208Y/120 bank construction

electrofelon

Senior Member
This is one of those things that comes up and I am baffled I have never thought about this before. To make a wye three phase bank you need 120v transformers. Seems inefficient to use the center tap of a standard 120/240 transformer, you are paying for a lot of transformer that you are not using. Granted that reduces your inventory. Is this pretty much always how its done? This is how Seattle City Light does it. See figure 3.1d and 4.

http://www.seattle.gov/light/engstd/docs2/0125.03.pdf

So when connected in this manner, the total system KVA is half the sum of the nameplates of the three individual units?

synchro

Senior Member
They are specifying secondary conductors with double the ampacity for 208 or 240 vs. those for 480. So they seem to be expecting to get the same KVA out of the tranny by drawing double the current from only half of the secondary winding for 208 or 240. Maybe the secondary windings are oversized to handle the extra current? But that would cost money.

jim dungar

Moderator
Staff member
Seems inefficient to use the center tap of a standard 120/240 transformer
Those do not look like standard center tapped transformers.The secondary looks to be 120V and 277V.

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electrofelon

Senior Member
Those do not look like standard center tapped transformers.The secondary looks to be 120V and 277V.
That would make sense: 277 transformers with a 120 tap, use for both 208y/120 and 480y/277, but that doesnt appear to be the way they are doing it. Look at "4.0 construction notes"

No, it’s all of it.
the picture fails to show the rest of the preparation.
The 120/240 transformers have to be opened up so the A&C leads can be put together and the B&D leads can be put together to make them straight 120V.
the two secondary coils are paralleled.
Ok got it. That was my first thought that it was 2 windings that could be paralleled, like your typical "general purpose" LV transformer, but I didnt see that paralleling or note on the spec. I see it now on page 7 it says "cut to 120". Cool!

Hv&Lv

Senior Member
That would make sense: 277 transformers with a 120 tap, use for both 208y/120 and 480y/277, but that doesnt appear to be the way they are doing it. Look at "4.0 construction notes"

Ok got it. That was my first thought that it was 2 windings that could be paralleled, like your typical "general purpose" LV transformer, but I didnt see that paralleling or note on the spec. I see it now on page 7 it says "cut to 120". Cool!
On a side note, a bunch of utilities are going to conventional transformers. That style reduces your inventory because they can be used for single phase service and three phase banks.
those that stick with the CSP transformers have to keep both types.
You don’t want to build a three phase bank with CSP transformers.

electrofelon

Senior Member
On a side note, a bunch of utilities are going to conventional transformers. That style reduces your inventory because they can be used for single phase service and three phase banks.
those that stick with the CSP transformers have to keep both types.
You don’t want to build a three phase bank with CSP transformers.
CSP?

Hv&Lv

Senior Member
Completely self protected.
Basically there’s a breaker inside for secondary faults, or we can turn the secondary off and on with it.
it isn’t very robust or reliable, so we don’t use it much.
lightning also pops them with regularity

Hv&Lv

Senior Member
There’s also a fuse link on the primary but it’s not field serviceable.

Open Neutral

Senior Member
Do all transformers have such a fusable link? I took these when PSE was inspecting the existing XFMR.
I wasn't sure if the cylindrical bushing had a link in it or not.

electrofelon

Senior Member
Do all transformers have such a fusable link? I took these when PSE was inspecting the existing XFMR.
I wasn't sure if the cylindrical bushing had a link in it or not.
I don't think so. Pads usually have a fuse, it's in the upper left of the first picture. The load break elbows don't have any sort of fuse link in them. The fitting to the right of the elbow is just a cap for the feed thru bushing (might be an arrestor but doesn't look like it to me).

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electrofelon

Senior Member
Cool pic. So what situation are those for? Some major failure on the primary side conductors or windings? Would a secondary bolted fault break them?

Hv&Lv

Senior Member
Cool pic. So what situation are those for? Some major failure on the primary side conductors or windings? Would a secondary bolted fault break them?
Secondary faults will blow the bay-o-net fuse, but not the internal link.
I know from experience, more than once...

only time i’ve seen the links blown was lightning. It was “rebuilt” (links replaced and tested) and sent back to us.