Currently working at a facility that has 2 Big Generators. Each Generator has a EM Distribution Panel a ways away from them inside the Facility on the top floor. Thier both 480/277 going to 2 identical Panel boards Main Breaker'd at 800 amps each. Each panel subfeeds 2 or 3 small ATS's. The two EM panels are mounted right beside each other. There is a extra 800 amp breaker in each panel making a jumper between the two. There's locks on the jumper breakers and are always locked off. Way it was designed so im told is if one GenSet Fails, its feed can be shut off at the main breaker in the panel. and the 2 aux breakers opened up. One GenSet take up the slack where the other failed. My concern or question is not with this setup. I actually like the idea. It's with the neutral conductors im questioned. There is 2 jumper wires going from panel to panel. Which is a good thing, when one GenSet is Locked out. But if things are working 100%, everythings normal. Isn't both panels neutral bonded together ? The facility's been up for 5+ years and they say there's no problems with it that they've noticed. Im just wondering if both GenSet's are running and both supplying power to thier individual panels like thier suspost to be doing. What are those 2 600MCM's going from panel to panel neutral bus doing to it. Shouldn't i need some kind of Switch to isolate the 2 neutral bus's? Im by far a engineer or know about harmonics and waveforms. Just looking for some advice and education. Thanks as always.... Hector/.
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Here's a Wild One. @ GenSets
- Thread starter HighLeg77
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Hey
Hey
Hey
First: It sounds like the gensets do not parallel, they have a bus tie breaker. If so, the two sources (genset source breakers) and the tie breaker require mechanical interlocks, usually Kirk Key deadbolts, to prevent the tie breaker from closing with more than one source (genset) breaker closed.
Your statement "its feed can be shut off at the main breaker in the panel. and the 2 aux breakers opened up." Should read one feed shut off and the tie breakers closed (using keyed mechanical interlocks).
Second: If the gensets are Y connected but the neutral never leaves the genset it is bonded, grounded, and earthed individually (per 250-30) at the genset and the feed out is 3 ph 3 wire no neutral. That's the clean way to do it, no worry about multiple neutral to ground connections and neutral current on a parallel ground path because the switchgear would be also 3 ph 3 wire no neutral (277 v to ground).
If the gensets are 3 ph 4 wire and the neutral comes with the feeder to the gear you have something to watch out for. The gensets are not "seperately derived systems" because of the shared neutral. All the neutrals connect together, utility and generator, but there can be only one connection from the neutral to the earth grounding system.
That's what I would look for. If you see any neutral to ground connections you would have to make sure it's the only one, a single point of connection. If there's more than one neutral to ground connection, very possible because of the multiple sources, you would have neutral current on the grounds paths and that would have to be corrected.
Hey
Hey
First: It sounds like the gensets do not parallel, they have a bus tie breaker. If so, the two sources (genset source breakers) and the tie breaker require mechanical interlocks, usually Kirk Key deadbolts, to prevent the tie breaker from closing with more than one source (genset) breaker closed.
Your statement "its feed can be shut off at the main breaker in the panel. and the 2 aux breakers opened up." Should read one feed shut off and the tie breakers closed (using keyed mechanical interlocks).
Second: If the gensets are Y connected but the neutral never leaves the genset it is bonded, grounded, and earthed individually (per 250-30) at the genset and the feed out is 3 ph 3 wire no neutral. That's the clean way to do it, no worry about multiple neutral to ground connections and neutral current on a parallel ground path because the switchgear would be also 3 ph 3 wire no neutral (277 v to ground).
If the gensets are 3 ph 4 wire and the neutral comes with the feeder to the gear you have something to watch out for. The gensets are not "seperately derived systems" because of the shared neutral. All the neutrals connect together, utility and generator, but there can be only one connection from the neutral to the earth grounding system.
That's what I would look for. If you see any neutral to ground connections you would have to make sure it's the only one, a single point of connection. If there's more than one neutral to ground connection, very possible because of the multiple sources, you would have neutral current on the grounds paths and that would have to be corrected.
