Stand-by System Short Circuit

[FaradayFF]

Greetings,
I've got a stand-by diesel generator feeding a small pump station. I'm trying to evaluate the system from system protection perspective. The caveat here is there is no neutral brought to the MCC, since there are not single phase loads.

I've got two scenarios:
1. For Y-connected load, for phase to neutral fault, I'm assuming all three phases will be affected, as there is no neutral wire to carry unbalanced current. In this scenario, genset breaker will pop on the overcurrent and/or phase unbalance.

2. For Y or Delta connected load, for ground fault, the system will be largely unaffected. My reasoning is that there is no system ground in the system and no neutral wire available for ground fault return path, thus there is no ground return path to the system source.

Please see attached my diagram. Please share your thoughts and good technical reference if you can recommend one.

Thanks,
EE

 

[zbang]

Greetings,
I've got a stand-by diesel generator feeding a small pump station. I'm trying to evaluate the system from system protection perspective. The caveat here is there is no neutral brought to the MCC, since there are not single phase loads.

How is the overall system grounded? Is the feed to the MCC completely ungrounded, corner-grounded, or what? If it's ungrounded, are there ground detectors? You need the generator to play nice with the main feed. You also either need to provide a fault current path or some way of recognizing the fault.

(A "Wye-connected" load generally implies having a connected neutral. If that point isn't brought out, it really looks like a delta load.)

And faults on the load-side of load breakers should be protected there, not by the generator's breaker.

 

[FaradayFF]

How is the overall system grounded? Is the feed to the MCC completely ungrounded, corner-grounded, or what? If it's ungrounded, are there ground detectors? You need the generator to play nice with the main feed. You also either need to provide a fault current path or some way of recognizing the fault.

(A "Wye-connected" load generally implies having a connected neutral. If that point isn't brought out, it really looks like a delta load.)

And faults on the load-side of load breakers should be protected there, not by the generator's breaker.

Hi zbang,

The feed to the MCC is completely ungrounded(only genset equipment grounding exists). There is no system ground established in the system when the system is fed from the back-up source. The ground detectors are not required, since the source rating is less than <1000 Amp per NEC. The thought was that for a L-N or L-L fault, the genset breaker would trip based on overcurrent in all three phases. The three phase conductors would carry substantially higher current due to absence of neutral return wire in the system. There is no L-N loads in the MCC either. I'll need to do a short circuit study to confirm this.

Regards,
EE

 

[Jraef]

Hi zbang,

The feed to the MCC is completely ungrounded(only genset equipment grounding exists). There is no system ground established in the system when the system is fed from the back-up source. The ground detectors are not required, since the source rating is less than <1000 Amp per NEC. The thought was that for a L-N or L-L fault, the genset breaker would trip based on overcurrent in all three phases. The three phase conductors would carry substantially higher current due to absence of neutral return wire in the system. There is no L-N loads in the MCC either. I'll need to do a short circuit study to confirm this.

Regards,
EE

I have some issues with what you have described.

It's very very common to not run a Neutral conductor out to an MCC if there are no neutral loads required, but that does NOT mean that it is an ungrounded system! If the service transformer is a grounded Wye secondary, then the SYSTEM is a grounded Wye, regardless of the presence of a neutral conductor.

IF you truly have a 3 wire system, then you MUST have EITHER a corner grounded delta system, or you must have Ground Fault Monitoring, IF the type of load is within the narrowly defined systems allowed to have no grounding as set forth in the NEC, which would not apply to a typical "pump station". Whether or not the Main is 1000A or not is an entirely DIFFERENT issue.

240.13 = GFPE requirement for 1000A services on solidly grounded wye services. Does not apply to Delta or ungrounded services.
250.21 (A) = short list of the ONLY places where you can have an UNGROUNDED system. Has no relationship to being under or over 1000A.

 

[AtTheKeyboard]

Greetings,
I've got a stand-by diesel generator feeding a small pump station. I'm trying to evaluate the system from system protection perspective. The caveat here is there is no neutral brought to the MCC, since there are not single phase loads.

I've got two scenarios:
1. For Y-connected load, for phase to neutral fault, I'm assuming all three phases will be affected, as there is no neutral wire to carry unbalanced current. In this scenario, genset breaker will pop on the overcurrent and/or phase unbalance.

2. For Y or Delta connected load, for ground fault, the system will be largely unaffected. My reasoning is that there is no system ground in the system and no neutral wire available for ground fault return path, thus there is no ground return path to the system source.

Please see attached my diagram. Please share your thoughts and good technical reference if you can recommend one.

Thanks,
EE

I don't have an answer for you, but I have a few thoughts, in case it helps:

• While most construction specifications prevent it, there are occasions when the conduit is used as an equipment grounding conductor.
• If the power source is grounded in the generator, it's effectively grounded everywhere else, unless later run through an isolation transformer.
• If it's a corner-grounded system, if I remember right, it will require fully-rated breakers, not "slash rated breakers" - i.e. 240V, not 120/240V. - I don't know what your system voltage is though. (System protection consideration too.)
• Wye (3-phase 4-wire) systems are frequently transmitted as 3-phase 3-wire, but that doesn't negate the upstream ground in any way. ...But it is a code violation to re-derive the neutral from the ground, although I've seen this an inappropriately-large number of times.
• With the want for 120V receptacles, lights, computers, etc., It's really common to do Wye systems for 208V - or sometimes High-Leg Delta for 240 V. Still, true delta systems exist, and this sounds like it could be one of those.

