Very good question. It is a little bit out of the expertise of the average lineman. Probably more in the realm of a good Electrical engineer. Usually with something of this sort I try to find a good reference book that I can hopefully understand. Usually the math required in things of this nature are a little bit beyond my ken.
The following are some excerps from a publication from GE that I picked up one place where I worked. It is a manual that is more geared toward Power Transformers or as some refer to them Sub Station Transformers. However the principles still hold true I'm sure. With this and what little I know maybe you can get a little better understanding. Probably someone else will chime in with some more info also.
Here's the GE stuff. " Although this connection delivers three-phase currents which are approximately symmetrical to a three-phase symetrical load, the currents flowing in the high voltage circuit are not equal nor are they 120 degrees apart.
The maximum safe output of the bank operating in this manner is 58% of a 3 pot Wye/Delta bank. The system is grossly unbalanced, both electrostatically and electromagnetically."
Here's what little I know about them. They will parallel with a 3 pot Wye/Delta bank. Done that many times. You can add an additional wing or power pot with out interuption. Done that a few times. Most companies limit their load to about 20 HP total 3 phase.
Here's something else I noticed in this book and I just thought I'd throw it in also as it's interesting. "Units of widely different impedances may be used to form a Y-delta bank without appreciably affecting the current division. Furthermore, there is no danger of excessive circulating currents in the delta, though the ratios are not the same, for these currents cannot be reproduced in the Y and therefore they are magnetizing currents and necessarily very small."
If you wish perhaps you might contact GE about such a book as this. It's not to be confused with the little GE Distribution handbook that is so common everywhere. This small book was compiled by the Power Transformer Department, Pittsfield,Massachusetts. dbrown20