It is used in where the transformer are connected in star delta.

One must check the design of the transformer windings. STAR - Delta or Star to star or Delta - star or Delta Delta. Wiring must be done accordingly.

The open Delta or V-Connection transformer is an improvement of the 1 three-phase transformer. The open Delta transformer can restore power in a Delta-Delta three-phase bank when one of the three single-phase transformers, which can be individually disconnected and replaced in case of failure, breaks down. The open Delta transformer then allows functionality to be restored, but at a reduced power of 57.7 of the original power.

With a transformer...

With a transformer...