I have touched on this subject before: Electrification of developing countries.

I was thinking of developing some ulta-cheap concept to distribute power. Yes, I know I'm insane, naive and all those things.

The SWER (Single Wire Earth Return) system has been debated before in the General area.

I was thinking of pushing this concept to the extreme: 40 kV to earth instead of 12.7 or 19.1kV

My idea is that it doesn't matter if the line above your head is 10kV or 40kV. If you touch it, you die anyway.

Using this insane voltage, I don't need aluminium wires. I can used steel wires. Steel is a poor conductor, R = 0.16 ohm m/mm2, but the high voltage offsets that. The good thing about a steel wire is that you can string it for longer distances than aluminium, 250 m or more between the poles. Much of the cost is in the poles.

Now, I only need a very small transformer as the each household only use a few hundred watts of power. Say a 10 kVA transformer. At 240V this corresponds to 40A or 20A per leg at 2 x 240V.

As meters are expensive, let's make the service very small and drop the meter. A one amp service seems like a good choice: Just over 200W per household.

I started calculating the voltage drop the wire from the transformer to the user. If you use 0.5 mm2 Cu wires and allow a voltage drop of about 10 V you get a maximum distance of about 150 m. That means it should be possible to cover a village with a single transformer in the middle.

Ideas and suggestions, please.

First thoughts: Steel is stronger than aluminum, but it's also much heavier. You might get away with greater spacing between poles/towers, but would they need to be stringer to support the extra weight?

Maybe one of ECN's linemen can come up with some figures on this ?

Wouldn't 40 kV be more prone to arching and therefore push up the price of the isolation material used?

C-H,
It's a good idea in theory.
Most aluminium power-lines already have a steel core, running through thier centre.
The further the poles are apart, the higher the strain on the poles gets and also you need to take into account wind drag, thermal effects, etc.
At 40kV, you'd need pretty big insulators and the lines would need to be well spaced, probably with a double cross-arm arrangement.
We are talking a huge pole for this type of installation!.

Paul,
>First thoughts: Steel is stronger than aluminum, but it's also much heavier. You
>might get away with greater spacing between poles/towers, but would they need to be
>stringer to support the extra weight?

I thought so too. Then I asked the guy next to me who is running the CAD and stress analysis softvare to make a thin (1 mm plate) round pole and hang 125m of 25mm2 steel wire on each side. To my suprise, the load was nearly nothing. The big load must come from the wind.

Belgian,
>Wouldn't 40 kV be more prone to arching and
>therefore push up the price of the isolation material used?

Yes. I'm counting on the extra cost to be small. I could be wrong.

Trumpy,
>It's a good idea in theory.

Thanks!

>Most aluminium power-lines already have a steel core, running through thier centre.

Yup.

>The further the poles are apart, the higher
>the strain on the poles gets and also you
>need to take into account wind drag,
>thermal effects, etc.

>At 40kV, you'd need pretty big insulators

Any idea how big?

>and the lines would need to be well spaced,
>probably with a double cross-arm arrangement.

SWER = Single wire...

>We are talking a huge pole for this type of installation!

Please explain why? I definitely need a high and strong pole to go these long distances. But huge?

[This message has been edited by C-H (edited 05-13-2003).]

On the website of OLEX cables, an Australian company, I found something called Aluminium clad steel wire.

From the data this appears to be an ideal form of conductor for a SWER system. Half the resistance of steel, much higher strength than aluminium, better corrosion resistance than steel.



Has anyone any experience with this type of wire? Is it expensive?

