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

[SteveFehr]

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?

 

[wareagle]

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.

 

[wareagle]

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

 

[SteveFehr]

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.

 

[DaveScott]

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...

 

[SteveFehr]

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...

 

[DanDel]

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.)

 

[SteveFehr]

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?

 

[DanDel]

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

 

[jghrist]

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.

 

[DanDel]

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.

 

[SteveFehr]

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.

 

[apowerengr]

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.

 

[waross]

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.

 

[Snoball]

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

 

[jghrist]

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?

 

[waross]

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

 

[SteveFehr]

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.

 

[DanDel]

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.

 

[SteveFehr]

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