There is a wide variety of electric traction systems around the world and these have been built according to the type of railway, its location and the technology available at the time of the installation. There has been a rapid acceleration in railway traction development. This has run in parallel with the development of power electronics and microprocessors. Many of these developments are highly technical and complex, the details of which are therefore beyond the scope of these texts.  Traction substations are used to convert electrical power as supplied by the power utility (or rail operators own network) to a form suitable for providing power to a rail system (via third rail or overhead line). Depending on the type of rail system this power would be either direct current (dc) or alternating current (ac).