Overhead XFMR Bank - Picture 1

[saladhawks]

Can someone provide some insight on the below Overhead XFMR Bank picture. Here is what is known (from top to bottom):

Top Conductors - 2400/4160Y, 4-wire, 3-Phase Primary
Middle Conductors - 120/240V, 3-wire, 1-Phase Secondary (taps are for house service drop from this pole)
Bottom Conductors - 240V, 3-Wire, 3-Phase Secondary
Each OH XFMR - 50 KVA (serve 240V, 3-Wire, 3-Phase Secondary)

My specific question involves the primary connection of the two 50KVA XFMRs.

I have been advised that the XFMRs are connected Open-Delta / Open-Delta.

It appears that one of the XFMRs is connected phase-to-phase (4160V), while the other XFMR is connected phase-to-neutral (2400V). What am I missing here?

 

[alehman]

It wouldn't be open delta if connected line-neutral. The individual transformer secondaries must be single-phase, not three-phase, because the primary is 2-wire (and 50kVA is a standard size for single-phase, not three-phase).

Open-delta/open-delta could provide the two secondary voltages you describe by using the center-tap of one of the secondaries.

Are you sure the bottom primary is a neutral?

 

[davidbeach]

The angle and lighting of the photo isn't great, but it appear that the two transformers are each connected phase-to-neutral. Why there are two neutrals on the top conductors is beyond me (is this somewhere where two-phase {quadrature} distribution still exists?) but notice the number and location of the fused cutouts. As far as I can see, there is one per transformer, which strongly suggests only one phase conductor to each transformer.

 

[cuky2000]

This appears to be a Scott (T) Connection with 2 single phase pole units. This type of transformer bank can be connected from 2 phase with 90 degree apart as shown in the enclosed sketches in the link below. This connection is infrequent and most likely is in old distribution systems.

Please verify if the primary side (Y, 3 phase).

http://cuky2000.250free.com/Transf Scott - 2 Ph to 3 Ph.tif

 

[cuky2000]

The above link may exceed the size allow. Enclosed is the same copy with lower resolution.

http://cuky2000.250free.com/Scott Transf 2 Ph to 3 Ph.jpg

 

[DanDel]

It can't be open-delta primary if there's two transformers and four connections; open-delta only uses two transformers and three connections. I've never seen a MV neutral carried along with the phase conductors, anyway.

I'm leaning towards the 2-phase theory already presented by cuky2000, and, as also previously stated, it must be an old distribution system. Where is the picture from?

 

[tinfoil]

You can have 3 phase open-wye delta just as readily as open-delta / open-delta, and that still allows for centre-tapping one of the secondaries to get 120/240.

So you may have either 2 neutrals amongst the four primary wires, or one of the three phases is there twice in an open-delta (say A phase on the top and bottom).

 

[saladhawks]

In response to the questions about the original post, I finally got a chance to visit the overhead pole in question.

With the help of the above posts, it took me about a 1 minute to figure out what was going on.

The picture below shows that the far right primary terminator does connect to either of the transformers.

It must be a remant from days past when there were actually 3 transformers mounted on this pole to provide 3-phase secondary service.

Thus, this is a simple Open-Wye / Open-Delta transformer configuration to provide 3-phase secondary service.

Thanks for the assistance. The key observation was the fact that there are only 2 primary cutouts.