Which of the following connections in a three-phase transformer is called open-delta connection?

Open-delta connection:

• An open delta connection transformer uses two single-phase transformers to provide a three-phase supply to the load.
• An open delta connection system is also called a V-V system.
• Open delta connection systems are usually only used in emergency conditions when one of the transformers in the Delta-Delta system is damaged, and removed from service.    
• Efficiency for the operation of open delta connection is low as compared to delta-delta systems which are used during standard operations.

Power supplied by open delta connection

\({P_{V - V}} = \sqrt 3 {V_{ph}}{I_{ph}}\)

Power supplied by open delta connection

\({P_{{\rm{\Delta }} - {\rm{\Delta }}}} = 3{V_{ph}}{I_{ph}}\)

Therefore, \({P_{V - V}} = \frac{1}{{\sqrt 3 }}{P_{{\rm{\Delta }} - {\rm{\Delta }}}}\)

\(\frac{{P_{V - V}}}{P_{{\Delta }-\Delta }} \ = 0.577\)

Hence, the total load supplied by the open delta system is 57.77% of the closed delta system.