OT: Overhead domestic powerline question

From

From the same wiki section as above: "In a three-phase system feeding a balanced and linear load, the sum of the instantaneous currents of the three conductors is zero. In other words, the current in each conductor is equal in magnitude to the sum of the currents in the other two, but with the opposite sign. The return path for the current in any phase conductor is the other two phase conductors. ...

Three-phase systems may have a fourth wire, common in low-voltage distribution. This is the neutral wire. The neutral allows three separate single-phase supplies to be provided at a constant voltage and is commonly used for supplying multiple single-phase loads. The connections are arranged so that, as far as possible in each group, equal power is drawn from each phase."

Not sure if that's what you were looking for, but it makes some sense. I see, by the way, that one of the leaders in 3-phase development was Nikola Tesla.

I think the efficiency does increase with higher number of phases, but diminishing returns mean there's little point going higher than three in practice.

Ideally the neutral could be zero diameter, if the phases were perfectly balanced, but in practice they have to accept some in-balance

If the load on thr three phases is balanced (as it should be) there will be no current at all flowing in the neutral. The neutral just carries any out of balance.

The neutral is a bit suspect - at least on the high voltage side - since normally the loads are strung between the phases. The neutral is only "created" at the substation on the LV side by being the centre tap of a star topology transformer. Local loads can then be strung from any of the stepped down 240V lines to neutral, or fed with 415V between any two phases.

Basically you get improvements in efficiency with more phases - the load is also easier to drive for the turbine (less "cogging" of the generator as the poles of the rotor pass the stator). However each new phase also introduces extra cost - more windings, more wires etc. So it was found that there is a big gain going from single to three phase, but then the extra efficiency gains thereafter don't offset the costs of adding extra phases.

Actually it could be the other way round...

If you look at the typical secondary of a substation transformer, the layout is similar to:

There is some of that as well - there will be minimum physical size required for the installation method chosen. The current carrying capacity (i.e. CSA of the cable) for the lines will be set based on the design load. The neutral will be oversized, but does allow for a completely unbalanced load (i.e. all on one phase) if needs be.

Not as much as many other things.

Notionally, the neutral wire carries no current! Its the same size as the other three to keep the number of wire-types at one.

PA

I suppose that a good analogy is cars. A 6-cylinder engine runs way more smoothly than a 4-cylinder engine.

Incidentally, a system can have a change of phase during the distribution system. A good example of that is our third-rail DC railways. Takes in 3-phase then converts it up to up to 24-phases!

PA

Thanks for all the replies. I am a wiser man!

That would only apply to premises supplied with three phase. Domestic premises (i.e. houses) would be connected to one of the three phases plus neutral.

It has more to do with the phases on the distribution transformers, the standard on UK networks is DY Delta Star, Delta (think the Greek symbol) on the HV and 3 pointed Star one for each phase and centre point neutral. Thus the 440 volt network has 4 conductors as you mention.

You need to look up the word "Notionally".

Yes think of the waveforms and their phase relationship to each other in your mind, and you can see why this works. As for why three phase overheads have another wire, I'm not sure. There are more than three or even four on the pylons near me, though I cannot see them now. I did wonder if the single wire was perhaps a local ground for those working up there, but I've never seen them doing this. Brian

I have and it doesn't say "if there are only three phase consumers".

As soon as a single phase line connects to other single phase lines on a different phase, the neutral currents upstream dissappear as they cancel each other out, mostly.

A large 100kV+ pylon has only a small earth/neutral running along the tower tops

It all depends on the transmission voltage. LV (240/415) networks require a neutral conductor for pd (potential difference) as they are Star networks (centre point 'earthed')

High voltage networks (11/33/132/400kv) do not require a neutral.

The 2/3/4 wires you see on pylons are grouped conductors on the same phase

It was usual to supply one phase to the first house in a row, a second phase to the second house, the remaining phase to the third house, before starting again with the first phase. It does mean that it might not be a good idea to run an extension lead from the next door neighbour though.

To someone knowing nothing about electrics, this was interesting, but left a point unanswered. The trick with 3 phase (120 degree shifted) requiring a minimal return, should also work with 2 phase (180 degree shifted). So why are two phase systems typically 90 degree shifted with a third return/common at a cycle maximum of 1.4 times the load of the Phase1 and Phase2 wires, rather than 180 degree shifted with a notional zero load return/common. In actual life, I think they may use four wires for two phase.

AIUI this is due to the design of induction motors needing some type of perpendicular electromagnetic load in order to start. 180 degrees means this will not happen. 90 degrees means it always happens, as does 120 degrees. Presumably, induction motor usages was the driver between this decision?

On 24/09/2023 10:54, Pancho wrote: Presumably, induction motor usages was the driver between this

Behind this decision, not between. I meant the reason they commonly use

Well 180 degree shift is, in a sense, still single phase. Merkins use it to get 220V having realised 110v is simply crap.

there is also 66kV and 275 kV networks too