Star Delta Connection – In star connection with earthed neutral, the maximum voltage of the phase winding to ground is 1 /.5 or 58% of the line voltage, while in delta connection this is equal to the line voltage in case of earthing of one of the lines during a fault. Therefore, for very high voltage transformers the star connection on the HV side is about 10% cheaper than delta connection on account of insulation cost. A delta-connected primary is necessary for a star-connected LV secondary feeding mixed 3-phase and 1-phase (line-to-neutral) loads. This is because the lines on the primary side can only carry current which add to zero. In the case of unbalanced 1-phase loads on secondary, delta-connected primary is needed to allow the flow of zero sequence current
This means that only positive and negative sequence currents flow in the lines on the delta side.
This could also be achieved by star-connected primary provided the primary and secondary star points are grounded. But this is not recommended on account of flow of ground current for unbalanced secondary loads.