# Three-phase transformer vs. three single-phase transformers

Some time ago a power plant here suffered a lightning strike which destroyed a transformer. A replacement was obtained from a decommissioned nuclear plant somewhere but required heroic efforts to move it to the plant. An employee there explained to me "The original equipment was banks of three single-phase transformers, but this one is a three-phase transformer. It's a lot bigger and heavier than anything they had to handle when they built the plant since it's basically 3 transformers in one box. That's why it's so hard to move it in."

Since then I've wondered what advantage there is to a single three-phase unit vs. three single-phase units suitably connected. Is there some subtlety in the design of the magnetic circuit which makes the heavier unit desirable?

• I think there's less iron in the big three phase transformer than in the three singles added together. – Jasen Dec 17 '15 at 4:50

• that is interesting. i understand how 3$\phi$ does that with 3 cables and a neutral return cable. if the loads are balanced, there is not much current in the return cable (or the return path in the earth). i hadn't thought of it that way with magnetic flux, but i can see how you can do that for the same reason. – robert bristow-johnson Dec 17 '15 at 5:04