Each GenSet has a Seperate 800amp 3ph 4wire feeders going to each seperate panel. 800 amp breaker in each generator and 800 amp main breaker in each panel. and 800 amp jumper breaker in each panel. the jumper breakers do not have key locks. Thier more of a bolt on style lock that gives room for a padlock to lock the breakers in the on or off position. One of the generators has the neutral bonded to gound at the generator, the other does not. im just concerned with the wires they have jumping between the 2 panels for connecting the neutral bus's together. When one generator is locked off... these jumpers are essential. when both generators are operating normally thats my concern.
Right
Right
Right
Ben: If the utility matched the gensets, Y secondary but the neutral never leaving the main, each source would be 3 ph 3 wire, the gear 3 ph 3 wire, no neutral, and each source earthed and grounded separately. That's what I was thinking as the clean way to do it, sorry I was not clear. In that scenario all the loads would be delta and single phase 277 volt lighting loads would have a separate transformer, 480 delta primary, 480/277 Y secondary. Looks like that's not what the OP has but if he did it would save him headaches (multiple neutral to ground connections and stray neutral current on the grounds).
Hector: First, the gensets do not ever run parallel because you have no paralleling equipment. You have dual main breaker with a tie busbar breaker. NEC requires mechanical interlocks on the mains and the tie breaker to prevent the tie from closing with more than one source connected. It's open transition switching, break to one source before closing to the other source (the tie breakers).
Second: Sounds like you have three sources, the utility and two gensets all 3 ph 4 wire, shared neutral (no neutral switching). Shared neutral is OK contingent on there being only one neutral to ground connection per 250-30 (A).
I'm sure the utility transformer and the service main have the neutral to ground bonding jumper per 250-30 (A) and so the gensets are NOT separately derived. The gensets neutrals must be always and effectively insulated from ground connection. That's what concerns me and what to look for, a second neutral to ground connection is also a code violation. The neutral bar to neutral bar cabling does not worry me, only if the neutral has a second connection to ground in addition to the one at the utility service.
If you see a second neutral to ground connection at the gensets, that could be placing the grounding system in a parallel path with the neutrals and neutral current will divide up and flow over it.
It sounds like you may want to pass this onto a PE EE who knows power distribution.
Right
Right
Ben: If the utility matched the gensets, Y secondary but the neutral never leaving the main, each source would be 3 ph 3 wire, the gear 3 ph 3 wire, no neutral, and each source earthed and grounded separately. That's what I was thinking as the clean way to do it, sorry I was not clear. In that scenario all the loads would be delta and single phase 277 volt lighting loads would have a separate transformer, 480 delta primary, 480/277 Y secondary. Looks like that's not what the OP has but if he did it would save him headaches (multiple neutral to ground connections and stray neutral current on the grounds).
Hector: First, the gensets do not ever run parallel because you have no paralleling equipment. You have dual main breaker with a tie busbar breaker. NEC requires mechanical interlocks on the mains and the tie breaker to prevent the tie from closing with more than one source connected. It's open transition switching, break to one source before closing to the other source (the tie breakers).
Second: Sounds like you have three sources, the utility and two gensets all 3 ph 4 wire, shared neutral (no neutral switching). Shared neutral is OK contingent on there being only one neutral to ground connection per 250-30 (A).
I'm sure the utility transformer and the service main have the neutral to ground bonding jumper per 250-30 (A) and so the gensets are NOT separately derived. The gensets neutrals must be always and effectively insulated from ground connection. That's what concerns me and what to look for, a second neutral to ground connection is also a code violation. The neutral bar to neutral bar cabling does not worry me, only if the neutral has a second connection to ground in addition to the one at the utility service.
If you see a second neutral to ground connection at the gensets, that could be placing the grounding system in a parallel path with the neutrals and neutral current will divide up and flow over it.
It sounds like you may want to pass this onto a PE EE who knows power distribution.
Looking at this again
Looking at this again
Looking at this again:
If the two generators are wired as separately derived the neutrals connect together with only one neutral to ground connection as your post says. You see one connection to ground at one generator.
If this is the case the problem is at the ATS, they must all be 4 pole and switch the neutral.
Something to try, at the 3 pole ATS try to amp clamp the neutrals and see what the current flow is. That by itself is not objectionable but neutral current on the ground is and there's probably no way to amp clamp the conduits. What clamping the neutral at the ATS will tell you is if the neutral current is significantly larger than expected for that circuit. You could be shunting to ground neutral current from the rest of the building trying to get back to the utility service.
