delta delta banks

[jkendrick]

I just recently started on a new system which is completely 12470 y primary. Today a lineman told me that the system has a few delta/ delta banks. I have never heard of anyone having a delta/ delta bank on a 12470 y primary. Is this possible?

 

[davidbeach]

Not a problem what so ever. Three phase conductors to the primary, three phase conductors from the secondary. No problems from an equipment or code standpoint for normal operation. On the other hand, other than a few very specific applications, why any body would want a delta secondary is a bit mystifying. That delta secondary leaves you with either an ungrounded system or a corner grounded system; a wye with high resistance ground would be far superior unless the associated phase shift was a problem.

 

[jkendrick]

If that is the case wouldn't they have to be very rare transformers. I mean they would have 7200 volts coming in both high side bushings opposed to 7200 on one bushing and a ground on the other. Why on earth would they even make such a transformer.

 

[davidbeach]

Delta-grounded wye would be extremely common, so delta on the high side is no big deal. Personally, I'd not do the low side delta.

 

[rcw retired EE]

"7200 volts coming in both high side bushings".

There would be 12,470 V between each of the bushings and across the winding, not 7200V.

 

[davidbeach]

True, good point.

 

[tinfoil]

Our 12470 Y-gnd system USED to be a 3-wire 12000V system, about 50 years ago.

We STILL have a few old Delta-delta services (and even one open-delta) that have lingered.

(Until a customer makes a change to his main service, there is no way to enforce an upgrade, and these legacy customers are unwilling to spend a dime to 'modernize')

 

[apowerengr]

Delta-delta bank is not uncommon. Some utilities having both 12470/7200 and 24900/14400 distribution systems will stock 14400 volt two-bushing transformers w/ taps down to 12470 to cover both systems.