Electrickery question

[journeyman Mick]

Disclaimer: I am definitely not touching this stuff myself, I'm not even getting a sparky in, I just want to know "why is this so?"

When I look at the power lines I note that the HV lines have three conductors, one for each phase. Yet the LV lines on the same poles have four wires, one for each phase and one for neutral.
Why is this so?:confused:

I would have assumed they would both have the same number of wires. Secondly, what does the neutral connect to at the transformer??

Mick

[Schtoo]

Because the HV is 3 phase, pure and simple.

The LV (under 1,000V) is 3 phase and neutral.

The LV side of the transformer is connected in a 'star' configuration. Each wire comes in, goes through the load (say, your toaster) and connects to the other 2 active wires. The neutral is formed in the middle of the 3 pointed star.

If you have a 3 phase device, it's often connected in a 'delta' configuration, where each wire comes in, goes through the load (motor windings?) and connects to one other wire. The whole thing looks like a triangle, hence the delta name. There is no neutral formed in this type of connection, since 3 phase does not need a neutral.

In delta connections, the voltage is 415V between each wire. In star, the voltage is 240V between each wire and the other two wires/neutral point.

For a picture, draw a 3 lines meeting in the middle. Halfway along each line draw a squiggle. The squiggle are toasters, the meeting point the neutral. This is star.

Draw a triangle, and draw squiggles between each point. The squiggles are motor windings, and each point is each incoming wire of the 3 phases. This is delta.


Does that help, or do I need to draw you a schematic?

[MurrayD99]

Schtoo, I thank you for your explanation because I too have wondered... but why am I feeling like I just attended a double period of Physics on a Friday afternoon? Is it Friday? Benny would know!

[Exador]

Because the HV is 3 phase, pure and simple.

The LV (under 1,000V) is 3 phase and neutral.

The LV side of the transformer is connected in a 'star' configuration. Each wire comes in, goes through the load (say, your toaster) and connects to the other 2 active wires. The neutral is formed in the middle of the 3 pointed star.

If you have a 3 phase device, it's often connected in a 'delta' configuration, where each wire comes in, goes through the load (motor windings?) and connects to one other wire. The whole thing looks like a triangle, hence the delta name. There is no neutral formed in this type of connection, since 3 phase does not need a neutral.

In delta connections, the voltage is 415V between each wire. In star, the voltage is 240V between each wire and the other two wires/neutral point.

For a picture, draw a 3 lines meeting in the middle. Halfway along each line draw a squiggle. The squiggle are toasters, the meeting point the neutral. This is star.

Draw a triangle, and draw squiggles between each point. The squiggles are motor windings, and each point is each incoming wire of the 3 phases. This is delta.


Does that help, or do I need to draw you a schematic?

Also, in LV, the neutral is connected to earth, which serves the multiple purpose of stabilising the reference (neutral) potential and providing a handy return path, thus removing the need for a return cable to the generator to complete the circuit. It also gives a nice spot to detect any untoward leakage between the actives and neutral in the final circuit, as the reference potential is fixed and so any leakage will lead to a current flowing between earth and neutral, which is easily and cheaply detected at currents well below those likely to cause harm to people (the ELCB and RCD safety switches).

[Iain]

And when one phase drops out you lose the power to your place but the neighbours still get to watch TV.
Having no neighbours we are never sure what is happening when the power goes off as we have no reference points (neighbours), this became evident a couple of years ago when the big fuse under the eave decided to suicide and we waited for a few hours before calling TXU who told us there was no power failure in the area.

[Schtoo]

Craig, I aced physics in high school, went to sparky school for 3 years and then passed the test first hit.

I really don't understand what you are getting at there... :confused: :confused: :confused:

[journeyman Mick]

Sorry Schtoo, still doesn't make sense to me.:confused: I understand about star and delta conections and how to draw them, but just don't understand why HV is 3 phase only and LV is 3 phase and neutral. What happens at the pole transformer? You've got 3 phases of HV going in, and 3 of LV + a neutral coming out, (I guess?) what's the neutral connected to?

Mick

[Ashore]

Mick the high volatge lines transfer power from one point to another ie sub station to sub station or power station to sub station etc so they don't need an earth. They use high voltage as there less losses over long distances
At the sub station the HV is converted to LV via transformers and for ease of use they use 3 phase and earth (neutral ) , it is proberly the best way to get power to indivudal homes as it can be 240 v ( 1 phase and a neutral ) or 415v (three phase) depending on your needs.


