Y/Y/Δ or Δ/Y/Δ transformer for a series connected 12-pulse rectifier?
A 3 winding power transformer isn't equal to a 12 pulse conver transformer. A 12 pulse transformer can have 4 windings. 2 HV and 2 LV windings. The HV and LV windings are arranged in double tier for better decoupling (still a coupling exsiting) of the winding systems and to have equal short circuit voltages.