Should i replace a 480V:240V Delta:Delta Transformer with a Delta:Wye?

sfscott

Member
Occupation
Controls Engineer
I have 240V skid based machine that gets shipped around the country and temporarily installed at our customer sites. Occasionally our customers only have 480V so someone at my company had purchased a 15kVA 480V:240V Delta:Delta transformer that we ship with the machine if need be.
I have spent some time on this site reading previous posts on the subject of grounding the secondary of a Delta:Delta transformer and I think I would be more comfortable knowing that any ground fault on my skid would open up the main circuit breaker in my skid panel.
The skid has no ground fault indication device currently.
Am I correct in saying that a leg to ground connection on my skid will NOT cause my main circuit breaker to open?
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I have 240V skid based machine that gets shipped around the country and temporarily installed at our customer sites. Occasionally our customers only have 480V so someone at my company had purchased a 15kVA 480V:240V Delta:Delta transformer that we ship with the machine if need be.
I have spent some time on this site reading previous posts on the subject of grounding the secondary of a Delta:Delta transformer and I think I would be more comfortable knowing that any ground fault on my skid would open up the main circuit breaker in my skid panel.
The skid has no ground fault indication device currently.
Am I correct in saying that a leg to ground connection on my skid will NOT cause my main circuit breaker to open?
That depends. Is your 240V delta grounded anywhere, as in a 240/120 3 phase 4 wire delta, or a 3 phase 3 wire corner grounded delta? If it is corner grounded one phase is already referenced to ground so another ground point on that leg is meaningless and you get a solid ground fault on either of the other two legs, the current will likely go high enough to trip your main. If it is 3P4W delta, you do have a ground connection so that too will let the current increase to trip the main. That's the reason for requiring the grounding somewhere.

If you are floating from ground on your 240V delta, then the NEC requires that you have a way to monitor for a ground fault. If you have none of the above, it is in violation of the code. So in a floating ground system, your first ground fault simply takes you to a corner grounded system and in that case, yes, your main may not trip. This was the reason people used to use ungrounded delta back in the day and you still can, IF you add ground fault monitoring. Notice that the NEC does not say that you must TRIP on a ground fault, only that you must MONITOR for it so that you can decide what to do.

Your third option is a 480 to 240Y137 V delta -wye transformer where you would reference that wye point to ground. The 137V isn't good for anything, it's just there for the safety / code compliance. Those are not often off-the-shelf transfers, but they can be ordered, most mfrs list them in their catalogs, often calling them "Drive Isolation Transformer" because that's the primary use for them. Drives don't like being connected to delta systems and it should be avoided if at all possible (some brands just say no, not ever). If your machine has a VFD, DC drive or servo on it, I highly recommend this option.
 
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sfscott

Member
Occupation
Controls Engineer
I will open the transformer up tomorrow but I think I can answer your first 2 questions;
The transformer has no center tapped leg so this is not a 240/120 3 phase 4 wire delta.
If I measure Ohms at the line side of my panel breaker I get high ohms on all 3 legs to ground so I do not think I have a corner ground.

sounds like to get back into compliance (before this thing ships out again) I will either install a ground fault indicator or replace the transformer. I am leaning towards replacing the transformer because even if I install a ground fault indicator all I can do is annunciate a ground fault, my main circuit breaker does not have a shunt trip.
 

sfscott

Member
Occupation
Controls Engineer
sorry jraef, I didn't initially read your 3rd paragraph. I am leaning towards replacing the transformer so thanks for the suggestion. Seems that the 480:208 delta wye are readily available. My skid has 2 VFD driven motors and a single phase heater. I know the motors are rated 208, I can look at the heater next for voltage rating. Can I also consider a 480V 208 delta wye?
 

Besoeker

Senior Member
Location
UK
I will open the transformer up tomorrow but I think I can answer your first 2 questions;
The transformer has no center tapped leg so this is not a 240/120 3 phase 4 wire delta.
If I measure Ohms at the line side of my panel breaker I get high ohms on all 3 legs to ground so I do not think I have a corner ground.

sounds like to get back into compliance (before this thing ships out again) I will either install a ground fault indicator or replace the transformer. I am leaning towards replacing the transformer because even if I install a ground fault indicator all I can do is annunciate a ground fault, my main circuit breaker does not have a shunt trip.
Just out of interest, what is the kVA rating of the transformer?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If the 240V delta is ungrounded, then a single phase to ground fault will not open a breaker.

You ask if you should change to a delta:wye transformer. That is one option, but not the only one.

You can 'corner ground' a delta transformer, which means that 2 of the phases are at 240V to ground and one of the phases is at ground potential. A fault from the grounded phase is like a fault on a normal neutral. A fault from one of the other phases will trip a breaker. If you do this you need breakers that are rated for the full 240V, not 'slash' rated.

You can change to a standard delta:wye transformer and ground the neutral.

Fargo transformers makes 'drive isolation transformers' which have multiple taps on the secondary, 480V in and 208 or 240 or 480V wye out. They could probably make a custom transformer that is the reverse of this, accepting 208 or 240 or 480V in and providing 240/139V wye for the output. You could ship this around with your machine and always know that you have a grounded wye supply.

-Jon
 

sfscott

Member
Occupation
Controls Engineer
existing transformer is GE 9T23B3881, 15kVA, 480V delta : 240V delta.
I just requested quotes on Hammond 15kVA 480V delta : 240V wye & 480V delta : 208V wye. This skid has 2 VFD driven motors (motors rated 480/230/208) and a heater. I am checking on the voltage rating of the heater element now.

Thanks for all thoughts and suggestions.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
sorry jraef, I didn't initially read your 3rd paragraph. I am leaning towards replacing the transformer so thanks for the suggestion. Seems that the 480:208 delta wye are readily available. My skid has 2 VFD driven motors and a single phase heater. I know the motors are rated 208, I can look at the heater next for voltage rating. Can I also consider a 480V 208 delta wye?
Yes, and that is easier to find for sure.
 

sfscott

Member
Occupation
Controls Engineer
I just pulled the trigger on a replacement transformer 15kVA 480V Delta to 208V Wye. Jraef and Jon thanks for advice.
 

Besoeker

Senior Member
Location
UK
Fargo transformers makes 'drive isolation transformers'
-Jon
Don't mean to hijack the thread.
But why would you want a drive isolation transformer?
I don't mean specifically for this application. I've seen it mentioned a few times on this forum.
It isn't something I have ever done purely for isolation.

Sorry mods.
 
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