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Old 09-01-2019, 07:00 AM   #1
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Default Grounded B phase - where to bond?

We lost our 3000 kva primary transformer yesterday, and discovered something that didn't seem quite right.

We have a 480 only corner grounded delta service. We own the transformer.

Currently, the secondary of the transformer enters our switchgear which has a disconnect for each of the 2 sides of the switchgear. B phase is bonded to ground at the transformer lugs, not in the switchgear.

So when the disconnects are open, the B phase is no longer grounded.


Now, in "normal" operation I suppose that is not an issue, but due to the failure of the transformer, we have had to use a rental generator for temporary power. This generator is a Y type.


This is not the 1st time that we have relied on this generator. 1st time, we bonded the generator ground to our plant ground. I assume the generator "ground" is the center of the Y, and neutral. This time, the ground was not connected, just the 3 phases.

Our generator feed enters the buss through a spare breaker and, the service disconnects are open. When the generator was "grounded" it was bonded to the plant ground grid, but not B phase.


So, questions...

Is our current location of the B phase bond correct? Or should it be after the disconnects? "point of 1st disconnect" Which would be on each of the two busses.


Provided we leave the generator neutral/ground unused, what would happen if we did have the B phase bonded to ground after the disconnects and then connected the Y generator? Therefor directly grounding one of the Y legs of the generator.

If we leave the bond point unchanged, what point is there to connecting the generator ground?

Anything else?

Thanks

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Old 09-01-2019, 07:08 AM   #2
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you lost me on this part:

the secondary of the transformer enters our switchgear which has a disconnect for each of the 2 sides of the switchgear.

So when the disconnects are open, the B phase is no longer grounded.
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Old 09-01-2019, 07:13 AM   #3
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If I were setting this up and you only had 480, I don't see how your system would know the difference. I have to assume the system is healthy and you have a phase monitor.
I dont think I would disconnect the neutral to ground.

JRaef or Micromind would be excellent members to comment on this one

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Old 09-01-2019, 09:01 AM   #4
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If the generator is a wye unit, if you bonded one of it's phases to ground, you would have a ground fault. The generator probably has it's own built in over current/ ground fault protection and should function fine on your system as connected and the generator should trip on any fault.

Your plant system is correct as wired, but a corner ground on that size system can have some crazy high fault current. A high resistance ground system could be a better option. Of course it has "always worked before". Could a major ground fault, on the ungrounded phases be the cause of the transformer failure?
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Old 09-01-2019, 09:37 AM   #5
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If the generator is a wye unit, if you bonded one of it's phases to ground, you would have a ground fault.

But. Wthout a ground on the trailer mounted generator, it would have no reference to plant ground.


Not sure yet why the xformer failed, but oil samples have had a history of moisture. About a month ago. We had the oil "dried". Discovered that moisture was/is entering through a cracked HV insulator. Its possible that a recent rain allowed additional moisture into the xformer. However, the HV fuse that was blowing was on a different phase than the cracked insulator.
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Old 09-01-2019, 09:44 AM   #6
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But. Wthout a ground on the trailer mounted generator, it would have no reference to plant ground.


Not sure yet why the xformer failed, but oil samples have had a history of moisture. About a month ago. We had the oil "dried". Discovered that moisture was/is entering through a cracked HV insulator. Its possible that a recent rain allowed additional moisture into the xformer. However, the HV fuse that was blowing was on a different phase than the cracked insulator.
Why would you not bond the generator to your building's ground? I'm pretty sure it is required in the NEC.

I would guess that you're on the right track with the cracked insulator letting in water. Unless you're going to do a post mortum on the transformer, you may never know. Maybe a megger on it before you remove it, might show you more?
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Old 09-01-2019, 09:48 AM   #7
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I do believe that our plan is to do an inspection of the transformer this week to determine if it can be repaired or needs replaced.

The oil had darkened significantly since the cleaning, but did not have any significant odor.

The missing bond was an oversight and has likely been remedied by now.
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Old 09-01-2019, 09:52 AM   #8
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But. Wthout a ground on the trailer mounted generator, it would have no reference to plant ground.


Not sure yet why the xformer failed, but oil samples have had a history of moisture. About a month ago. We had the oil "dried". Discovered that moisture was/is entering through a cracked HV insulator. Its possible that a recent rain allowed additional moisture into the xformer. However, the HV fuse that was blowing was on a different phase than the cracked insulator.
You said that you bonded the generator to the plant ground grid in the OP. This is as it should be.
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Old 09-01-2019, 10:55 AM   #9
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Keep in mind that in most corner founded installations B phase has no fuses in the motor disconnects.
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Old 09-01-2019, 12:02 PM   #10
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A grounded wye generator couldn't work on a corner grounded system without leaving the generator ground unconnected. Otherwise there would be a direct short.

However, the frame of the generator relative to the plant ground is now hot, so theres a danger in that. The only way to safely use a wye generator on that system would be through a transformer.
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Old 09-01-2019, 04:49 PM   #11
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Keep in mind that in most corner founded installations B phase has no fuses in the motor disconnects.
Yes, this would be the only issue with this temporary set up.

I assume that with both of the service disconnects locked off, the bonding to ground of "B" phase does not now exist?

