The answer to that question could be very long and involved. There are just too many factors to answer this question briefly. A short introduction is available at
and I'm sure there are many others on the internet. If you have more specific questions, you may want to post them on the Electric Power Engineering forum of this site. For example, if you're considering delta/star versus star/star auxiliary transformers and want to hear feedback from users you could ask that. I'll warn you that you will get more questions, such as: Is the high side of the transformer connected to a grounded or ungrounded system? What kinds of loads will be powered? ... and more. Good luck.
Original Poster: Would you please tell all your current responsibility for the subject devices? Maybe we can better understand your needs with respect to this nontrivial subject.
