Electrickery question

[Schtoo]

So, you needed a schematic huh?

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.

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 ) 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.

Cheers
Pulse