400kW Wye Generator - Delta Loads

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botz

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Hi, I have a 400kw, wye generator, 480/277V, 3phase. All Loads fed from this generator are delta. Utility is 480V, 3phase delta. Currently neutral and ground are linked at the generator and only phase conductors are fed into a 3 pole transfer switch(no neutral cables). I was advised to remove the generator neutral ground link at the generator to increase system reliability and leave neutral floating. Idea behind this is to keep the generator main breaker from tripping in case of a ground fault. A ground fault monitor will be installed to find ground faults.

I would like to ask, does this sound like a good engineering approach, has anyone seen this? Which standards should I refer to get more information on this topic? Thank you.
 
It seems to me that in general the idea is sound.

Is the utility power grounded in any way?

if the incoming utility power is ungrounded delta why would they have bought a 480/277V generator? that makes me want to look a lot closer at just what was going on here.

What does the engineer that spec'ed all this stuff and did the drawings say about it?
 
No, the utility is not grounded. Ground fault monitors and typical wye-broken delta type xfmrs are are installed at substations to derive a neutral and measure overvoltage on gf. so ungrounded system.
People who bought these generators have retired or left. And this issue came up in a reliability study.
Its something I have not done before so confused.
 
I would think that a genset of this size would be a standard 12 lead machine. Can't you just reconfigure the connection to create a 480 ungrounded delta to match your system?
 
There is no issue leaving the neutral floated and connecting the wye generator to the ungrounded delta system. The equipment is looking for 480 volts line to line and does not care if the source is wye or delta.
 
don_resqcapt19 said:
There is no issue leaving the neutral floated and connecting the wye generator to the ungrounded delta system. The equipment is looking for 480 volts line to line and does not care if the source is wye or delta.
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I would agree. It would seem to me that in this case (short of reconfiguring it as suggested in my previous post), that floating the neutral would be REQUIRED for safety as it is being connected to an ungrounded system. You can't supply an ungrounded system from a grounded system.
 
texie said:
I would think that a genset of this size would be a standard 12 lead machine. Can't you just reconfigure the connection to create a 480 ungrounded delta to match your system?
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Can you do that? I don't see how you can get both 480 delta and 480 wye out of the same set of 6 coils. You would need 240 volt coils to get 480 delta, but if you reconnect those coils in a wye you would have a 415/240 volt system.
 
don_resqcapt19 said:
Can you do that? I don't see how you can get both 480 delta and 480 wye out of the same set of 6 coils. You would need 240 volt coils to get 480 delta, but if you reconnect those coils in a wye you would have a 415/240 volt system.
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I think I stand corrected. 480 would have to be a wye connection. I was thinking 240 volt delta. I still believe that it must be ungrounded for the OP's case.
 
texie said:
I think I stand corrected. 480 would have to be a Wye connection. I was thinking 240 volt delta. I still believe that it must be ungrounded for the OP's case.
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Do worry it very common for those to think there is a delta 480 generator but all the ones I have seen all have an X0, while it is possible its just not very common for a 480 delta generator when they can make one to serve both 480 delta and 277 loads, I think this comes from people thinking about a 120/208 Wye verses a 240 volt delta, in the 480 system it is the lower voltage that is used for lighting but the higher voltage stays the same delta or Wye where 120/208 its the higher voltage that is changed between a Wye and delta, if we had a 240 volt Wye we would have a 139 volt X0 connection, while most generators can be wire like this to utilize the X0 for fault protection its not a useful voltage for line to neutral loads.

Heres the difference with a delta to delta transformer bank if a primary phase was lost you wouldn't single phase the plant as you would now just have an open delta and the reason delta transformer banks were requested over a Wye for industrial installations with allot of three phase motors, but with a generator you don't have this protection so it makes no sense to request a delta generator as if you loose a phase you will still single phase the plant.
 
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