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Delta from a wye generator- ground the neutral or float?

Delta from a wye generator- ground the neutral or float?

Delta from a wye generator- ground the neutral or float?

(OP)
We just put in a 480/277V Y generator feeding a 480V Delta switchboard.  During the inspection, we discovered the contractor had grounded the neutral in the generator with 3/0 (same size as the NEC-specced ground wire- looks comically small next to the line conductors).  In speaking to the Caterpillar reps on-site, we found out the same N-G connection was also made internally.  There is appx 25' of cabling between this generator and the first OCP in the switchboard.

Our senior technician feels strongly that both N-G jumpers should be removed and the delta left to float ungrounded.  He's worried a L-G fault will fry the generator.  I don't like that the line voltage could float all over the place during normal operation or that an L-G fault could potentially go unnoticed if one should occur.  Our lead electrician agreed with me, which is why he wired it up that way.

What's best practice here?  Any other pros/cons for either option?

RE: Delta from a wye generator- ground the neutral or float?

Generally you will find that the mfg will install a neutral to frame ground. If you are using a transfer switch and do not switch the neutral you need to remove the jumper. Since you are not doing this, the generator is providing service
as would a utility. The neutral to gnd connection is required in case there is a ground fault. You need to install and EGC to the transformer and a ground rod(or some
approved electrode) at the generator.

RE: Delta from a wye generator- ground the neutral or float?

If the load is connected delta what are you doing with the
neutral?

RE: Delta from a wye generator- ground the neutral or float?

(OP)
The load is delta.  We did not connect the neutral, the T0 terminal of the generator is grounded with no other connection.  The discussion between my technician and I was whether or not the neutral should be grounded or left to float.

The generator is otherwise grounded and installed properly.

RE: Delta from a wye generator- ground the neutral or float?

One problem that could be experienced with an ungrounded system, is that when there is a fault, then the voltage from two phases to machine ground is suddenly root 3 more, which could test your insulation...

RE: Delta from a wye generator- ground the neutral or float?

(OP)
At 480/277V, unintentional corner grounding would give 480V high-legs, which (hopefully) shouldn't be an issue on THHN and is done intentionally in some circles.

Is the standard 600V insulation rating on low-voltage cable rated for 600V RMS or 600V peak?  480V RMS is 678V peak...

RE: Delta from a wye generator- ground the neutral or float?

In any case, you should not leave both N-G connections.

I would recommend leaving the generator N-G connection, and removing the added external N-G connection, unless there is a specific reason why you need an ungrounded system. And in that case you should have some kind of warning system for when a phase is shorted to ground (phase lights, etc.)

RE: Delta from a wye generator- ground the neutral or float?

(OP)
Well, it turns out that the UPS, the main load on this switchboard, requires a grounded delta input.  Per the UPS manufacturer's engineering rep, the MOVs and static switch are not designed to cope with fluctuating line voltages (with respect to ground) and could potentially fault, especially during a ground fault, which would raise the other two lines to 480V.

  And overcurrent protective relaying built into the generator should theoretically protect it from an L-G fault, which was our other main concern.

Dan, why would two N-G jumpers inside the generator be an issue?

RE: Delta from a wye generator- ground the neutral or float?

More than one neutral-ground point will cause circulating current, among other issues.

RE: Delta from a wye generator- ground the neutral or float?

I don't think circulating current will be an issue with both neutral-ground connections at the same place.  There isn't any neutral conductor.

RE: Delta from a wye generator- ground the neutral or float?

Obviously, if both conductors were in the same place, it would simply be two parallel conductors grounding the neutral. My understanding was that one N-G cable was installed internally inside the generator, while the other was in the lug connection cabinet.

While this system is probably not a critical situation, you do not need a connected neutral back to the switchboard to have circulating current.

Any time you have two conductors connecting the same components (neutral bus in the generator, & ground bus/metal cabinet), you have a conductive loop. Any time you have a conductive loop near AC current flow or the magnetic field of an AC generator, you will have induced current through the loop.

RE: Delta from a wye generator- ground the neutral or float?

(OP)
Both N-G connections are inside the generator enclosure- one is in the generator compartment, and the other is right on the other side of the sheet metal separating it from the lug compartment.  There's maybe 2' of cabling between the two.  It's essentially a parallel conductor.  There IS potential for circulating current, though, but I'd tend to discount it as negligible.

RE: Delta from a wye generator- ground the neutral or float?

I'd suggest you need to carry a grounded conductor to the switchboard for a ground-fault current path since the generator is 4 wire.  It does not matter if you use or need it as a neutral or not.  One N-G connection point would also be correct.

RE: Delta from a wye generator- ground the neutral or float?

