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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.