True or False: The polarity of each transformer used in a three-phase bank must be the same.

• A. True
• B. Not applicable
• C. Sometimes
• D. False

Answer: (D)

Polarity tells you which end of a winding is positive at any instant, and it must be coordinated with how the three windings are connected to produce the correct phasor relationships. In a three-phase bank you’re aiming for voltages that are 120 degrees apart with the right phase sequence, and the way you tie the windings together (wye, delta, and the chosen vector group) determines which ends of which windings are connected to which buses. Because of that, you may need some windings to be connected with opposite polarity relative to others to get the correct overall phasing. So there’s no requirement that every transformer in the bank have the same polarity. The bank is configured to achieve the desired phase relationships, and reversing polarity as needed helps accomplish that without causing improper phasing or circulating currents.