Neutral derived from 3-wire system
Neutral derived from 3-wire system
(OP)
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
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






RE: Neutral derived from 3-wire system
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.
RE: Neutral derived from 3-wire system
Haveing two grounds is a problem, they need to be connected.
RE: Neutral derived from 3-wire system
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Neutral derived from 3-wire system
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.
RE: Neutral derived from 3-wire system
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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Neutral derived from 3-wire system
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
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"Why not the best?"
Jimmy Carter
RE: Neutral derived from 3-wire system
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.
RE: Neutral derived from 3-wire system
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.
RE: Neutral derived from 3-wire system
thanks for your input. I have requested that these transformers are removed and Dyn fitted.