When a three-phase load is comparatively small so that the installation does not warrant a 3-transformer bank. Which of the following 3-phase transformer connection is used?

Question

Question

This question was previously asked in

DFCCIL Executive Electrical 2016 Official Paper

Answer 1

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.
When the three-phase load is comparatively small so that the installation does not warrant a 3-transformer bank. Then open delta connection is also used.

Power supplied by open delta connection

Therefore,

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