high voltage three wire to low voltage 4 wire?

Can someone explain to me please how a substation converts high voltage three wire to low voltage 4 wire?

Think about how transformer windings are connected.
There are three types of connection Delta, Star and zigzag
Doing a sketch will help you work it out.

R-fur said:

Think about how transformer windings are connected.
There are three types of connection Delta, Star and zigzag
Doing a sketch will help you work it out.

Click to expand...

im looking at star-detla now but im just stuck on this today for some reason

try delta-star

R-fur said:

try delta-star

Click to expand...

sorry thats what i meant

so the 3 wires coming in would be the higher voltage on the delta then would go through a transformer and come out as "star" providing an extra 4th wire (the neutral) ?

askawaysparky said:

so the 3 wires coming in would be the higher voltage on the delta then would go through a transformer and come out as "star" providing an extra 4th wire (the neutral) ?

Click to expand...

exactly. with delta ther is no neutral butonce converted to star via the transformer ( as the psecondary is connected as star) the star point is your neutral.

telectrix said:

exactly. with delta ther is no neutral butonce converted to star via the transformer ( as the psecondary is connected as star) the star point is your neutral.

Click to expand...

The star point is neutral?

yes, the teory is too complex for a forum post, but basically. if the loads on all 3 phases are exactly balanced (equal), the the neutral current is zero. there is a formula for unbalanced loads.


N current = Square root of /(L1^2 + L2^2 + L3^2) - (L1*L2 + L1*L3 + L2*L3)

W

telectrix said:

yes, the teory is too complex for a forum post, but basically. if the loads on all 3 phases are exactly balanced (equal), the the neutral current is zero. there is a formula for unbalanced loads.


N current = Square root of /(L1^2 + L2^2 + L3^2) - (L1*L2 + L1*L3 + L2*L3

Click to expand...

Would the star balance the load because of the neutral causing there to be a balanced load that the delta could take the ?

no. the balabce of the load on the star supply is determined by what loads are connected. you could have 110A on L1, 70A on L2, and 30A on L3. that would be unbalanced, and there would be current in the neutral accordingly.

telectrix said:

no. the balabce of the load on the star supply is determined by what loads are connected. you could have 110A on L1, 70A on L2, and 30A on L3. that would be unbalanced, and there would be current in

Click to expand...

telectrix said:

no. the balabce of the load on the star supply is determined by what loads are connected. you could have 110A on L1, 70A on L2, and 30A on L3. that would be unbalanced, and there would be current in the neutral accordingly.

Click to expand...

Ahh yes Iv seen that on a diagram where you connect the phases using the N and have them all measured accordingly, I take it delta would only be used for distribution and we would transform to a star for buildings and such, because of the unbalanced loads we would be most certain to pull?

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.

I should add that there are other factors affecting the choice of star or delta connection for various applications, including how they handle harmonic currents. Because not all loads are pure resistances, even with a pure sinusoidal voltage the load current may not be sinusoidal, i.e. it may be distorted and contain harmonics. Like low-power-factor 'wattless' current, harmonic currents can add to system losses if they flow through cable resistance without delivering useful power. Because the supply impedance is non-zero, harmonic currents impress harmonic voltages on the system too, making it non-sinusoidal and possibly degrading motor performance etc.

In a 3-phase system, zero-sequence harmonics (those whose phasor does not rotate relative to the fundamental) aka triplens, 3rd, 6th etc. have the property of only flowing along the neutral, since they are in-phase in all three lines, but they can circulate around a delta winding. Therefore in a delta-star transformer, any triplen secondary current demanded by the load sets up a circulating current in the delta primary but not along the transmission lines back towards the energy source. Transmission losses and voltage distortion due to triplens are thus minimised by delta-star transformation near the point of use