Does it look anything like this:
This is a "Rectifier Transformer" - part of an elaborate UPS System.
Although the one shown in the drawing does not have a 4 wire Wye Secondary scheme, all that is needed to make the Wye section a 4 Wire type is to tap into the "Common Jumper" which connects the left sides of each coil on the Wye side, and extend that tapped point with a suitable sized Conductor.
This is the way that _ALL_ 4 Wire Wyes are done.
The Delta winding is used for cancelling Harmonics. It is likely connected to some external load.
Need to refresh myself, by grabbing the Electrical Engineering Handbooks!
Here's another type of Rectifier Transformer I have run across:
This has Three (3) 3 Phase Secondaries - resulting in a 9 Phase Secondary Transformer.
These drawings are from the Technical Reference section, and may be found under the topic: 6 and 9 Phase Transformers
I thought I had uploaded some pictures of these Transformers, but not sure where and when - if at all!
Plan to make a new topic in the Technical Reference section tonight, which will have pictures of these "Specialty Transformers" I have come across. Once this is done, will reply to this thread with link(s).