Should I Ground the Neutral?

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SteveJ55

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I am currently involved with the installation of a 10-generator paralleled system where each generator is 480V wye at 375 KW. There are no neutral conductors going from the generators or coming from the utility transformer. There are no neutral conductors in any of the six transfer switches. I am told the utility is a delta transformer. My question is, do we need to ground the neutrals at each of the 10 generators to provide a ground fault path back to the source (2005 NEC 250.2)? The owner of this particular facility says that we are not to ground the neutral of the generators. What do you think?
 
SteveJ55 said:
My question is, do we need to ground the neutrals at each of the 10 generators to provide a ground fault path back to the source (2005 NEC 250.2)?
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In my opinion you are correct.
 
1. What did the utility do in their transformers?
2. What did you do at the Main Service?
3. What does the engineer of record say?
 
These are all good questions to which I do not know the answer. I am just the generator start-up man, and this question was asked of me by the electricians who are doing the installation.
 
IMO 1, 2 and Especially 3 is the solution to this project. Seems a job this big the electricians would have figured this out before start-up!
 
SteveJ55 said:
I am currently involved with the installation of a 10-generator paralleled system where each generator is 480V wye at 375 KW. There are no neutral conductors going from the generators or coming from the utility transformer. There are no neutral conductors in any of the six transfer switches. I am told the utility is a delta transformer. My question is, do we need to ground the neutrals at each of the 10 generators to provide a ground fault path back to the source (2005 NEC 250.2)? The owner of this particular facility says that we are not to ground the neutral of the generators. What do you think?
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To not ground the generators, IMO, is not an option. Since these are relatively small generators, solidly grounding the system might be reasonable. Are there any 277 Volt loads? Evidently not, if the utility didn't have a neutral either. If there were 277 Volt loads, then there is no option. The generator must be grounded.

The generators could be grounded through high resistance systems, but this can be pretty expensive by the time you figure in all the monitoring hardware.

A literal (what other kind is there???) reading of 250.20 (B) would seem to indicate that you do not need to ground the neutral point of the generators.

(1) doesn't apply because the voltages are not limited to below 150
(2) doesn't apply because, although this is a WYE, there is no neutral conductor that is being used as a circuit conductor
(3) doesn't apply because it is not a high-leg system


250.20 may not require it, but it sure seems like 250.4 would require it.

If they hold to their position of not connecting a system bonding jumper (Your original post said 'ground' but then you stated that it was for providing a fault path) then i'd start asking lots of questions.

"If i don't install a system bonding jumper, then how will the system clear in the case of a ground fault"? and on and on until all the questions are answered.

Good luck. Let us know what happens.
 
Are all of the gens identical? If not, are they at least the same pitch?

In my experience, grounded wye gens don't get along all that well with each other. There will be circulating currents between them, more pronounced when they're cold, diminishing as they warm up. VAR controllers can help alot here.

If the POCO service is a delta, how are ground faults detected? If there's a grounding xfmr involved, can it be placed on the load side of the synch gear? (If it's not already). If so, there's no need to ground the center of the wyes on any of the gens. The only time they'll float is when they're running, but not synched in yet. Not really an issue.

Look carefully at the PT's in the synch gear. They're likely 2 xfmr open delta, I've seen instances where the utility side of a delta service was grounded B, and said PT's were used for ground fault detection. If this is the case, and the gen's are grounded wyes, and they parell the utility, there'll be a fight, but not for long! lol.

Since the gens and the utility are different systems, the answer can get complicated. Just because it's drawn a certain way doesn't mean it'll work, you'll need to verify for yourself.
 
micromind said:
Are all of the gens identical? If not, are they at least the same pitch?

In my experience, grounded wye gens don't get along all that well with each other.
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Correct-a-mundo (Of course, i don't mean to imply that anyone who is not of this world, reading this post, is incorrect; but i digress...)

And if you handed the generators scissors, they would probably run with them. (Pun intended)

But seriously,
This type of set-up would take some pretty serious control equipment. I'm assuming that there will be a synchronization relay that wil be used to close the ties on each generator. I don't see why anyone would attempt this without the VAR controllers. Without them, the generators would spend more time 'talking' to each other than they would getting the 'message' out to the outside world.

Metaphorically yours...
 
All 10 generators are identical. They are bifuel, that is to say, they run on both diesel and natural gas. When the generators are started with no or little load, they run on 100% diesel, but as the load increases, the natural gas begins to flow up to the point where 90% natural gas, and 10% diesel is being used which is when the gens are 90-100% under load. The generators interact with each other and with a centralized controller via RS-485 communications.
 
don_resqcapt19 said:
Eric,

What in 250.4 would require this to be a grounded system?
Don
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Nothing, actually.

I saw, as i was writing the post, that my mind drifted from 'grounding' to 'installing a system bonding jumper.' The original post was somewhat ambiguous as the word 'grounding' was used and then the concept of 'fault current' was presented.

So, 250.20 does not require the system to be 'grounded.'

250.4(3), however, requires the bonding of the system such that fault current has a place to go.
 
250.21(4) seems to indicate that ungrounded electrical systems, such as delta-connected, 3-phase, 3-wire, providing 480 volts are not required to have a circuit conductor that is intentionally grounded. The reason for this is to avoid an automatic unplanned opening of the supply current as this may be a dangerous situation (in this case a county jail). However, some type of ground fault detection should be employed, I would think, to alert maintenance personnel of a ground fault.
 
Ten generators in parallel? As has been said, they will have kvar sharing. They will also have kw sharing, fixed pf - fixed kw, isoc, droop, and likely a batch of other modes.

Why not ground the WYE point of each gen? I can think of a couple of reasons:
Multiple small generators operating in parallel can easily be in a position where if one has a fault, the others will deliver enough short circuit current to damage the faulted gen past repair. This is different from being in parallel with a utility. Ther one has the utility transformer impedance limiting the SSC.

If the utility system is ungrounded delta, why would you want to to ground the gen wye point? What would that benefit?

carl
 
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