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Can autotransformer be used for distribution?

1305 Views 13 Replies 5 Participants Last post by  Breakfasteatre
Can I use a 600/208 autotransformer for distribution? I have 347/600Y supply coming to a machine shop and need to step down to 120/208y for 400 amp 3-phase panel. 150 KVA autotransformer is readily available. Rule CEC26-266 2(a) seems to say I can and i can't. I do have a solidly grounded neutral available from the primary feed. Is this setup acceptable in Canada?
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· Chief Flunky
Field Service Engineer
4,095 Posts
Gotcha. Thought you meant available, as in at the supplier. Also thought you'd said 200a.

Not sure if there is anything in the cec on it.(I'm without a code book at the moment) But I was always told it's a bad idea to use them as a step down. Faults will run right through them causing high circulating currents. If a winding shorted you could end up with higher than expected voltage on the low side as well.
Only ever wired small ones to step up voltage at a specific piece of machinery.
Step down is fine.

Passing faults is the case with any wye wye. The bigger danger in step down is there is the minuscule potential with a loss of neutral that you will see HV on the “LV” line. It is far more unlikely in a standard transformer because it can only happen with a HV to LV short. The problem is that utility equipment tends to be full of various faults since it is all wye wye and the equipment is built with megavolts of insulation. Imagine what it takes to jump line to line or down an insulator and across a cross arm. That’s not the kind of construction non-utilities use and we don’t have 100+ kV BIL ratings. Our grounds are also vastly superior to any utility ground so with no impedance every utility fault grounds through your transformer and plant system. That’s a nonissue here.

Circulating currents is also true with wye wye. It’s a fact of life. But millions of utility transformers are wye wye. One advantage is unlike deltas it acts like 3 single phase transformers so the propensity to back feed on a single disconnected phase is much less. Not that you would drop only one phase considering the intended use. Juxtapose this small loss against the crazy low %Z and lower losses of an autotranaformer. Just don’t attempt to float the neutral unless you have significant surge protection: we launched parts in a tea stand that way (13.5 kV around 4 MVA) when the oil filled RC filter boiled and misted everything…can you say fuel air bomb? We launched bolts 30 feet away.
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