Just a brain dump - hope it helps.

My two favorite books for these kinds of things are the NEC (with this forum backing it up) and the IEEE 141. That Red Book covers lots of good stuff. You should at least sift through it if it happens to be in your office.

 

[zbang]

It's very very common to not run a Neutral conductor out to an MCC if there are no neutral loads required, but that does NOT mean that it is an ungrounded system! If the service transformer is a grounded Wye secondary, then the SYSTEM is a grounded Wye, regardless of the presence of a neutral conductor.

That's what I was getting at, Jraef said it better; the likelihood of an un-grounded feed to the MCC is quite small.

That also plays to the generator questions, if the source's (be it a service or an SDS) neutral doesn't extend to the MCC, you need to either bring it there or use a 4-pole transfer and bond the gen's neutral to the GES. (assuming a wye-connected source, corner-grounded delta would be similar in concept)

Remember- only one neutral/ground bond at a time and make sure there's a fault current path from the equipment ground to the source.

 

[FaradayFF]

I have some issues with what you have described.

It's very very common to not run a Neutral conductor out to an MCC if there are no neutral loads required, but that does NOT mean that it is an ungrounded system! If the service transformer is a grounded Wye secondary, then the SYSTEM is a grounded Wye, regardless of the presence of a neutral conductor.

IF you truly have a 3 wire system, then you MUST have EITHER a corner grounded delta system, or you must have Ground Fault Monitoring, IF the type of load is within the narrowly defined systems allowed to have no grounding as set forth in the NEC, which would not apply to a typical "pump station". Whether or not the Main is 1000A or not is an entirely DIFFERENT issue.

240.13 = GFPE requirement for 1000A services on solidly grounded wye services. Does not apply to Delta or ungrounded services.
250.21 (A) = short list of the ONLY places where you can have an UNGROUNDED system. Has no relationship to being under or over 1000A.

Hi Jraef,

The utility brings 3 phase conductors and neutral to MCC. The neutral is grounded at MCC via ground rod. The xfmer may also have the neutral point of Y-connected secondary grounded locally via ground rod. I'm not sure I agree that the system is a grounded WYE when stand-by generator is feeding the MCC - the generator doesn't have it's neutral point grounded or may even be DELTA connected. If there is no return path back to the source for a ground fault, how can we call the system grounded. The generator enclosure will be grounded, but that is different from system ground.

I'll study NEC references you've listed more closely.
Best Regards,
EE

 

[ron]

The neutral is grounded at MCC via ground rod. The xfmer may also have the neutral point of Y-connected secondary grounded locally via ground rod. I'm not sure I agree that the system is a grounded WYE when stand-by generator is feeding the MCC - the generator doesn't have it's neutral point grounded or may even be DELTA connected. If there is no return path back to the source for a ground fault, how can we call the system grounded. The generator enclosure will be grounded, but that is different from system ground.

If the utility transformer neutral point (XO) is grounded, then the utility is considered solidly grounded.
Is there no neutral brought to the MCC from the transfer switch or no neutral brought out of the generator to the transfer switch at all?
If there is a neutral brought to the transfer switch, and if the transfer switch between the utility and the generator is 3 pole, meaning the neutral is not switched, then the system is solidly grounded (hopefully no N-G bond in the gen alternator).

The fact that a neutral is not brought to the MCC (load) is irrespective of the system be a solidly grounded system.

 

[zbang]

The utility brings 3 phase conductors and neutral to MCC. The neutral is grounded at MCC via ground rod. The xfmer may also have the neutral point of Y-connected secondary grounded locally via ground rod.

OK, you have a solidly-grounded wye system. Period. The system, which includes the MCC and whatever it's feeding. (Where is the service disconnect? In the MCC? Perhaps we'd be better off calling it the service switchboard instead of an MCC.)

Also, in an earlier message, it surely sounded like there wasn't a neutral at the MCC (post #3).

I'm not sure I agree that the system is a grounded WYE when stand-by generator is feeding the MCC - the generator doesn't have it's neutral point grounded or may even be DELTA connected.

If the main system is wye-connected, the standby generator must then also be wye-connected, you need that to get the fault current path. I don't see how you would have a choice. And most generators can be wired for either wye or delta as conditions require.

Since you do have a neutral at the MCC, bring the generator's neutral in, bond that, and move on. Simple, easy, uses a 3-pole transfer switch; that's pretty much what everyone else does.

If there is no return path back to the source for a ground fault, how can we call the system grounded.

If there is no fault current path, then you have a design problem, and we're trying to tell you ways to solve that.

The generator enclosure will be grounded, but that is different from system ground.

All equipment grounds must tie together and then to the grounding electrode system (GES). If the generator has it's own set of electrodes, those must connect to the overall GES.

I think you may need to re-read article 250 sections II, III, and V.

 

[FaradayFF]

Thank you everyone who has responded to this thread!

-EE