Neutral derived from 3-wire system

[BritEng]

There is a current project underway on the site I am currently working and the following problem has reared its head.
A 3-core cable was installed to supply a DC UPS system and when they came to commission the system it was realised they actually needed a 4-core cable , 3 phase & neutral.
To overcome this "oversight" they are proposing to install a local autotransformer of vector group Zn0 to generate a neutral for the load. The "new" neutral will not be earthed at this point however the source where the 3-phase eminates from is a 415v transformer with solidly earthed star point.
they are proposing this to save running a new cable (excavation required). I've never seen this before and have a few questions.
Has anyone had experience of this type of "fix"

Regards

 

[jghrist]

An autotransformer will not work, but the vector group you reference if for a zig-zag earthing transformer not an autotransformer.

An unearthed zig-zag earthing transformer would do nothing. I'm not sure what the implications would be to earth the zig-zag transformer. This would establish two ground sources on the circuit with no connection between them except through earth. I wouldn't advise it.

An autotransformer is not a ground source and a connection to the earthed source will be required. A 1:1 autotransformer would do nothing anyway because the primary and secondary would be connected at the same point on the winding.

You should install a delta-wye transformer to establish a neutral. If you do, it should be earthed at the new transformer secondary.

 

[cranky108]

If the auto had a delta tert. winding, or you were using wye-delta transformer. The delta voltage dosen't matter, but the size does.
Haveing two grounds is a problem, they need to be connected.

 

[Skogsgurra]

The real question seems to be what the neutral is needed for. If the system was installed without neutral and most of your load is happy with that, but some single phase loads need to be connected phase-neutral, then one or two transformers can produce the single phase system needed.

If that is impractical, you need a Z transformer to get a neutral. If local code says that neutral shall be grounded, then local code differs between countries. Some countries use a TT system (local ground rods close to load) and some use TN (ground needs to be connected all the way to the transformer ground). Or IT (local ground, again - and neutral floating). So the answer differs a lot. Where are you? BTW.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[burnt2x]

Skogs,
I think I've read a "DC UPS" in the OP!
The 3-phase UPS requires a reference neutral that's why they need a neutral! I did experience a UPS unit not firing just because the neutral was a little bit off the ground potential. (Is your UPS a GE?)
BritEng,
I think jghrist(last paragraph) is best. Make sure you install a good ground connection.

 

[Skogsgurra]

OK. So the UPS needs a neutral itself?

If that is for the filters, then an inductive component like a transformer (Z or D/Y) will not work because of leakage inductance.

For what is the neutral actually needed?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

I suggest a delta:wye transformer. Three individual transformers to generate a neutral will not work. The voltages will be inversely proportional to the loads on each phase to neutral.
You may have a perfectly balanced load, but it is dangerous to assume that loads will never become unbalanced.
A three phase transformer with a three legged core may work. A three legged core acts as if it has an internal delta winding because of the flux return. The phantom delta winding will stabilize the phase voltages, but I have never done this. I have no idea how much neutral displacement you may get with unbalance phase to neutral voltages. I wouldn't depend on a three legged core transformer to do the job without a lot of research. Perhaps someone else will comment on this part of the issue.
BUT, when you start looking for a three phase 1:1 autotransformer I will be surprised if you can find one.
I suspect that you will end up with a conventional transformer and only use one winding. In that case, go for a 416:240/416 transformer and connect it in delta/star.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BritEng]

I have attached a drawing to explain the installation in question, I am based in the UK. The UPS system is made up multiple sngle phase chargers (hence the neutral requirement)spread across the 3 phases.
There wasn't a neutral in the local DB therfore they have installed the transformer in question to generate the neutral. The nameplate is stamped as an autotransformer.

 

[jghrist]

Curious. I certainly wouldn't call this an autotransformer. The fact that there is no connection from neutral to ground means that there are not really two grounding sources on the same circuit. The zig-zag neutral is not connected to the supply transformer neutral.

This might work, but I'd have to spend more beard scratching time before I'd be convinced.

A Delta-GrdY would have been a lot more straightforward.

 

[BritEng]

Gents,

thanks for your input. I have requested that these transformers are removed and Dyn fitted.