Reverse fed 480/277-2300 delta secondary

[jminer99er]

I need some help understanding a ground fault on this transformers secondary. I don't deal with 480/277-2300 transformers often.

I will attach the name plate info and a few pictures. This transformer is being reverse fed. It's stepping up 480/277 to 2300V in what I believe is an ungrounded delta secondary. I did confirm from the manufacturer that the delta isn't corner grounded internally. The mfg engineer couldn't answer my specific questions because he said he's not a power systems eng.

The 2300V secondary travels about 1500' on a steel bridge to a switch (I think) and steps down to 480/277V

The primary feed is 3 phase 4 wire 480/277. The neutral lands on the XO and has a bonding jumper strap to the enclosure, which is tied in to ground rods.

The secondary has 3 medium voltage cables going into conduit with a green #4 thhn conductor. (I think this is acting like the equipment ground?) but the #4 thhn is also bonded to the enclosure.

Does this transformer need ground fault monitoring on the secondary for unintentional ground?

If a phase unintentionally goes to steel, will the 2.3kV comes back via egc to the enclosure, which the 480/277 neutral is tied to, does it put 2300V on the gear, and is it rated for that voltage, or will the primary protection open?

Does the primary even need this neutral?

Thanks

 

[DrSparks]

I need some help understanding a ground fault on this transformers secondary. I don't deal with 480/277-2300 transformers often.

I will attach the name plate info and a few pictures. This transformer is being reverse fed. It's stepping up 480/277 to 2300V in what I believe is an ungrounded delta secondary. I did confirm from the manufacturer that the delta isn't corner grounded internally. The mfg engineer couldn't answer my specific questions because he said he's not a power systems eng.

The 2300V secondary travels about 1500' on a steel bridge to a switch (I think) and steps down to 480/277V

The primary feed is 3 phase 4 wire 480/277. The neutral lands on the XO and has a bonding jumper strap to the enclosure, which is tied in to ground rods.

The secondary has 3 medium voltage cables going into conduit with a green #4 thhn conductor. (I think this is acting like the equipment ground?) but the #4 thhn is also bonded to the enclosure.

Does this transformer need ground fault monitoring on the secondary for unintentional ground?

If a phase unintentionally goes to steel, will the 2.3kV comes back via egc to the enclosure, which the 480/277 neutral is tied to, does it put 2300V on the gear, and is it rated for that voltage, or will the primary protection open?

Does the primary even need this neutral?

Thanks

View attachment 2557403 View attachment 2557404 View attachment 2557405

First, is this transformer specifically listed for back feeding?

Sent from my SM-A326U using Tapatalk

 

[hillbilly1]

Is the primary feed from a grounded 480 volt system? If yes, you do not want XO bonded on that transformer. On the load end, yes, you will need it bonded on the LV secondary.

 

[DrSparks]

You can run ungrounded, but you'll need ground fault monitoring equipment definitely and there are a lot of NFPA 70e requirements too.

Sent from my SM-A326U using Tapatalk

 

[LarryFine]

X-0

The primary feed is 3 phase 4 wire 480/277. The neutral lands on the XO and has a bonding jumper strap to the enclosure, which is tied in to ground rods.

Does the primary even need this neutral?

No, that connection should be broken. X-0 should be left floating.

In other words, the primary should be wired as if it was a delta.

Technically speaking, the feeder need not have the neutral, just an EGC.

 

[DrSparks]

You need the neutral on the 480 side or you wouldn't have a complete circuit.

No, the fault current on an ungrounded system would not return to the xfmr. That's the whole point of an ungrounded system--fault current mitigation.

Sent from my SM-A326U using Tapatalk

 

[LarryFine]

You need the neutral on the 480 side or you wouldn't have a complete circuit.

Not true; every 3ph L-L load works without a neutral.

 

[hillbilly1]

You need the neutral on the 480 side or you wouldn't have a complete circuit.

No, the fault current on an ungrounded system would not return to the xfmr. That's the whole point of an ungrounded system--fault current mitigation.

Sent from my SM-A326U using Tapatalk

Load yes, but input, no. You will need to bond the transformer case on the input side though, so a fault to the case will activate the ocp.

 

[jminer99er]

That's what I'm trying to figure out for the 2.3kV side

If this installation correct? It was put in 4 years ago, and recently dropped on my lap.

There is 70e stickers

 

[jminer99er]

Yes mfg said xfrm is OK to backfeed

 

[DrSparks]

Not true; every 3ph L-L load works without a neutral.

You're right. Yep, took me a minute to picture it my head. The neutral is only for sqrt 3 voltage.

Sent from my SM-A326U using Tapatalk

 

[xptpcrewx]

Does this transformer need ground fault monitoring on the secondary for unintentional ground?

Ungrounded systems generally need ground fault detectors.

If a phase unintentionally goes to steel, will the 2.3kV comes back via egc to the enclosure, which the 480/277 neutral is tied to, does it put 2300V on the gear, and is it rated for that voltage, or will the primary protection open?

A single ground fault on an ungrounded system just turns that system into an unintentional grounded system (kinda like a corner grounded system). Properly bonded equipment is “technically” equipotential so it doesn’t really impress dangerous voltage on the rest of the system unless there is current. What does happen though, is the line-to-ground potentials of the ungrounded system rise to line-to-line values.

Does the primary even need this neutral?

What do you mean by primary neutral?

 

[LarryFine]

Yes mfg said xfrm is OK to backfeed

The windings are shown as HV and LV, which is usually an indication.

 

[LarryFine]

What do you mean by primary neutral?

The white wires in the feeder. They could be made green and repurposed.

 

[xptpcrewx]

The white wires in the feeder. They could be made green and repurposed.

At which transformer?

 

[jminer99er]

So what is the purpose of the green #4 thhn traveling with the medium voltage cable?

Am I understanding the above replys:

1- The XO neutral on the primary should be removed? (What can happen if it stays?)

2- The 2300V secondary is ungrounded and need ground fault detection?

 

[xptpcrewx]

So what is the purpose of the green #4 thhn traveling with the medium voltage cable?

Am I understanding the above replys:

1- The XO neutral on the primary should be removed? (What can happen if it stays?)

2- The 2300V secondary is ungrounded and need ground fault detection?

The EGC is for bonding. It’s required.

The neutral to the X0 helps with voltage regulation.

The ungrounded MV system needs ground fault detection.

 

[LarryFine]

At which transformer?

The feed end, that's there now. From above:

 

[xptpcrewx]

The feed end, that's there now. From above:

I understand this to be the secondary downstream.

 

[jminer99er]

I understand this to be the secondary downstream.

No this is the primary