Which of the following 3-phase connections of a transformer is rarely used?

Concept:Star-star type of transformer is rarely USED due to problems with UNBALANCED loads.It is economical for small high voltage transformers as the number of turns per phase and the amount of insulation required is less. Applications of different three-phase transformers:Delta – star CONNECTION type three-phase transformer is used for both LARGE and low voltage rating transformers.Delta – star transformer is used at the GENERATOR side to step up the voltage levelsStar – delta transformer is used at the load side of distribution systems to step down the voltage levels.Star – star connection transformers are used for small, high voltage transformers.Delta – delta connection transformers are used for large, low voltage transformers.