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.
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 ?
Re: Wiring developing countries#136926 05/13/0304:35 AM05/13/0304:35 AM
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.
>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
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).]
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
Re: Wiring developing countries#136931 05/15/0311:07 AM05/15/0311:07 AM
At 40kV, you'd need 7-Fin Pyro-glass insulators, these are 300mm in diameter and about 800mm long.
That's huge. I had no idea. Thanks for the reality check, mate!
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.
What, topple the poles? Please tell me you're pulling my leg now...
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.
That sounds really scary. What happens then?
Regarding your post on Alu-steel wire, it's not expensive in itself, but the jointing hardware is.
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.