When paralleling a Delta-Delta bank with a Wye-Delta bank, what issue prevents proper paralleling?

• A. Voltage level mismatch
• B. Phase displacement of 30 degrees
• C. Overheating
• D. Ground fault risk

Answer: (B)

Paralleling transformers works best when their output voltages are in step with each other, meaning they share the same phase angle and direction. A Delta-Delta bank and a Wye-Delta bank have different vector relationships between their primary and secondary voltages. The Delta-Delta arrangement produces essentially zero phase displacement between its line voltages, while a Wye-Delta arrangement introduces a 30-degree phase shift. That makes the two banks’ voltages out of step when tied together in parallel, so they push current into each other rather than sharing the load. This circulating current prevents proper paralleling and can cause heating and voltage regulation problems. Other issues like voltage level mismatch or overheating can occur in general, but the fundamental obstacle here is the 30-degree phase displacement.