Generator/Service Grounding Application

[Doug98]

Background: I am involved with an existing data center project with following equip:
(1) 12.47kv / 277/480 delta wye utility transformer connected to 3ph/4w, 3,000A service switchboards "1" and "2"(swbd 2 currently fed from swbd 1 via breaker)

Currently two generators (only one on-line at a time) connect to swbd 1 via 3 pole util and gen breakers with transfer controls at swbd. Generator neutral solidly connected to swbd 1.

(2) Due to capacity, an additional utility service transformer will be added to swichboard "2" and tie to "1" removed. Swbd 2 is set up with util and gen breakers and transfer control similar to swbd 1.

(3) Customer wants to parallel the (2) 2MW generators and connect through parallel swgr to each swbd.

(4) Problem: Adding generators with solidly connected neutral will create code violation, interconnecting grounded neutral of two systems, illegal n-g bond.
Adding 4 pole transfer switches is not practical due to costs and space limitation.

(5) Potential solution: Since most of Swbd "1" and "2" loads are 3 phase, provide separate transformer for the 1 phase load (mostly lighting) and change Swbd "1" and "2" and paralleled generators to 3ph/3wire.

Q1) Exist swbds are listed as 3ph/4wire, does anyone have experience with a smilar change, I presume it would be required to have affected swbds field certified as 3ph/3 wire by UL.

Q2) Further, would it be possible to leave swbd #1 and #2 grounded neutral connection as is, remove all neutral load conductors, and connect to 3ph/3w generators. Under this scenario, swbd 1 and 2 would still be listed as 3ph/4w. Since there is no neutral load, generator parallel swbd could be 3 wire. Is anyone aware if this is a code violation? What are implications of ground fault sensing with 4wire but no neutral load.

 

[alehman]

Q1 - are you proposing to operate the system ungrounded? I think this could be done in the field, but it may be complicated because you will have to remove ground fault protection. I would not recommend an ungrounded system for reasons discussed here many times.

Q2 - it's not a problem to have no neutral load. In terms of the listing and application of the switchboards, you should not leave it as 4-wire if the neutral is not connected. This could be dangerous if someone years from now connects something to that neutral bus.

You may be able to circumvent this problem with a carefully designed differential ground fault scheme.

 

[Doug98]

Thank you for your comments.

1) Local inspectors have indicated the grounded neutral must remain at service switchboards since fed from grounded wye transformer (250-24B)

2) If switchboards remain as 3ph/4w with no neutral loads. I would plan to connect the paralleled generators to each switchboard with 3 wire feeders. Neutral not required since no 3 phase load. In this case it is not clear whether the generators will require ground fault protection.

 

[alehman]

The system you propose would be effectively ungounded while operating on generator power (because there is no return path to the generators) and grounded when fed from the transformers. There is no way (or need) to apply ground fault sensing to ungrounded generators. I don't think this would be a good idea unless there was some way to assure that someone in the future does not connect to the neutral bus thinking it is usable. It would probably violate the NRTL listing.

If you are considering coverting to ungrounded, I would always prefer a resistance grounded system instead.

Perhaps you could arrange it such that the generators feed 3-wire boards downstream of your main switchboards. You still have a system that is ungrounded on generator but you avoid the problems with the main switchboards. Then you can also ground the generator neutrals at the paralleling board to avoid the problems of an ungrounded system.

The NEC permits a common neutral with a single grouding connection for double-fed systems. You may find this article useful.

http://ecatalog.squared.com/techlib/docdetail.cfm?oid=09008926800a89b8

 

[mc5w]

What you are essentially doing is to turn your double ended switchboard into a quad ended swithcboard. As long as you are using source return ground fault protection using only 1 neutral to ground connection at the tie breaker there should be no problem with ground fault protection or continuing to use phase to neutral loads. Your generators SHOULD use 4 pole circuit breakers with ground fault protection so that a generator fault can be isolated. Unlike the utility transformers which have primary fuses to interrupt a secondary fault the only protection for a generator is to interrupt the neutral and shut down the generator.

You might, however, want to insert a 3-phase inductance into the hot wires of your 277/480 volt generators that uses a trifiliar winding on 1 core leg to increase the zero sequence impedance so that you do not get circulating current under less than perfect balance conditions which would be 100% of the time. Starting with 2005 National Electrical Code you will NOT be allowed to put such an inductance into the neutral.


Mike Cole, mc5w@earthlink.net

 

[Doug98]

Thanks alehman,
The application is not a double ended service, however, using partial differential ground fault relaying could effectively compensate for the solid neutral connection between the two switchoards through the generator paralleling swgr if 3ph/4wire systems are used.

After further review, the approach I will take is to have each service switchboard grounded with no neutral load conductors. The generators will be 3ph/4w, separately derived, neutral connected to ground electrode at the paralleling switchgear. Connection to each switchboard will be 3 wire (no neutral) since swbds only are connected to 3 wire loads. Plaques will be added at switchboards warning that no conductors shall be connected to panel neutral bus.

Any thoughts?

 

[alehman]

I think what you are proposing is technically in violation of the NEC because you still have an unusable neutral bus (If there is neutral running current while on generator, it would be forced to return to the generator via ground).

In reality, I think it would be functional. You must have an adequately sized ground path between the switchboards and the paralleling board. Ground fault protection on the generators could be 3-wire residual or ground return.

As Mike suggested, if you could use 4-pole breakers, that might eliminate the problem.

 

[RonShap]

Is there ground fault protection at the generators? If not, remove the neutral to ground bond at the generators. Since there is a solid neutral connection at the service, then the generator is not running ungrounded.

 

[Doug98]

Thanks for comments.

The main issue I am looking to solve is the solid neutral connection between two 3ph/4w individual service switchboards through a common generator. Although this technically is an additional neutral-ground connection (code violation) in reality is it an issue?

Since the switchboards/services will be grounded through the common building ground electrode, the switchboard neutrals will already be interconnected. Each switchboard includes full size grounded neutral conductor (sized same as phase conductors) routed to the respective service transformer. The service feeder distance is much shorter than the connection to the gen switchgear. The effect of the neutral connection on ground fault operation should not be significant.

I would like to leave the solid connected neutral between each swbd and the generator switchboard. Generators would be solidly connected to neutral, not separately derived.

Any thoughts?

 

[dw51]

You've indicated your local inspector has already offered direction on his requirements, is he likely to accept the violation you want to install? Establishing parallel paths for ground faults can have unexpected effects on low-voltage ground fault systems. Remember, one of the primary reasons for the GF requirement was due to higher impedance arcing ground faults which resulted in a number of switchboards burning down.