Have two wrong sources, need a correct one.

[itsmoked]

We have a 3-phase 240V Delta source.

It is used, via a set of three transformers to create a 3-phase 120V Wye source.

We have to run a standard USA 240V range.

Unfortunately the range uses 120V and needs a neutral for the 120V heat settings and the controls.

If the 120 Wye is used, only 208V is available which doesn't get the job done.

How would you guys solve this?
What's the most economical solution.

Suggestions....

Thanks in advance.

 

[alehman]

Install a 240:120/240V single-phase transformer to power the range. Another option would be to install a single-phase boost transformer to convert two legs of the 208 service to 240.

You may be able to use two legs and the neutral from the 120/208Y system. The range will work at somewhat reduced (208^2/240^2 = 75%) power.

 

[itsmoked]

Thanks alehman. Can this neutral be tied to the 120V Wye neutral? My guess is that they are not in phase.

 

[alehman]

The neutrals can (and should) be connected, but only at the grounding electrode.

One thing I didn't think about - if the range has any motors in it that are connected line-to-line, they may not like two phases of the three-phase system due to the phase angle (120deg vs. 180deg. for a true single-phase source).

 

[davidbeach]

If a load is connected to two phases of a wye system, it has no way of determining the angle of the phases. The load sees one voltage and cannot tell how that voltage is derived.

 

[itsmoked]

Okay what can we say about this structure?

Can this neutral point be connected to the same safety ground provided by the Delta's generator?

Is the KVA rating requirement of the transformer only a function of the neutral current?

The biggest neutral current would only be during the largest single phase imbalance?

What happens when one leg of the transformer opens?

 

[fsmyth]

Yes.
No.
Yes.
For either of the two top windings, not much.
For the winding it is in parallel with, it will depend
on the size of the transformer, since it is now the
bridge between the other windings. Think open-delta.
Would you connect a load across the open in that?
<als>

 

[alehman]

david - I agree if it is connected to only the two legs, which would probably be the case.

fsmyth - isn't VA = In x 120V?

 

[itsmoked]

Adding some clarifications..

The transformer is only being used as an auto transformer.

It would seem to me the transformer's VA is only the step-down VA required by the 120V loads.

 

[edison123]

itsmoked,

Looks like you are loading only one phase of the generator. What about negative sequence heating of the rotor ? If I remember right, the unbalance should be restricted to 20%.

 

[itsmoked]

Hi edison123;

Good point. The Range is 14kW max with everything cooking (a rare situation). The generator is, I believe, 50kW.

So we are at 28% (rarely).

It looks like the "warmer" and the "convection boost" elements are 5A 120V each and that other 120V loads amount to maybe 2A (lights, fan and controls). All other elements are duty-cycled 240V.

 

[edison123]

If the heaters in the range are of multiple units, may be you can rewire them into 3 phase blocks and then use the 120 V controls in the phase that is least loaded. This way a balanced loading of the generator is assured.

 

[ccjersey]

Are you sure the generator can't provide a neutral tap in one of the (probably) series delta circuits?

Then you could have 120/240 3 phase 4 wire service and not need the delta-wye transformer bank for the 120 V loads. Or if you are dealing with a reconnectable set, you could set it up single phase 120/240 with either the low zigzag or double delta connection. either would be simpler than the transformer arrangements.

 

[alehman]

If the elements are duty-cycled they are probably single segments, wired at 240V. In that case, there should little neutral current (controls and fan, maybe). You still need the neutral, however.

What cc said about reconnecting the generator. I would be slightly surprised if there isn't some way to get 120/240 with a neutral.

 

[itsmoked]

edison123; Good idea on the internal wiring. I will look into it. Not sure it can work because one leg is run to the control boards and the other is run directly to the elements so there may not be a way to mix legs. It should be noted that the generator is always running other single phase and three phase loads and that the range would be put on the least loaded existing single phase side.

ccjersey,alehman; The three phase triangle I show is on a daily basis shifted from a delta only 50kW generator to multi-megawatt HEP (Head-End-Power from an Amtrak Locomotive provided as Delta via a 480->240 transformer) to utility 240V open delta.

The interest in using the auto transformer scheme is a effort to avoid hanging a 15kW transformer in a very cramped, moving structure, that is subject to multi-G shocks.

 

[alehman]

I think you can size your auto-transf to match the maximum neutral current. I've seen similar arrangements in very old buildings where a small 120/240 distribution tranf. is used for this purpose.

NEC would require OC protection to trip the *load* if the load on the auto-transf. execeeds 125% of its rating. Also, you must have a means to trip the upstream OC if there is a single-phase condition or internal fault. This is detailed in NEC Article 450 (but then I guess being a railroad, you don't have to comply with the NEC).

 

[itsmoked]

I would rather conform to NEC so I will dig mine out and read 450. Thanks alehman that's what I was looking for.