EVERY so often, a problem occurs - the requirement for a power supply is over there and the power supply point is over here.The distance between the two points can be as much as 5km. The maths is quite simple: say you want to supply a load of 220W at 220V 1km from the supply point.
Using 2,5mm2 two-core cable with the armouring as earth, the voltage drop will be about 13% so the load voltage will be about 206V (and you won't actually be supplying 200W but let's not get too technical).Now 220V at 220W is 1A and you can easily see that to supply 5A will result in a load voltage of about 160V, which will make the circuit unusable.
Even if you make the cable 4mm2, you have an unacceptable volt drop. But anyway, one has to be more practical - nobody is going to lay 1 000m of 2,5 mm2 copper-cored steel wire armoured cable to supply a few amps - apart from the initial cost, there's the hassle of what to do when the cable gets stolen.
Assuming you can cope with the theft aspect, what are the alternatives? Putting up the supply voltage is one of them. If you get hold of two transformers, which have say, a 220/230V primary and 380/400V secondary, then you can step up the transmitted voltage to 400V and step down with another transformer at the other end. And the volt drops look much better - you can send 800W of power with a just acceptable volt drop of about 7%.
Even better, if you can get 220/1 000V transformers you can send 7 000. Since PVCSWA cable (in RSA) is rated at 1 000V, you don't need any special cables. With modern technology, there are still further options: it so happens that if you transmit power as three-phase rather than single-phase, then the volt drops greatly improve.
If you only have a single-phase supply, not to worry - you can buy a converter, which will convert from single-phase to three-phase at one end and back again at the other; and this will allow you to transmit three times the power than at single-phase.
Further tricks: the volt drop of a cable is due to the drop due to inductance and the drop due to resistance. So, if you put a capacitor of suitable size at the load end, then the capacitor will compensate for the volt drop due to inductance and will ‘support' the load end voltage.
If you want to transmit lots of power any distance, you have to send it via a power line rated at 6 600V or above. The simplest and safest way to do this is to build a line using 9m treated power poles at about 50m intervals, each pole buried about 1,5m into the ground and stabilised using a sand cement mix. You should use aerial bundle conductor to transmit the power and string it up according to the specifications of the manufacturer - it so happens that the conductor will expand and contract with temperature and, if not allowed to do so, can cause the power line poles to bend out of line.
A handy tip, if you are doing this yourself (which you can only do if you are a registered electrician or have one to sign off) is to wrap a stainless steel wire loosely around the conductor, say one turn every 10m.
Then, when the thief in the night comes to steel the conductor, he will find - surprise, surprise - that the stainless steel wire prevents the conductor being cut. Ag shame.
Most importantly, if you are planning to do any of the above, remember I am just putting forward a few suggestions.... You don't have to spend a fortune to send power a long distance but you do have to have a system that is safe and only a registered person can sign it off.
So do employ one.
machoy@iafrica.com