C-H,
At 40kV, you'd need 7-Fin Pyro-glass insulators, these are 300mm in diameter and about 800mm long.
Maybe huge, was overstating the size, but pretty big would describe the poles better,
one thing you have to design into any power system, is what wiil happen with Electro-mechanical Stress, should there be a short-circuit on the system, in HV and EHV systems, this can topple poles over, literally like matchsticks.
The only other thing that would worry me about the voltage level on the HV side of the line, is, what would happen if the if the transformer shorted between the HV and LV windings, this can (and does) happen, therefore you need to have adequately selected Primary(HV) fusing on the line side of the tranny.
Regarding your post on Alu-steel wire, it's not expensive in itself, but the jointing hardware is.
It also doesn't go too well, where there is a lot of ambient moisture in the air, this really shortens it's life!.
Hope this helps

That's huge. I had no idea. Thanks for the reality check, mate!


What, topple the poles? Please tell me you're pulling my leg now...


That sounds really scary. What happens then?


How are joints made?

Have you noticed I haven't got a clue about this?

C-H
Regarding your reply,
Insulators from HV lines look large enough when they are up on the poles, but when you see one next to you on the ground, the difference in size is quite amazing, the same could also be said of Cross-arms!.
With respect to electro-mechanical stresses on poles and lines, should there ever be a direct short-circuit to Earth,
the Fault currents can be such that, the wires up-stream of the break can be seen to twist and snake as though they were possessed by some evil force, it's not a nice sight to see either!.
This is why I would stipulate that the poles be reasonably close together, it just gives the lines more support.
Regarding jointing lines, common practice these days dictates that the simplest, cheapest method is used, hence the use of crimp type sleeves, these are bare aluminium sleeves, filled with conductive grease.
Now here is the expensive bit, the proper tool for crimping these sleeves, operates by a Hydraulic system, because of the high crimp forces needed to enable an effective crimp, these tools cost around NZ$4000 each and require a different set of crimp dies for each sized sleeve.
Hope this helps!.

C-H,
I also have some misgivings about the SWER system of supply,
having worked on these systems before, in the Rural areas around where I live.
The protection on a transformer in an SWER system, will only operate if the Primary(HV) side winding in the transformer short circuits.
But it will not however operate if there is a short circuit between the HV and LV windings inside the transformer core.
Should this event occur(And thankfully, it is rare, but does happen), the full Primary voltage can be sent down the Service Main to the switchboard in the installation,I've seen the effects of this and it wasn't pretty!.
During a fault in a transformer on an SWER system, the Step Voltages around the supplying pole can be lethal, where voltages above 22kV are used, but this goes on the proviso that the HV protection has not operated.
By thier virtue,(being the only metallic object for miles around and up high) SWER systems attract lightning strikes like a magnet, this
can increase the cost of maintenance and repairs.
Using such high voltages can cause a few other problems too, it can affect nearby live-stock and also the wires over a certain length of run can induce dangerous voltages into wire fences(where these run parallel to the lines) and other such metallic objects.
Finally, rural areas are not really renowned for up to the minute standards of wiring, and this concerns me the most as the Earthing in an SWER system is the most important wire in the system and I've seen them in all sorts of states!.
Hope that this helps.

Would a small fuse size eliminate this problem? The biggest fuse would be around 25A.


$4000 is a lot of money. What other metods are there of jointing lines? (Requiring less expensive tools)


That's going to be rather nasty. If the primary side is fused with a small "fuse" compared to the secondary, wouldn't this mean that the it works like an earth fault: Shorting to earth via the windings on the secondary side?

But the point is taken. The system will anyway need some form of surge protection, in case of lightning. If correctly designed it should be possible to protect the outgoing LV lines from the abovementioned fault as well.


You mean if the "fuse" or whatever protection there is fails to operate? That's one of the scary parts of the system: Earth fault detection just isn't possible. But, on the other hand, electric trains work in the same way.


And I want to use steel poles... Like putting up a big sign "Strike me!"

This is a major concern. As there is no "lightning conductor" above the lines (many HV have these) the strike will go directly into the transformers...


*gulp* Why isn't anything simple?


Hmmm... I was in fact thinking of not having an earth conductor, but to rely entirely on the pole. At least it won't go away. (I've seen what happens to the earth bonding of fences and poles at train stations and substations.)