If the generator has ground fault detection, It should trip if a ground fault exceeds the trip threshold of the trip unit setting. Since all of the loads are phase to phase, the plant over current protection should work normally. (Think of the old two overload motor starters.)

As mentioned above, if you wished to maintain a corner grounded system on the generator, it would require a wye to delta transformer connection between the generator and the plant. Delta rental generators may exist, but I would not think that they would be readily available. Of course, needing a transformer was your original problem.

Since "B" or any other phase is no longer intentionally grounded, the generator frame would not be "energized" as mentioned above.

This situation seems to be one of those "do what is necessary to keep the plant running".
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Old 09-02-2019, 06:46 AM   #12
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Oh, I see that the phase is grounded in the transformer. I missed that the first time through. Disconnect open = no grounded phase.
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Old 09-02-2019, 07:13 AM   #13
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A grounded wye generator couldn't work on a corner grounded system without leaving the generator ground unconnected. Otherwise there would be a direct short.

However, the frame of the generator relative to the plant ground is now hot, so theres a danger in that. The only way to safely use a wye generator on that system would be through a transformer.
If the generator is grounded at the generator it will be a problem, if the generator XO is not bonded in the generator then you have a B Phase grounded Wye.

And if this is the setup while not legal the frame is not "hot".

Why not reconnect the generator in a Delta? And then mirror the plant setup.
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Old 09-02-2019, 12:46 PM   #14
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Keep in mind that in most corner founded installations B phase has no fuses in the motor disconnects.
Three fuses and overloads is the minimum allowed since UL changed the Listing requirememt back around 1970. You must follow manufacturers instructions. It's the easy way to date old equipment.

It's a grounded phase (neutral) but it's not just carrying the unbalanced current in three phase circuits like it is in mixed single/three phase loads like at an MDP. So if the motor is running at say 10 A what is the current on the grounded phase? How about an ungrounded phases? And what is the voltage? Would you undersize the ampacity because it's a neutral? Did you lift the ground on the control power so ground faults on field wiring stop blowing fuses too? If I have two motors going to a junction box, I can save space by just running one neutral back to the starters since it's just a neutral with zero Volts, right?


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Old 09-02-2019, 09:16 PM   #15
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Originally Posted by paulengr View Post
Three fuses and overloads is the minimum allowed since UL changed the Listing requirememt back around 1970. You must follow manufacturers instructions. It's the easy way to date old equipment.

It's a grounded phase (neutral) but it's not just carrying the unbalanced current in three phase circuits like it is in mixed single/three phase loads like at an MDP. So if the motor is running at say 10 A what is the current on the grounded phase? How about an ungrounded phases? And what is the voltage? Would you undersize the ampacity because it's a neutral? Did you lift the ground on the control power so ground faults on field wiring stop blowing fuses too? If I have two motors going to a junction box, I can save space by just running one neutral back to the starters since it's just a neutral with zero Volts, right?


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A "grounded phase" in a corner delta system is not a neutral.



So the rest of your questions should now be answered.
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Old 09-02-2019, 09:38 PM   #16
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Quote:
Originally Posted by acro View Post
We lost our 3000 kva primary transformer yesterday, and discovered something that didn't seem quite right.

We have a 480 only corner grounded delta service. We own the transformer.

Currently, the secondary of the transformer enters our switchgear which has a disconnect for each of the 2 sides of the switchgear. B phase is bonded to ground at the transformer lugs, not in the switchgear.

So when the disconnects are open, the B phase is no longer grounded.


Now, in "normal" operation I suppose that is not an issue, but due to the failure of the transformer, we have had to use a rental generator for temporary power. This generator is a Y type.


This is not the 1st time that we have relied on this generator. 1st time, we bonded the generator ground to our plant ground. I assume the generator "ground" is the center of the Y, and neutral. This time, the ground was not connected, just the 3 phases.

Our generator feed enters the buss through a spare breaker and, the service disconnects are open. When the generator was "grounded" it was bonded to the plant ground grid, but not B phase.


So, questions...

Is our current location of the B phase bond correct? Or should it be after the disconnects? "point of 1st disconnect" Which would be on each of the two busses.


Provided we leave the generator neutral/ground unused, what would happen if we did have the B phase bonded to ground after the disconnects and then connected the Y generator? Therefor directly grounding one of the Y legs of the generator.

If we leave the bond point unchanged, what point is there to connecting the generator ground?

Anything else?

Thanks

disconnect for each of the 2 sides of the switchgear. <-Not sure what yo mean here.


The first time when you grounded the generator to the plant grid you were correct in doing so. You just don't need to run the "neutral" into the plant because you do not need it.



The generator gets grounded to the XO in it's setup as there is no more corner bonding done in your plant.



Now here is the "catch-22" in using this setup.



The GFI protection in the generator will protect your plant and personnel from any faults as before... BUT


Since it was a corner grounded delta system all your breakers and fuses for the B phase should have been direct connections.



Therefore; any shorts to ground on the B phase will shut down the generator, and therefor the entire plant. Phase A and C will blow breaker/fuses.



In your current setup without the Generator bonded to the plant grid you are essentially running an Isolated system. Needs indicator lights at least.



Shorts to B phase will go undetected and cause a voltage potential between the plant and the frame of the generator. And other weird things...
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