I am engineer developing powerline communication systems for helping people in villages communicate via existing power lines.
Since I have no experience on the working of these systems
kindly send me circuit ideas and block diagrams to develop this system,. It's for a good cause.
Since these wires are owned by the power distribution company or ministry, you should inquire of them about permission to use them and what frequency and power limitations you will be held to.
In Europe there are very extensive regulations on this method of communication and there exist commercially made ICs for this purpose.
As Flatulent points out - the power lines are not public property in the sense that you can use them as you please. You need permission.
Furthermore, most of today's powerline communication methods involve high frequency up to ultra high frequency RF that can interfere with other systems including firefighting, emergency medical, military, etc. You need to talk with your local service providers and see what their concerns might be.
Interference is a very major problem. In fact, many trials in US had to be cut short due to this. In US, the amateur radio operators were the bunch that most affected. In the developing country, this part of spectrum can be the vital link between the villages. So double check before going ahead.