I need a quick refresher. When hooking up transformers in a DELTA configuration what are the pro's and con's (other than cost) of using "Closed Delta" or "Open Delta".

If one transformer of a delta-delta connected bank becomes defective, it can be disconnected, and the system will continue to function as an open-delta, but can only deliver 58% of its closed-delta rating.

In my area they were used when transformer insulation wasn't as good as it is today.

They used to install two transformers, connected open-delta, and add the third, (in other words "close the delta") at some point in the future when the load increased.

Ed