The issue with multiple ground to neutral connections is not circulating currents, but neutral current flowing in the grounding system.
A chronic, unwanted current in the ground system can cause serious degradation of the ground system.
One possible issue is local heating in the earth in proximity to the ground electrodes. This can result in abnormally high ground resistance. Under fault conditions the abnormally high ground resistance may result in excessive step and touch potentials, and elevated voltages to ground on the surface of grounded equipment.
Another issue that may result from chronic unwanted currents in the grounding system is accelerated corrosion of the grounding electrodes.
A third possible issue with multiple ground connections on the neutral may be protection issues. Multiple ground paths sometimes bypass protection CTs so that fault currents divide between the neutral and the ground and part of the current bypasses the protection CTs.

If this is a stand alone installation, I would suggest leaving the original ground, and removing the contractors ground.
You may want to add a CT, current relay,  and a shunt trip on the main breaker, so that you get faster clearing on ground faults.

If this is a standby installation with an automatic transfer switch, there are other considerations.

You may consider high impedance grounding, but this will leave the high voltage issues in the event of a ground fault.

The magnetic fields that cause circulating currents tend to cancel from phase to phase. They can be minimized by keeping the phase conductors as close together as possible and keeping any ground or neutral conductors relatively far from the phase conductors.
respectfully.

RE: Delta from a wye generator- ground the neutral or float?

I think you'll need to run the neutral everywhere to insure fault current path.

RE: Delta from a wye generator- ground the neutral or float?

HugoKane,
A grounding conductor, not a neutral (grounded conductor) needs to be run to ensure a fault current path.

Waross,
With only delta connected loads, where does the chronic current in the ground system come from?

RE: Delta from a wye generator- ground the neutral or float?

You are correct jghrist.
My comment was a general comment on multiple grounds and may not apply in this situation.
Chronic current may come from circulating currents, but both of us doubt that circulating currents will be an issue.
respectfully

RE: Delta from a wye generator- ground the neutral or float?

(OP)
Man, if we're getting this much disagreement on a 2' neutral jumper, what do you all think of NEC 2005 250.30(A)(1) which allows neutrals of separately derived systems to be bonded together?  If this was a wye switchboard, the neutral would have to be bonded at both the generator and the substation, and the two bonded together on the neutral bus of the switchboard.   Nice big loop right there!

It's tough to do anything else on systems with make-before-break paralleling generators and UPS systems- switching the neutral is moot when both sides are connected.

RE: Delta from a wye generator- ground the neutral or float?

SteveFehr, I believe 250.30(A)1 allows two bonding jumpers only "where doing so does not establish a parallel path for the grounded conductor", i.e. when there is a 4-pole ATS in use.

RE: Delta from a wye generator- ground the neutral or float?

(OP)
Dan, 250.30(A)1 exception does not set such a limitation on the two bonding jumpers; it allows direct bonding of neutrals even when it DOES create a parallel ground path.  In fact, that's the whole point of the exception.

This is important for parallel systems and make-before-break connections.  For example, if you have an N+1 system with 3 parallel wye generators, normal operating condition in storm or peak shaving operation is for all 3 generator neutrals and the utility neutral to be connected at the switchboard neutral bus simultanously.  250.30(A)1 allows this.  

But as noted, it creates a condition where current will move through the ground- neutral current for generator #1 can conceivably travel through the neutral to the utility transformer, to the ground at the xfmr, back through the xfmr ground conductor to the switchboard, through the generator ground conductor back to the generator, and through the bonding jumper to the generator neutral.  This has high impedance relative to the actual neutral and thus reduces the amount of current- actual current levels would be was less than proportionate to the normal current level over the neutral.  But there would still be some current

RE: Delta from a wye generator- ground the neutral or float?

Quote (NEC 250.30(A)1):

Exception No. 2: A system bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded conductor. Where a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.

Note that it specifically states "...does not establish a parallel path..."

Why are you saying that it permits a parallel path?

RE: Delta from a wye generator- ground the neutral or float?

SteveFehr, Art. 250.30(A)1, which you reference, states that the system bonding jumper connection "shall be made at any single point on the separately derived system"

Only Exception 2 allows two jumpers per my previous post, (See davidbeach's helpful post.

RE: Delta from a wye generator- ground the neutral or float?

The exception to 250.30(A)(1) does not allow bonding the grounded neutral conductor at both the service entrance and at a generator if the grounded neutral conductor is not switched.  In this case, the generator is not a separately derived source.  If the neutral is switched by an ATS, then the generator is a separately derived source and the grounded neutral conductor of that source is bonded at the generator.

RE: Delta from a wye generator- ground the neutral or float?

(OP)
I was not referring to exception 2, I was referring to exception 1:

NEC 2005, 250.30(A)(1) Exception 1: For separately derived systems that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single system bonding jumper connection to the tie point of the grounded circuit conductors from each power source shall be permitted.

This exception excepts 250.30(A)1 and the single point.

RE: Delta from a wye generator- ground the neutral or float?

No.  Exception 1 wouldn't apply, and where it does apply it means that two sources don't each need their own bonding jumper but can use a single bonding jumper for both.  So it is the opposite of what you are suggesting, it is permission to go from two bonding jumpers to one bonding jumper.

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