There one more variation of the open-delta arrangement used by electric utilities, and fairly common in some areas—that is open-wye grounded primary, open-delta secondary. At:
http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm you can visualize it looking at Fig.1-1 Y-∆ layout. Now remove the primary winding at the ten o’clock position that is connected to L2. The hi-side wye point has to be connected to the hi-side neutral conductor [not shown on 1-1.] so there are still three conductors serving on the primary side of the bank. Now, on the secondary, delete the winding at the 2 o’clock position. [Sometimes this is incorrectly called ‘two-phase,’ but that designation is reserved specifically for 90° phase-displaced voltages.]
In areas where the multigrounded common neutral is used, it saves a fused cutout and wire for the unneeded third phase. {For geeks only—In some areas where lo-side delta voltages are needed, a grounded wye-delta bank is prohibited, for it acts as an undesirable ground source and can induce high currents in the common-neutral conductor during hi-side faults.}
[This message has been edited by Bjarney (edited 06-08-2002).]
