transformer for auxiliary purposes. Can we connect the neutral on the 0.44kv side of the transformer to earthmat where 11kv side neutral of the 110/11kv transformer is grounded? Some say 0.44kv side neutral should be grounded seperately and should not be connected to 11kv side neutral grounding earthmat. Is this correct? If correct, what is the reason? I request your valuable comments.
With regards.
appunni

 

[RajT]

Offcourse you can connect the 440V side ground to the same ground as the 11kV ground, the reason being that "Earth" is same no matter where.

 

[dpc]

It's generally safer to bond the two grounds together to reduce the risk of a high step or touch potential in the substation during a fault. Without a metallic connection between the two, a potential difference will exist between the two "grounds" during a fault. The resistance of the earth will be much higher than a ground wire, so the the voltage drop could be much higher without a copper connection.

I'd tie everthing together.

 

[ScottyUK]

RajT,

The problem with your statement is that "earth" most definitely isn't the same everywhere during a fault - during moments of high current flow, there is a voltage gradient within the earth which you could measure if you were brave enough to put voltmeter probes at two locations along the current path of the fault. Personally I'd like to be well away from the area during the fault!


Scotty.

 

[RajT]

Scotty,

What I said was that it is technically acceptable ( and often a must ) to bond both the 11kV and 440V together. So on the assumption that all is engineered and installed well, then the voltage gradient( albeit a small one) should not be an issue. Also the duration of fault currents are relatively small so even if you were in the area I doubt if you would notice an appearence/clearence of an earth fault.

 

[jbartos]

Suggestion/Questions:
1. What are the dimensions of the substation compound?
2. Is the grounding mat under the entire compound?
3. Is there a requirement for the "quiet" earth/ground (without noise)?

 

[rbulsara]

appunni:

When I was in India (not sure where u are at), this was a common requirement of the local utility co (In Gujarat). I beleive it was due to the fact that in many dry soil locations the ground resistnace is high and the utiltiy co does not trust the users to keep the ground at the electrode connection moist!


If the grounding R is high, a ground fault on the primary side would impress damaging voltages on the secondary side via the neutral. As long as the phycial separation between the secondary N grounding electrode and primary side electrode (or mat) is such that it cannot be spanned by a person, keeping the two neutrals isolated (and therefore total electrical separation between the primary and seconday systems) is not a bad idea and has no downside.

 

[jghrist]

rbulsara wrote:
"As long as the phycial separation between the secondary N grounding electrode and primary side electrode (or mat) is such that it cannot be spanned by a person, keeping the two neutrals isolated (and therefore total electrical separation between the primary and seconday systems) is not a bad idea and has no downside."

There definitely can be a downside. During a ground fault on the 110 kV system, the substation ground grid will rise in potential with respect to remote earth (Ground Potential Rise or GPR). Point on the surface of the substation will be at various potentials with respect to the grid, but if the ground grid is designed properly, the potential difference between the grid and points on the surface will be kept below the allowable touch potential. The separate 0.44 kV ground electrode will be at some potential different from the ground grid, determined by it's location and the currents flowing through the earth. Someone standing on the substation surface will have his feet at one voltage and if he touches something grounded to the 0.44 kV ground electrode, he may be exposed to a fatally high voltage. The grounding design could conceivably keep this voltage to a safe level, but this would involve unusual and complicated calculations. It is much safer to bond all ground electrodes together.