Eskom generate electrical energy at power stations, a large amount of which are grouped next to coal mines in Mpumalanga and the Northern state. This is not everyplace most of the electrical energy is used, however. The huge load centres are in places such as Gauteng, the Western Cape as well as KwaZulu Natal.

So the power must be sent from the power station to the load centres in the most consistent and cost effective method. Investigation has shown that this is finest done by distributing the power over high-voltage power lines.

As it passes the power station, the electricity is boosted by a apparatus called a step up transformer to voltages such as 132 000 volts (132kV) or 400kV or 765kV.

Refer: http://www.themercury.co.za/index.php?fArticleId=4222264