Converting Delta Delta to Grounded-Wye

[adamscb]

Hi all,

We have a 4160v/480v, delta/delta transformer that we're considering changing out to a delta/grounded-wye (long story short - we may think the TX may have a turn-turn or winding-winding short, still yet to put the TTR to it). I think the capacity is 500kVA, all it feeds is two motor starters for our well pumps.

A question I had is when converting from delta-delta to grounded-wye, really all you have to do is to run an earth grounding conductor from the fourth bushing on the secondary of the transformer, to the grounding lug where the two grounds from the motor starters are coming from, is that correct? Of course in addition to re-connecting the transformer metal housing to earth ground.

Also as far as sizing this earth grounding conductor, from my understanding since there are parallel 500's on the secondary, this earth grounding conductor would be sized for 2/0, per table 250.66

Just checking that my thought process is headed in the right direction...thanks in advance.

 

[synchro]

Of course, double check that the delta secondary has not been corner grounded. If it was then any bonding to ground on that line would need to be removed, and overcurrent protection added on the previously grounded conductor if it's not already there. This is all probably obvious but I'm mentioning it since you've asked what else might need to be done.

 

[adamscb]

Of course, double check that the delta secondary has not been corner grounded. If it was then any bonding to ground on that line would need to be removed, and overcurrent protection added on the previously grounded conductor if it's not already there. This is all probably obvious but I'm mentioning it since you've asked what else might need to be done.

It's an ungrounded delta, not corner-grounded...but a good point.

 

[dkidd]

A question I had is when converting from delta-delta to grounded-wye, really all you have to do is to run an earth grounding conductor from the fourth bushing on the secondary of the transformer, to the grounding lug where the two grounds from the motor starters are coming from, is that correct? Of course in addition to re-connecting the transformer metal housing to earth ground.

You need to determine (see nameplate?) if the secondary is delta or wye wound. If it is delta, the fourth bushing, if present, won't help.

You may need to look at a zig-zag transformer to derive a goundable neutral point.

 

[MrJLH]

Based on what you described for clarification, are both motor starters tapped directly off the transformers?

Also you will need to install some kind of ground grid so that the transformer and motor starters are at the same reference point.

 

[kwired]

You need to determine (see nameplate?) if the secondary is delta or wye wound. If it is delta, the fourth bushing, if present, won't help.

You may need to look at a zig-zag transformer to derive a goundable neutral point.

If secondary is delta and it has a fourth bushing, it might be a high leg delta configured secondary.

 

[Jraef]

If secondary is delta and it has a fourth bushing, it might be a high leg delta configured secondary.

Not common at 480V, but possible. kwired brings up a good point though; why would a delta secondary have a 4th bushing at all? So that is one reason, the other is that it is not a delta secondary, but they are running it as a ungrounded (floating) Wye.

 

[kwired]

Not common at 480V, but possible. kwired brings up a good point though; why would a delta secondary have a 4th bushing at all? So that is one reason, the other is that it is not a delta secondary, but they are running it as a ungrounded (floating) Wye.

True it wouldn't be too common on 480 volts. I see many 480 volt high leg systems, but they are always from systems comprised of multiple single phase transformers and not a single unit type.

Have run into POCO padmounts that are high leg 240 delta, usually as replacements where existing was high leg 240. POCO usually somewhat reluctant to install these but sometimes they will anyway - I think they cost them more than 208/120 transformer of similar rating is main reason they don't want to install them.

 

[electrofelon]

I think OP is talking about changing the transformer guys.

 

[kwired]

I think OP is talking about changing the transformer guys.

That has been my assumption, and he has told us it was ungrounded delta and will be grounded wye when done. If ungrounded delta were done correctly there should have been EGC's and GES installed anyway, all he should need to do is connect the neutral of the new system to the existing grounding conductors if it was done properly to begin with.

 

[Jraef]

If this is a new XFMR, then the problem I see with the OPs plan is this:

...really all you have to do is to run an earth grounding conductor from the fourth bushing on the secondary of the transformer, to the grounding lug where the two grounds from the motor starters are coming from, is that correct?

Since this is now the SERVICE entrance, you would not run the X0 of the transformer to two separate ground rods at the motor starters. You would need an acceptable Grounding Electrode for the service and a Grounding Electrode Conductor(GEC) running from the X0 terminal to the electrode. Then you would ALSO need Equipment Grounding Conductors (EGCs) to the cases of the motor starters and the case of the new transformer.

Rather than repeat it all, read this older thread:
https://forums.mikeholt.com/showthread.php?t=182318

 

[infinity]

I think OP is talking about changing the transformer guys.

 

[kwired]

If this is a new XFMR, then the problem I see with the OPs plan is this:



Since this is now the SERVICE entrance, you would not run the X0 of the transformer to two separate ground rods at the motor starters. You would need an acceptable Grounding Electrode for the service and a Grounding Electrode Conductor(GEC) running from the X0 terminal to the electrode. Then you would ALSO need Equipment Grounding Conductors (EGCs) to the cases of the motor starters and the case of the new transformer.

Rather than repeat it all, read this older thread:
https://forums.mikeholt.com/showthread.php?t=182318

I don't think it is all that clear if this is service transformer or a SDS. If service and it was only 3ph 3w before, we now need a 4th wire with service conductors. Beyond the service equipment there should have been separate EGC's if done correctly in the first place.

Also isn't clear exactly what these motor starters are. If they are simply a contactor and overload assembly - we probably have some code issues, but they could be pump panels or combination starters with disconnect/overcurrent protection. Pump panels are suitable for use as service equipment.

 

[Jraef]

My point was that he does not have a GEC now because it was a delta secondary, he only had two EGCs for the starters and that would have been fine. But I don’t believe he can use those EGCs as the GEC for the new wye transformer as he seemed to be proposing, they must remain as the EGCs and he must add a GEC.

 

[electrofelon]

My point was that he does not have a GEC now because it was a delta secondary, he only had two EGCs for the starters and that would have been fine. But I don’t believe he can use those EGCs as the GEC for the new wye transformer as he seemed to be proposing, they must remain as the EGCs and he must add a GEC.

An ungrounded system should still have a GEC, although its not really clear if the OP has one. In theory, after changing the transformer, all the OP would do is add a MBJ/SBJ.

 

[kwired]

An ungrounded system should still have a GEC, although its not really clear if the OP has one. In theory, after changing the transformer, all the OP would do is add a MBJ/SBJ.

Yes, should have had a GES in the first place if was done correctly.