electricalforum.nz

I'm connecting three phase step down auto transformer. 400V input and 230V output. It's to supply a piece of equipment brought over from America and the load won't be balanced.

My question is the star point common for the line and load side of the neutral. I'm not 100% sure on this one.

Cheers

Yes, like this. But that doesn't mean that the input side needs to have a neutral connected. As long as any unbalanced loads (single phase loads) on the output have a neutral connected, the output voltage will still be balanced.

Oh right, I had originally connected as in the picture with only the output side neutral connected to the star point, powered it up and all seemed fine. But after I left site, I started second guessing whether the input side neutral would be needed to carry back any unbalanced loads and keep the star point at zero potential. From your explanation it sounds like it doesn't matter either way which does make sense.
I've since managed to make it back and connect the input side neutral just to be sure!
Cheers for the response

The diagram is incorrect

If you have 3 windings connected in star on a 3-phase 400 V supply; there can only be 230 V (approx) across each of them.
The 400 V is between phases, not from each phase to earth. L-E voltage is 230 V.
Therefore the line terminals should be labelled as 230 V, not 400 V

Any tappings on the windings will be at less than 230 V - presumably in this case 110 V (not 230 V as shown).

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Without the N connection from supply to star point, the load currents have to be considered as combinations of loads in series between each pair of phases. Which is fine as long as the load between each pair of phases is the same.

But if there is any variation - for example, having a number of single-phase loads;
variations in connected load on any phase will result in the actual voltage applied to the equipment varying; up to a maximum 400 V worst case.
Plenty of cases where failing to recognise this has resulted in damaged equipment.

OP stated the load is unbalanced; therefore connecting the incoming N is crucial.

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I hope that you have worked through the fault and SC protection requirements for the 110 V circuits;
ie can the tx supply enough fault current for operation within time limits.

@AlecK

Maybe thinking about it this way?
400V phase to phase for primary.
230V phase to phase for secondary
115V phase to neutral secondary

Yes, the fault protection needs to be correct.

But for the unbalanced secondary load, it will be 115V load which was unbalanced, and having a secondary neutral connected to the star point should ensure that the voltage remains correct.

Or is my brain still in lockdown and I've totally screwed this up?

Yes I have screwed it up.
If it's 230V phase to phase on the secondary then it's 230 ÷ √3 to neutral which is 132V.

So I'm not sure why there's a neutral point at all in that case, or where the "unbalanced load" could come into it.

Hard to tell without seeing more of the wiring diagram.

Read NZS 6114 A standard dealing with no-standard supply voltages and give guidance.

Tho OP didn't state the nature of the equipment; other than it's from America.
Therefore will be designed for operation on 60 Hz supply.
auto transformer (any transformer) will provide 50Hz output; which may not be suitable.

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The need for neutral reference becomes clear if you forget about the transformer altogether; and just consider a selection of single-phase loads on a three-phase supply.
With N, as loads vary (being switched on or off) the out-of-balance current flows in N, and voltage in each phase remains steady.
Without N, switch off all loads in one phase, and the loads on the other 2 phases become effectively 2 sets of loads in series on a 400 V supply.
The voltage across each lot of "single phase" loads will be proportional to the impedance of each. Only if both sets of loads are equal will the voltage across each group be the same, and as individual loads are switched on or off the voltages will be constantly varying.
With all three phases

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Connecting a "STAR connected" primary winding to a MEN supply can caused high currents in the incoming neutral connections and is generaly not recommended. The cause of the high cureents being harmonic currents, in particular the 3rd and 5th harmonics. It is likely to be more of a problem if the rst of the electrical installation has non-linear control by use of SCR's VSD's etc. The usual method of fixing the high haromic currents is to leave the "Star point"of the primary winding floating.