Three phase autotranformers or transformers with yy Connection are usually provided with tertiary winding in delta connection The rating of the tertiary winding is usually between 15% and 30% Of the rating of the main windings. The transformer is designed with respect of the short circuit
impedance between the main windings. The short circuit impedancees between the tertiary winding and at least one of the main windings is very low, so that the the tertiary winding is usually not able to carry out the short-circuit current, if the supply is strong. The usage of reactors to
adjust the short circuit impedance makes the transformer complicated, beary and troublesome. The tertiary winding must be checked relative to short circuit current and if it is not able to carry out this current care must be done not to set it out the dangers of the short circuit current.