240v open delta to 230v wye transformer grounding/bonding

[SSDriver]

I have a job that is 240v open Delta with B phase grounded and I need to install a drive isolation transformer that has a wye secondary. I was curious about potentially having a grounding/bonding issue. The B phase is grounded on my primary and normally I would be bonding XO on the secondary side. I wasn't sure if I was going to have an issue due to having 2 sides of the transformer bonded. The secondary is going to a fused disconnect, then to another room to a double throw safety switch that will chose one of two drives that it will supply power to.

 

[Hv&Lv]

240v open Delta with B phase grounded

I'm assuming it’s a corner grounded delta?

 

[texie]

I have a job that is 240v open Delta with B phase grounded and I need to install a drive isolation transformer that has a wye secondary. I was curious about potentially having a grounding/bonding issue. The B phase is grounded on my primary and normally I would be bonding XO on the secondary side. I wasn't sure if I was going to have an issue due to having 2 sides of the transformer bonded. The secondary is going to a fused disconnect, then to another room to a double throw safety switch that will chose one of two drives that it will supply power to. View attachment 2554309

Supply the xformer with a 2 pole breaker along with grounded B phase. You don't bond anything on the primary. On the secondary go to a 3 pole breaker or fusible switch. Bond secondary like any SDS and install a GES.

 

[ptonsparky]

Now I would be using a three pole breaker. Take the EG that should have been developed at the SE to the transformer case as normal. Bond and ground the secondary as is needed

 

[texie]

I'm assuming it’s a corner grounded delta?

The OP indicates so but it's worth asking if he is sure this is not a center tapped 240/120 4 wire service as would be far more common.

 

[LarryFine]

I have a job that is 240v open Delta with B phase grounded . . .

This is a non sequitur. An open Delta is (almost) always a high-leg added to a typical 120/240v 1ph to supply a smaller 3ph load.

A grounded Delta would have one phase solidly grounded, as with any grounded neutral, but would supply only 3ph loads.

 

[SSDriver]

B phase is solidly grounded 0 volts from B to ground.. 240v from A -B, B-C, A-C. All loads are 3 phase only, customer told me POCO says it is open delta. I do not have access to the transformer(s) feeding the building as they are behind locked gates. They have a separate service for single phase loads. I just wanted to make sure Im not going to have an issue with the primary side having a solid ground on the B phase, and then grounding the XO on the secondary side. The existing motor is currently fed with a 3 Pole breaker. I was planning to use that to supply power to the new transformer, and then I was going to have a fused disconnect on the load side of the transformer. Thank you all of the input so far

 

[LarryFine]

I just wanted to make sure Im not going to have an issue with the primary side having a solid ground on the B phase, and then grounding the XO on the secondary side.

No, there's no issue. That's exactly why you are using a D-to-Y transformer: for primary-to-secondary isolation.

One conductor of a separately-derived system can (and usually must) be grounded; in your case, the neutral.

Added: To me, a corner-grounded, open-Delta supply is weird, but there's no electrical reason why it can't be.

 

[SSDriver]

Thank you, I have done a bunch of transformers but this is the first one with a grounded primary phase conductor and wanted to make sure I'm wasn't missing something dumb. I have never worked on a system with a corner grounded open delta before.

 

[winnie]

I just wanted to make sure Im not going to have an issue with the primary side having a solid ground on the B phase, and then grounding the XO on the secondary side.

I agree with LarryFine: what the transformer cares about is the supply line-line voltage, not the line-ground voltage (as long as the line-ground voltage is not so high as to over-stress the insulation system. As long as the primary gets 240V line-line, it doesn't matter if that is corner ground delta, high leg delta, or 240/138V wye.

The existing motor is currently fed with a 3 Pole breaker. I was planning to use that to supply power to the new transformer, and then I was going to have a fused disconnect on the load side of the transformer. Thank you all of the input so far

Sounds reasonable. If you were to put fuses on the primary side, then you would need a 'blank' in the B phase, because you are not supposed to interrupt a grounded conductor with a fuse. But you are fine with a 3 pole _breaker_ since that interrupts all phases simultaneously.

-Jon

 

[SSDriver]

On a side note how would you identify the wires different for delta and wye even though they are both 230v? I'm not a huge fan of tags. Primary delta currently is BLACK-WHITE-BLUE. would the secondary wye have to be something other than BLACK-RED-BLUE-WHITE? What would be the least confusing way to do it if needed.

 

[LarryFine]

What would be the least confusing way to do it if needed.

If wye with a neutral: black/red/blue; if delta with a neutral: black/orange/blue.

If you need to differentiate further, pick any other non-grounded/ing colors.

 

[Hv&Lv]

On a side note how would you identify the wires different for delta and wye even though they are both 230v? I'm not a huge fan of tags. Primary delta currently is BLACK-WHITE-BLUE. would the secondary wye have to be something other than BLACK-RED-BLUE-WHITE? What would be the least confusing way to do it if needed.

That must have been the way you were taught..
Personally, I wouldn’t use white for anything other than a neutral (grounded conductor)
Orange for nothing other than a high leg. (208 or 416 high legs)

 

[SSDriver]

That must have been the way you were taught..
Personally, I wouldn’t use white for anything other than a neutral (grounded conductor)
Orange for nothing other than a high leg. (208 or 416 high legs)

The white on the delta was for the B phase on the open delta system with the grounded B phase. Not a high leg. Sorry for the confusion. I wasn't sure if the colors had to be completely different on the primary vs secondary due to one side being delta vs wye.