Generator/Transformer Grounding - When does distance become an issue?

[healyx]

Hi All,

I am considering a transformer with emergency generator combination. I have read some threads of pros and cons of using a 3 pole vs a 4 pole ATS.

3 pole seems to avoid some issues with potential floating neutrals but usually comes with the warning that having genset and transformer on same grounding system requires gensets to be close.

I have the following.
(1) a single neutral to ground/earth bond (NE link) at the main switch board (MSB) which houses the transformer/generator incoming cables and the ATS.
(2) I am deliberately NOT grounding the generator neutral until it meets the single NE link at the Switchboard.
(3) I am grounding the generator frame to a local earth electrode(s).
(4) I am running an equipment ground/earth cable from the MSB earth bar back to the generator frame.
(5) Using a reasonably small generator (say under 200kVA), maybe not worth extensive relaying?

My question is, why would generator distance from the MSB affect this scheme? I would have thought the issue would be a stator to frame fault which would cause current to flow down the equipment ground cable to the MSB the back through the neutral. Given a stator fault is before the breaker feeding the cable to the MSB, this current wouldn't trip a breaker and tripping a breaker wouldn't stop the fault. Is distance an issue because we need to make sure the equipment ground cable to the genset frame needs to be large so that the fault current is large enough to cause a significant disturbance and stop the generator (via some other internal sensing), rather than silently sit with a high impedance touch potential hazard? I guess various relaying arrangements could detect this, but what is the outcome? Do they shut down the generator? None of this seems related to generator distance to the NE link.

I'm in IEC land if that helps.

 

[fx007]

Where is this part coming from: "3 pole seems to avoid some issues with potential floating neutrals but usually comes with the warning that having genset and transformer on same grounding system requires gensets to be close."
I have had some training at ASCO and I never heard of this before. 3 or 4 pole comes to your decision and user experience. NEC land is more towards 3 pole, while IEC towards 4 pole (switched neutral). My recommendation: If you design for IEC, stick to 4. That's what they are used to.

 

[jghrist]

The difference is that, depending on the location, the return current of a ground fault may have to flow through a neutral that is separated from the phase conductor that is carrying the current. This increases the impedance and could reduce the fault current to the point where it won't trip the circuit.

See

http://ecmweb.com/content/grounding-non-separately-derived-generator-systems