3 Ph.Transformer Backfeed

[Plinko]

I'm looking for reference material on 3 phase transformer backfeed. For instants, I had a three phase bank 120/240 (gnd Y/closed delta...75/37.5/37.5 kVA) and lost one phase on the high side of the bank. Oddly enough, the phase that the cutout had isolated was actually at a higher voltage then other two phases. It's a classic delta backfeed situation. However, I'd like to show how this happens on paper (phasors, etc.)...for this and other TFR connections.
Thanks in advance....

 

[busbar]

See §4 of ANSI/IEEE C62.92.4-1991 …Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution

For the described reason, it is listed in IEEE C57.105-1978 …Application of Transformer Connections… as a “problem connection” for it acts as a very good zero-sequence source on the wye side.