Rgds

[journeyman Mick]

Mick the high volatge lines transfer power from one point to another ie sub station to sub station or power station to sub station etc so they don't need an earth. They use high voltage as there less losses over long distances
At the sub station the HV is converted to LV via transformers and for ease of use they use 3 phase and earth (neutral ) , it is proberly the best way to get power to indivudal homes as it can be 240 v ( 1 phase and a neutral ) or 415v (three phase) depending on your needs.


Rgds

Yep, I understand why they use HV and that it needs to be brought down with a transformer, but where does the neutral come into it and where's it connected to at the transformer?:confused:

Mick

[chrisp]

Mick,

The neutral is connected to the "star" point (i.e. centre intersection) of the "star" side of the transformer. This is also tied to "Earth" at the household.

Chris

[Exador]

Craig, I aced physics in high school, went to sparky school for 3 years and then passed the test first hit.

I really don't understand what you are getting at there... :confused: :confused: :confused:

Sorry mate, 5 o'clock in the morning or whatever that was isn't the best time to post if I want to make myself understood . I'm not entirely sure what I was getting at either:eek:.
I believe I was trying to point out that in a LV single-phase setup, the earth is bonded to neutral because of the safety factor of keeping one side at the same potential as people, but also that because of that, the safety switches have a conveniently stable reference potential (earth) with which to sense any faults.

Also, the earth does provide a nice return path to the generator.

[journeyman Mick]

Mick,

The neutral is connected to the "star" point (i.e. centre intersection) of the "star" side of the transformer. This is also tied to "Earth" at the household.

Chris

So at the transformer are the three LV terminals connected in a star formation to the centre neutral terminal? Wouldn't this just mean a dead short to earth seeing how the neutral is bonded to earth at the house? I've got a feeling that this is one of those things I'm just not going to understand, ever ( a bit like why women think they way they do :confused: )

Mick

[Ashore]

Mick the power supplied to your house , say single phase would be two wires one a power cable and the other a neutral , when you connect up an appliance one end to the power and the other to the neutral and switch it on you are completing a circuit and electricity flows through the appliance.

If the power cable comming to your house breaks and touches the ground which is also connected to the neutral you get a dead short

Hard to follow I know but harder for me to put into words, perhaps
http://www.allaboutcircuits.com/vol_2/chpt_10/2.html
may be of some help its the simplest description I could find on the net that doesn't waffel on too much with mathamatical theory etc

It is not however harder to follow than a womans logic or reasons for doing what they do


Rgds

[journeyman Mick]

Ashore,
I know how the circuits run to my house (2 phases to house, 3 to shed) and how most stuff is one phase and requires the neutral to complete the circuit. I may even have (under strict, direct supervision from a sparky of course ) have completely wired four single phase power circuits, one lighting circuit and five three phase circuits in my shed as well as running the three phase + neutral + earth feed from house board to shed.

What I don't understand is how the three cables for the HV lines become 3 phase + neutral on the LV side of the transformer and why no neutral is needed on the HV side.

If, for some unknown reason, for the sake of arguement, you needed to run a appliance at 22KV (or whatever the HV is) and it only needed to be single phase, and this appliance was right next to the generator plant, would you just run a single phase to appliance and a neutral return to the generator? IS the reason there's no neutral return on the HV that all the HV load is 3 phase, and doesn't require a neutral?

For instance, if I had no lights in my shed, and I only had machines that required 3 phase without neutral (ie 3 phse motors only, no lights or other bits on the machines), would this mean that I'd only require 3 phase + earth to the shed. If the answer is yes, then I can understand (sort of,) why HV doesn't require a neutral.
It still doesn't answer where the neutral connects at the transformer.

Mick (the dim witted)

[Ashore]

For instance, if I had no lights in my shed, and I only had machines that required 3 phase without neutral (ie 3 phse motors only, no lights or other bits on the machines), would this mean that I'd only require 3 phase + earth to the shed. If the answer is yes, then I can understand (sort of,) why HV doesn't require a neutral.

Yes

Have a look at that web link at the bottom of each page click on the forward to go to the next section it realy does explain it much better than I can , including how a transformer, generator and motor work electrically.

Rgds