If it is to be used in rural Africa, it must be simpler than simple.


Sure do!

Trumpy,
are you sure about the size of the insulator? I looked through the drawings of insulator manufacturers on the net. I found a number of pin type insulators from 33kV to 38kV. All were less than 300 mm long and roughly equally wide.

It seems 40kV wasn't a very good choice. 33kV or thereabouts seems to be the standard all over the world. I'll adjust the voltage to 33kV instead.

Isn't the 40kV ø-ground, and 33kV ø-ø, or ~19kV ø-ground?

Bjarney,
You're right there about 33kV being Phase-Phase.
Over here we normally only use 11kV for SWER lines, the rest is all 3.3 or 6.6kV 3 Phase.
C-H,
What sort of climate are we talking about here?.
Having steel poles on any sort of HV supply system, is just asking for trouble.
Also using a pole as an earth stake, is not something I would recommend, unless it's Galvanised, otherwise it will melt away like an iceblock in the sun.
Regarding the jointing of lines, I'll see what I can dig up on jointing methods.
One system that has just been released over here, is a clamp system with a set of set-screws that "break" when tightened to the correct torque and cannot be over-tightened and they are inexpensive to fit(you only need an Allen Key Spanner.
They are made especially for Aluminium conductors, but can also be used to join Al to Cu, if need be.

If it helps to visualize the difference, here are a couple of pics of local HV lines here in England.

The first is 11kV, so phase-to-ground will be about 6.4kV:


The second is a 33kV line which, as Scott said, will actually be around 19kV phase to ground:


There's quite a difference in size just between these two types of insulators, so a line at 33 to 40kV to ground would be quite substantial.

We have some 66kV transmission lines (~38kV to ground) here, but none nearby so I don't have any photos, but the insulator stacks are pretty large.

Bjarney and Paul: You are of course right. The current SWER systems use 33kV/19.1kV to ground.

My original idea was to use 40kV to ground with 69kV phase to phase. But as you have pointed out, the insulators become very large. The question is if that is a problem. What problems does a big insulator cause?

Now I'm thinking of widening the envelope by making it a corner grounded delta at 33kV. Why, you ask.

It would allow me to distribute 3-phase with only two wires: Two phases in the air and the remaining phase in the ground. The transformers are then connected in delta configuration on the primary. The ordinary off the shelf 33kV transformers should work with this system, shouldn't they?

Should the need arise, an extra wire can be added to make a "conventional" 3-phase system. (This to limit radio interference and eliminate the need for a very good ground)

Look at the images Paul posted:
What are the problems with these poles?

They have a crossarm. Costs extra and the horisontal arrangement requires a wide separation of the wires to avoid clashing.

The pole is made of wood. Trouble is, wood of suitable quality is found in Europe and North America, not in the countries close to the equator. Furthermore, a wood pole is heavy compared to steel. If you don't have a truck and a crane, it is not easy to get them to the site. The lack of roads in poor countries means that the workers have to carry the poles.


Mostly tropical.


Why? I can see that it will be very difficult (dangerous) to do repairs while the line is live. All those 400kV lines are on steel towers?


Melt away? {Thinks hard for a minute} Ah, you mean rust because of the current flowing in it? Good point. The transformers could need a separate wire to earth.


Sounds interesting. I think I've seen something like this for insulators?

C-H,
What sort of system are we talking about now?.
SWER, 2 Phase, 3 Phase?.

C-H,
I wouldn't personally mix O/H and U/G wiring within the same system, it is frowned upon, because of the difficulties encountered during Faultfinding.
Also, if you are using a Delta transmission system, you will need Delta-Star transformers, as you will not have a Neutral point for your take-off point to houses.
A distribution system is not something that can be done cheaply, there are all sorts of things that have to go into the design and installation of any power system, especially a High Voltage one.