For a wild suggestion you could also try walking near the ATS's with a compass or gauss meter to see if you have stray magnetic fields. I don't know if that will work but that's the idea, neutral current flow on the grounds will make magnetic fields.
Again, multiple neutral to ground connections would be the problem.
Looking at this again
Looking at this again:
If the two generators are wired as separately derived the neutrals connect together with only one neutral to ground connection as your post says. You see one connection to ground at one generator.
If this is the case the problem is at the ATS, they must all be 4 pole and switch the neutral.
Something to try, at the 3 pole ATS try to amp clamp the neutrals and see what the current flow is. That by itself is not objectionable but neutral current on the ground is and there's probably no way to amp clamp the conduits. What clamping the neutral at the ATS will tell you is if the neutral current is significantly larger than expected for that circuit. You could be shunting to ground neutral current from the rest of the building trying to get back to the utility service.
For a wild suggestion you could also try walking near the ATS's with a compass or gauss meter to see if you have stray magnetic fields. I don't know if that will work but that's the idea, neutral current flow on the grounds will make magnetic fields.
Again, multiple neutral to ground connections would be the problem.
What are the loads
What are the loads
What are the loads.
Hector, what are the loads. If the loads are all or mostly delta, motors or delta primary distribution transformers, one of the ways to clean it up would be to go to all 3 ph 3 wire switchgear, no neutral connected loads, no interconnection of the source neutrals, and each source Y neutral earthed and grounded, then insulated, and not leaving the source equipment. Grounded Y with 3 ph 3 wire distribution, 3 pole ATS's would be OK.
What are the loads
What are the loads.
Hector, what are the loads. If the loads are all or mostly delta, motors or delta primary distribution transformers, one of the ways to clean it up would be to go to all 3 ph 3 wire switchgear, no neutral connected loads, no interconnection of the source neutrals, and each source Y neutral earthed and grounded, then insulated, and not leaving the source equipment. Grounded Y with 3 ph 3 wire distribution, 3 pole ATS's would be OK.
Hey Dan, Thanks alot for your time and input i really appreciate it. The Loads are mostly sad to say very sensitive Data and Com Equipment. There are a few 30 & 45KVA xfrmers that bust the 480/277 down to 120/208. Quite a few UPS's and also supplemental air cooling data rooms, certian lights in certian areas. The EM lighting ckt is gen backed up. Going without a neutral conductor would be nice, i havnt thought of that yet till u said it, but won't be a option.
Really liking the idea of amp clamping and doing some research that way. Your probally right. Id imagine the utility power is definatly neutral to ground bonded. And for sure one of the Gens is. Thats really the only 2 i can think of that are upstream of these 2 panelboards. As for downstream, every Xfrmer is im positive. but i dont think thats a issue. Then again 99.9 percent of the time these 2 panelboards have no voltage on them. Only time thier live is when the generators kick on.
I donno about three sources. Your statement thou sounds right to me. " the utility and two gensets all 3 ph 4 wire, shared neutral (no neutral switching)."
Think i will take your advice and get this EE out here to explain a thing or 2. His name is stamped all over these drawings i just inheriated. I definatly think something needs to be done. im still concerned with them 2 jumper neutral wires going between the 2 panels. youd think a means of switching these open or closed should have been considered, atleast id think.
Thanks again Hector/.
Really liking the idea of amp clamping and doing some research that way. Your probally right. Id imagine the utility power is definatly neutral to ground bonded. And for sure one of the Gens is. Thats really the only 2 i can think of that are upstream of these 2 panelboards. As for downstream, every Xfrmer is im positive. but i dont think thats a issue. Then again 99.9 percent of the time these 2 panelboards have no voltage on them. Only time thier live is when the generators kick on.
I donno about three sources. Your statement thou sounds right to me. " the utility and two gensets all 3 ph 4 wire, shared neutral (no neutral switching)."
Think i will take your advice and get this EE out here to explain a thing or 2. His name is stamped all over these drawings i just inheriated. I definatly think something needs to be done. im still concerned with them 2 jumper neutral wires going between the 2 panels. youd think a means of switching these open or closed should have been considered, atleast id think.
Thanks again Hector/.
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