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. :D

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. :D

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 :o. 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

[Schtoo]

So, you needed a schematic huh? :rolleyes:

In the case of your 22kV appliance, a single phase thing hooked up to a single line of 22kV would not work, because there is no neutral. :D

HV does not need a neutral. Strict 3 phase does not need a neutral. Anything that only needs 240V does need a neutral.

The neutral is a locally created thing, each transformer that pumps out 415/240V creates a neutral point by the way it's hooked up, ie; the centre of the star.

When you connect something 3 phase up as star, it kinda ceases to be 3 phase, instead becoming single phase x 3. The appliance runs it's 3 circuits at only 240v each, not the 415v that exists between each phase that you would get in a delta connection.

In the case of the lightless shed, you would use 3 actives and an earth, no neutral. You can buy outlets that have the 3 phases and earth (missing the centre pin) and cable that is also sans neutral (3C + E).

The earth is connected to the neutral, as often mentioned. It is not a return wire for anything, until something goes wrong. Not worth worrying about other than make sure it's present and working.


As a closing example, a couple of things I have hooked up in the past.

Two battery chargers for forklifts. Different buildings, different brand chargers.

One needed 3 phases and neutral, the other didn't.

The one that needed neutral used it only for the control wiring, ie; lights and relays inside the charger.

The one that didn't either had a small transformer that created the a lower voltage for the control gear or created it's own neutral inside the box. Either way would work quite alright (even though it did have a small tranny making 32V for the control gear).

What I am trying to say (and if it sticks, the light bulb will illuminate) is that the neutral is a locally created thing, nothing more. The generators don't need a neutral, the tranformers don't need a neutral, nothing really needs a neutral. But being bitten by 415v is a lot less nice than being bitten by 240v, and it's cheaper/easier to keep 240v safe than keeping 415v under control. ;)

[chrisp]

Mick,

There seems to be two questions you are asking: (i) Why there is no neutral on the HV side, and (ii) where does the neutral come from.

Three phase power isn't too hard to understand but it helps to be able to do some mathematics to fully understand it. (I know of licenced electricians who don't understand three-phase :eek: so don't feel dumb about not understanding it). The "star" and "delta" not only the winding formation of the transformer, but also are kind of a shorthand notation for "phasors" which are a geometrical representaion of the sinewaves.

I think it helps to understand WHY we use three-phase - this isn't often explained. The reason three-phase is used is that you can get twice as much power over three wires than what you can get over two wires. To rephase (pun not intended), by using 50% extra copper (or whatever conductor), you can get 100% more power. This may not seem important to domestic power users, but to a power distributor it is a significant saving.

In three-wire three-phase (as used in distribution) one phase returns via the other two phases. (It can help to draw the three phases on paper to see how this is possible - you will find one or two of the other phases are negative while the other is positive.) No neutral is required - a further cost saving in cable.

When distributing to domestic houses, the final transformer is a delta-star transformer - three-wire three-phase in and four-wire three-phase out. The fourth wire being the neutral.

If you measure between two of the (three) phases you will measure 415V [=sqrt(3) * 240V]. If you measure between a phase and the neutral you will measure 240V.

In Australia, typically each house will connected to one (single) phase and the neutral. If you walk down a typical street with overhead power lines, you will also notice that the actual phase used alternates between houses - this is an attempt to "balance" the load on each phase - and this in turn reduces the total current in the "neutral" conductor.

In Australia we use a Multiple Earthed Neutral (MEN) system - at each house we have an earth stake which is the "earth" conductor in our power points. The earth is also bonded (only once) to our house's neutral bar (for the reasons given by Ashore).

I hope this is of help.

Chris

[journeyman Mick]

Schtoo, Chris & Ashore,
I checked out the site that Ashore pointed out and it's making sense (mostly). I understand the concepts, but some things still puzzle me - probably because I tend to think of electricity more on low voltage DC terms as that's what I first learnt about and tinkered with. So I do understand now where the neutral is "created" in the centre of the "star" on the pole transformer, but the fact that the earth is bonded to it at the meter box has me confused now. Why only in one spot, and if they're connected there, would it make any difference if they were connected at, say every power point? (Don't worry, I'm not going to do it:p ) I'm guessing it would be a no-no. but I can't understand why. A connection is a connection. Connecting the earth and neutral together at one point effectively makes them one continuous bit of copper doesn't it? So why woulkd it matter if there was more than one connection point?

Oh, and Schtoo, this mythical 22KV single phase appliance of mine that I'm trying to get going, what if we created a star connection at the generator output and hooked a neutral up to that? Would it work then?

I'd better stop now, my head is starting to hurt.;)

Oh, and when we've got that sorted out, how does that single wire, return to earth that they have in remote rural areas work??:confused:

Mick (slightly more enlightened now)

[Schtoo]

I know why the earth is bonded at one point, I really do, but since Wednesday is a busy day, my brain is fried and I have been away from sparkying for long enough that I have to dig for info.

And I am not digging tonight. :)

The mystical 22KV device wouldn't work if you just made a star at the genny and hooked it up. You would be connecting 2 phases directly to each other, that being a dead short would not result in smoke but in orange liquid presented in a nice spray pattern. ;)

There needs to be a load between the incoming juice and the created neutral. In a tranny, the windings present a suitable load that things don't fry, even if your toaster is unplugged.

An isolation transformer (same input as output) might work, but I wouldn't want to be doing it myself...

[Pulse]

Hi Mick, things getting pretty complicated here. I think part of the answer is that the supply is MEN or multiple earth neutral.


At your switchboard the earth and neutral are connected, the neutral on the LV 3 phase street power lines is also earthed at regular intervals. I suppose on average with a large number of houses on the three phases the net flow in the neutral is close to zero anyway. As mentioned it would also save on distribution costs.

I think the whole reason your neutral is connected to earth at the switchboard is to provide a low impedance fault path to trip the breakers in case you earth electrode is not good enough.

Some more info here (http://www.epanorama.net/phpBB2/viewtopic.php?t=1821).

Cheers
Pulse