Delta on a Wye system

[bertisevil]

What are the ramifications of connecting a delta transformer to a grounded wye system?

Available primary is 12,470 GrdY/7200. The 3 phase padmount is 12,470 delta.
Secondary is 120/240 delta with one grounded leg.

The voltages are correct and the transformer will work. I am concerned about other areas like safety, backfeeds, fault scenarios, switching, etc.

All other padmounts and overhead banks are grounded wye.

Thanks for any input.

Bob

 

[dpc]

It is done all the time. The delta-delta configuration should not create any problems for you on the primary side.

This sounds like a utility distribution feeder?

 

[bertisevil]

DPC,

You are correct, distribution feeder.

Waross,

Almost correct, the secondaries will yield 120 ph-gd and 240 ph-ph on the leg with the center tap grounded.

I am mostly concerned with primary side issues.

Thanks for the feedback, guys.

Bob

 

[davidbeach]

208V to ground from the wild leg. Bill's keyboard must not have been cooperating very well. ;-)

 

[bertisevil]

This is a closed delta 240 padmount with a center tap on the third leg. Gives 120/240 on that leg. Open delta would give you a 208 leg.
Bob

 

[bertisevil]

Actually, now that I think about it.... in the closed and open delta banks you can get the 208 volt from the other legs to ground. My bad. Wasn't planning on using it that way but it is there.

B

 

[apowerengr]

The connection is fine as far as the transformer is concerned but you may have to mitigate ferroresonance issues on the distribution feeder. There are several other threads dealing with the ferroresonance topic.

 

[waross]

In a wye:wye connection, line to ground fault current on the secondary will be reflected as a line to ground current on the primary.
In a delta:wye, line to ground fault current on the secondary will be seen as line to line current on the primary.
It may make a difference to your protection planning or awareness if you add a delta:wye transformer to a predominantly wye:wye system.
Then again it may not make any difference at all.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rempman]

The biggest issue may be if you are feeding the transformer with any length of Underground Cable. We had such a ferroresonance issue that we now spec all of our pad-mounted transformers with LBOR switches.

If that is not possible use a three phase device to protect the primary cable system like a recloser or sectionalizer.

The ferroresonence occurs when one cable fuse blows or when the worker closes in one fuse.

Another solution would be to add load to the secondary of the transformer, such as a load box, when closing in the fuses.

 

[bertisevil]

Thanks for all your replies. If this was a new station service, it would be a Gnd Y - Gnd Y and I would not have any concerns.

B