Voltage on a transformer

[Brian Stevens]

I have a pad mount 3 phase transformer which from phase to phase I have 480 volts. On A &C phase to ground I am getting 480 volts and on B phase I have nothing.(1.4 volts). What direction should I go in to start troubleshooting this? I believe the transformer is bad but cannot open it to check anything as it is a 2300kva transformer and the contractor has no PPE gear for them to open it.

 

[Dennis Alwon]

Is that a delta trany?

 

[jim dungar]

I have a pad mount 3 phase transformer which from phase to phase I have 480 volts. On A &C phase to ground I am getting 480 volts and on B phase I have nothing.(1.4 volts). What direction should I go in to start troubleshooting this? I believe the transformer is bad but cannot open it to check anything as it is a 2300kva transformer and the contractor has no PPE gear for them to open it.

I do not believe there is anything wrong with this transfromer, based on these voltages. It is most likely a corner-grounded transformer, like Dennis asked, or it is an ungrounded transformer that is experiencing a ground fault.

there a several normal voltage other than the 480Y/277 you may be familiar with. You need to know what your transformer is intended to deliver, before you can begin troubleshooting.

Do you have a GF indicating scheme?

 

[Brian Stevens]

Is that a delta trany?

No its a Y

 

[jim dungar]

No its a Y

Is it a grounded wye or an ungrounded one?

If it is solidly grounded at the neutral point then you can not have the voltages you described.

You said:
Vab = 480V
Vbc = 480V
Vca = 480V
Vag = 480V
Vbg = ~0V
Vcg = 480V

What about?
Van
Vbn
Vcn
Vng

Do the voltages change depending on which breaker/switches are turned on or off?

 

[Brian Stevens]

After looking into it a little further, it is not wired for a y tranny because there is no neutral. Is it possible that we lost the bonding in the transformer?

 

[jim dungar]

After looking into it a little further, it is not wired for a y tranny because there is no neutral. Is it possible that we lost the bonding in the transformer?

Are you sure that this has not been wired as a ungrounded-wye with an intentionally Grounded-B phase?

Maybe you have an ungrounded-wye with an unintentionally grounded (i.e. faulted) phase.

The odds of loosing a N-G bond on a 3-wire service is pretty low. Although, it could be possible if there was enough L-G current flowing, but you would expect the faulted phase to be arcing with >0Vlg unless the fault has become 'bolted'.

 

[hillbilly1]

Could be a resistance ground wye, with the resistance opened due to a severe fault that is still conected on the ground end.

 

[Smart $]

Could be a resistance ground wye, with the resistance opened due to a severe fault that is still conected on the ground end.

Umm... still "connected" b-phase-to-ground, i.e. "bolted" as Jim mentioned, likely where the fault occurred... or rather is continuing to occur.

No mention was made whether this is a service transformer or SDS. If service, the lack of neutral conductor would make it a non-compliant installation and, if it set up as an ungrounded system, there should be a ground detector.

 

[hillbilly1]

Umm... still "connected" b-phase-to-ground, i.e. "bolted" as Jim mentioned, likely where the fault occurred... or rather is continuing to occur.

No mention was made whether this is a service transformer or SDS. If service, the lack of neutral conductor would make it a non-compliant installation and, if it set up as an ungrounded system, there should be a ground detector.

Umm..... I know Jim stated a possible "bolted fault", but that was for an ungrounded wye, also someone mentioned a bolted fault with a solidly grounded wye, but nobody has mentioned a resistance ground system which was what I was stating.

 

[Hv&Lv]

Have you made sure all three fuses are in at the riser pole? One primary phase could be dead, although with a common core, I would expect the voltage to be a little higher unless the loads are just right. I swear that sounds like a corner grounded delta though...

 

[infinity]

Sounds like a corner grounded (B phase) to me.

 

[texie]

I hope I don't get beat up over this, so I'll be gentle. I have to wonder if it is prudent for the OP to be working on this if he does not have a full understanding of the system in question. Is it a delta, wye, grounded, ungrounded, resistance grounded, etc. This sounds like a good way to get injured or do some serious damage.

 

[jim dungar]

Umm..... I know Jim stated a possible "bolted fault", but that was for an ungrounded wye, also someone mentioned a bolted fault with a solidly grounded wye, but nobody has mentioned a resistance ground system which was what I was stating.

If a resistor is still in service, then the volategs in the OP could not exist. If a resistor is out of service, then it is the ungrounded wye I mentioned.

 

[jim dungar]

If service, the lack of neutral conductor would make it a non-compliant installation and, if it set up as an ungrounded system, there should be a ground detector.

If the wye point is bonded by the utility, then yes it would be non-compliant. If the wye point was left floating by the utility (unlikely) then the '277V' neutral point would not need to be brought to the service.

 

[Smart $]

If the wye point is bonded by the utility, then yes it would be non-compliant. If the wye point was left floating by the utility (unlikely) then the '277V' neutral point would not need to be brought to the service.

Completely agree. Yet my reply was to hillbilly1 regarding a resistance [high impedance] grounded wye system, and if it's a service transformer, a neutral would have to be brought to the service equipment.

 

[Hv&Lv]

If the wye point is bonded by the utility, then yes it would be non-compliant. If the wye point was left floating by the utility (unlikely) then the '277V' neutral point would not need to be brought to the service.

How would it be non compliant if set up by the utility?

 

[Smart $]

Umm..... I know Jim stated a possible "bolted fault", but that was for an ungrounded wye, also someone mentioned a bolted fault with a solidly grounded wye, but nobody has mentioned a resistance ground system which was what I was stating.

I know that.

I was commenting about you saying it was still connected on the ground end. I assumed you meant the resistor (or what is left of it) is still connected on the ground end. But that wouldn't explain OPer's reading's. There still has to be a bolted fault somewhere down the line.

 

[jim dungar]

How would it be non compliant if set up by the utility?

If the utility establishes a 480Y/277 service, then the NEC requires the neutral to be brought into the service equipment even though it is not required by the load.

 

[Hv&Lv]

If the utility establishes a 480Y/277 service, then the NEC requires the neutral to be brought into the service equipment even though it is not required by the load.

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