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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?

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

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  • \$\begingroup\$ 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. \$\endgroup\$ – robert bristow-johnson Dec 17 '15 at 5:04
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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.

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