high voltage three wire to low voltage 4 wire?

[askawaysparky]

No, it's nothing to do with balance.

We use delta for transmission because line plant is expensive and the lowest number of wires wins. For a 3-phase system that's 3 wires.

In theory we could use 3-wire delta for distribution to individual installations and some systems work like that, without a neutral at all. Each single-phase circuit is connected between two of the three lines. E.g. one load is connected between L1 and L2, another is connected L2 to L3 etc. There's no need for the loads to be balanced, if they are unbalanced the three line currents are simply different. Shipboard domestic power is often delta, historically it was popular in parts of Europe.

But for consumers, 3-phase 3-wire delta has drawbacks. Everything has to be double-pole switched and fused because both wires of a single-phase circuit are lines. More importantly, it misses out on an important advantage of 3-phase 4-wire which is that you can take power at one voltage L-N and a different voltage L-L according to application. E.g. with 3-phase 4-wire one supply can provide 230V single-phase power with the convenience and safety of an earthed neutral, and at the same time 400V 3-phase for industrial loads with the advantage of reducing the current by sqrt(3) saving on copper and losses. These advantages of the 3-phase 4-wire system come at the cost of providing four wires instead of three, and for installations of buildings that extra cost is well justified.

So we have standardised (at least in the UK and many countries) on 3-phase 3-wire for transmission on grounds of economy, and 3-phase 4-wire for distribution to normal consumers on grounds of flexibility. Hence, most local substation transformers are wound delta-star.

this is brilliant thankyou, very comprehensive, the second part you posted I had no idea about.

 

[pc1966]

Also to add something Lucian pointed out before but not mentioned in this post - the neutral is defined by the fact it is linked to earth at some point.

Traditionally that was at the substation where the star-centre was earthed, so your LV distribution was in fact 5 "wires" (3*L, N, and the E/CPC) but often the earth/CPC is the cable armour. This is TN-S

Today it is more common to have the N & E combined and earthed at multiple points, then separated at the installation, the TN-C-S system.

In Europe it is more common not to distribute the CPC, so both the transformer star and the installation have separate links to earth locally to the equipment/installation, this is TT.

But in all cases the conductor called neutral due to its low potential to earth is that way because it is deliberately linked to earth (sometimes by a current-limiting impedance though).

 

[RenatSha]

In Russia, we have 3 wires for a voltage of 10 kV!

 

[RenatSha]

In Russia, there are three wires in front of the transformer on the 10 kV side, there is no neutral. There is a neutral after the transformer on the 0.4 kV side.

 

[pc1966]

In Russia, there are three wires in front of the transformer on the 10 kV side, there is no neutral. There is a neutral after the transformer on the 0.4 kV side.

That is very similar to the most common UK setup for final distribution transformers with a 3-wire 11kV delta-wound primary, and a nominal 400V/230V star-wound secondary with neutral.

Others on here will know much more about the UK distribution network but I think the most common transformer size is 0.5-1MVA for urban areas, typically feeding around 100 homes.

It is rare in the UK to have domestic 3-phase supplies, though becoming more common as folk get high power electric vehicle chargers.

 

[brianmoooore]

The above is for urban and built up areas. In the rural area where I live, 15kVA are by far the most common, with plenty of 25kVA around as well.