why 30 degree phase shift in transformer?

A 30-degree phase shift in transformers, particularly in delta-wye (?-Y) configurations, is primarily used to manage harmonics and improve system stability. This phase shift helps in balancing the load across phases, reducing neutral currents, and minimizing the impact of harmonics generated by non-linear loads. Additionally, it aids in the proper functioning of three-phase systems by ensuring that the voltage and current relationships are maintained, enhancing overall system performance.