home-made neutral

[SparkyTris]

I was asked to have a look at an electrical installation recently:

there is a "normal" PME 240VAC supply feeding a medium sized solid-state 3-phase 400VAC generator (based on VFD inverter I think)
this machine outputs 3 conductors L1 L2 L3, being the three phases; 415VAC interphase.

as an afterthought, a "neutral generator" (not my words) has been installed.

this is a machine in a vented steel case into which the three phase lines are connected, and out of which comes a Neutral line. I believe there is a transformer inside this steel case.

there is 240VAC from each phase to this neutral conductor. (which is what the so-called neutral generator was installed for)

there is 160VAC between the incoming PME 240VAC neutral and the abovementioned 3-phase "synthesised neutral".

Why is there a significant voltage difference between the incoming DNO neutral and the 3-phase neutral?

 

[SparkyTris]

what I did not do of course was to see what the PD between the incoming L and the 3phase Neutral was

 

[JohnW2]

... there is 240VAC from each phase to this neutral conductor. (which is what the so-called neutral generator was installed for) ... there is 160VAC between the incoming PME 240VAC neutral and the abovementioned 3-phase "synthesised neutral". ... Why is there a significant voltage difference between the incoming DNO neutral and the 3-phase neutral?

I would have thought that, assuming there is a transformer (or other isolating means) in the 1-phase-3-pase concersion, such that the 3-phase output is 'floating' then the potential difference between the 1-phase neutral and the 'synthesised' 3-phase neutral could be "anything" (dependent primarily on stray capacitances etc.).

I would therefore suspect that if you connected some '230V' load (e.g. a 230V incandescent light bulb) between those two neutrals (or between the 'synthesised neutral' and earth), the potential difference between them would probably fall to near-zero.

However, others may have other ideas!

Kind Regards, John

 

[bernardgreen]

Why is there a significant voltage difference between the incoming DNO neutral and the 3-phase neutral?

Probably because the entire 3 phase system has not been given any means of referencing its neutral to ground. Maybe the 3 phase system is intended to NOT be referenced to ground and thus be fully floating or maybe the installation is incorrect. Seek advice from the technical department of the manufacturer of the invertor.

 

[SparkyTris]

I would therefore suspect that if you connected some '230V' load (e.g. a 230V incandescent light bulb) between those two neutrals (or between the 'synthesised neutral' and earth), the potential difference between them would probably fall to near-zero.

I thought exactly the same, so I did exactly as you suggested, and the bulb lit, with a convincing 160VAC. So that 160VAC is a true, "stiff" voltage!

 

[JohnW2]

I thought exactly the same, so I did exactly as you suggested, and the bulb lit, with a convincing 160VAC. So that 160VAC is a true, "stiff" voltage!

Ah. That does surprise me, assuming that it was an incandescent bulb (and not a LED/CFL etc.) - was that the case?

Kind Regards, John

 

[SparkyTris]

Probably because the entire 3 phase system has not been given any means of referencing its neutral to ground. Maybe the 3 phase system is intended to NOT be referenced to ground and thus be fully floating or maybe the installation is incorrect. Seek advice from the technical department of the manufacturer of the invertor.

Thanks, yes yes and yes.
What would be a practical arrangement of "referencing" such a system to ground?
If one simply connects the Neutral from this neutral generator (I suspect this is simply a star-wound transformer, with the central point being called Neutral) onto a convenient earth point (such as the neutral of the DNO), then the 160VAC will drive a considerable current through it.

Would the invertor have to be set up such that one of its 3 phases its exactly in-phase with the DNO Line? (although I would have thought that this would occur anyway for design convenience)

 

[SparkyTris]

Yes, took care to use an incandescent.

 

[bernardgreen]

For most installations ( generator or invertor ) that I have experienced the midpoint of the output was connected to a local ground rod and the CPC of the 3 phase system to ensure the majority of faults would trip at least one safety cut off device. ( MCB or RCD ) .

 

[JohnW2]

Yes, took care to use an incandescent.

Strange.

It sounds as if the 3-phase output must in some way be earth-referenced. I cant help feeling that the fact that 400-240 = 160 might well be relevant, but I can't think quite how. I initially wondered whether one of the 3-phase outputs had somehow been connected to earth, but can't really see how that could explain what you are observing. However, have you tried measuring from each of the 3-phase outputs to the incoming (single-phase) neutral (or earth)?

Kind Regards, John

 

[JohnD]

Would the invertor have to be set up such that one of its 3 phases its exactly in-phase with the DNO Line? (although I would have thought that this would occur anyway for design convenience)

It must be completely isolated from the DNO line.

 

[ericmark]

Neutral as name implies has no or little polarity to earth, however around the world there are different ways to tie the supply to ground, it seems in USA they have a hot wire system where the transformer is delta wound but one winding is centre tapped.

The problem is when grounding any cable is ensuring some one has not already grounded a different point. I had this with a duel voltage Honda generator, it had two windings, one was centre tapped, in parrellel you had 110 volt the centre tap on one winding giving 55-0-55 however in 220 volt mode in series you got 55 - 0 - 165 i.e. neither out put was earthen and if you earthen the output marked neutral it would be a direct short.

The fact the inverter and neutral auto transformer are separate means we can only guess how the two are internally wired.

 

[AdrianUK]

The "neutral generator" could be a zig-zag or interconnected star transformer. This a three phase, three limb transformer, with two coils on earth limb. The coils are connected across the phases in such a manner as to create a high impedance to positive & negative sequence currents but a very low impedance to zero sequence currents.

These are more commonly found in transmission systems are as way to earth reference a delta system but could be used in the manner above to provide a way of providing a neutral from a system with a delta source.

 

[ericmark]

Seen zig zag option with 12 winding generator, where windings could be arranged to give single or three phase with voltage options. Not seen zig zag used with a transformer, I can,t see why a three phase transformer could not be wired to use all three phases for a single phase supply, in the same way as a generator, however in this case it's three phase in and three phase out.

With centre tapped delta transformer be it auto or isolating one could use the USA hot wire neutral method, however also it could simply be the transformers can be wired star or delta and some one has made an error.

Only option is to open it and look.

 

[AdrianUK]

Seen zig zag option with 12 winding generator, where windings could be arranged to give single or three phase with voltage options. Not seen zig zag used with a transformer, I can,t see why a three phase transformer could not be wired to use all three phases for a single phase supply, in the same way as a generator, however in this case it's three phase in and three phase out.

With centre tapped delta transformer be it auto or isolating one could use the USA hot wire neutral method, however also it could simply be the transformers can be wired star or delta and some one has made an error.

Only option is to open it and look.

Its not quite the same as the zig-zag way of configuring a genny to get a single phase supply. There is no way to configure a transformer to supply a single phase load from a three phase system and load the phases equally.

A zig-zag earthing transformer isn't a transformer in the sense that it has a primary & a secondary. It only has a primary. The coils are connected so that each limb has one coil fed from its 'own' phase & a second coil in anti-phase fed from one of the other phases. The final three ends form the artificial star point. For a better description & some vector diagrams, google "zig-zag earthing transformer"

This is a way of creating a "star point" in the middle of a delta system, as an alternative to using corner grounding. Its used alot HV transmission systems to enable ground fault detection