Three-phase transformer vs. three single-phase transformers

Question

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?

Answer 1

Imagine each single-phase transformer has an 'O'-shaped core. When you make a 3-phase, you combine one side of each 'O'. If the 3-phase system is balanced (equal magnitudes of currents in each leg), then the total flux through the combined core sums to zero -- basically the centre leg is not necessary. In practice, it is usually present -- it helps in case there is some imbalance in the phase currents, or even a shorted fault in one phase.

So, instead of needing three complete 'O' cores, you just need 3 'C' cores. This saves about half the iron.

Answer 2

It differs in behaviour to manage harmonic currents multipliers of 3, the strongest is 150Hz (US 180Hz). As those harmonics come in exact phase, their flux collide and cancells out, in 3 single transformers all the harmonics have a free path.
This is also the reason why the neutral connection is important in the 3 single phase conf., to carry those harmonic balancing currents.