Can you parallel a Delta-Delta bank with a Wye-Delta bank?

• A. Only with a special coupling
• B. Yes
• C. No
• D. Only if the angular displacement is corrected

Answer: (C)

Paralleling transformers requires that their output voltages be in the same phase angle so their currents add up instead of fighting each other. A Delta-Delta bank has no phase shift between its primary and secondary voltages, while a Wye-Delta bank introduces a 30-degree phase displacement. If you try to connect these two banks in parallel to the same bus, their voltages aren’t aligned in phase, so circulating currents flow between the banks and load sharing is not even. This causes overheating, unequal loading, and possible protective trips. The only way to parallel them would be to introduce a device that corrects or compensates the phase angle (such as a phase-shifting transformer) or to reconfigure one bank to match the other, but directly paralleling them isn’t feasible.