Roozbeh Asad - Electrical Engineering Department, Iran University of Science and Technology, Tehran,
Ahad Kazemi - Electrical Engineering Department, Iran University of Science and Technology

Electrical system mainly generates, transmits and distributes electrical energy through ac voltage for more than a century. But, during the recent decades, some newmodifications in electrical system, like the fast promising growth of renewable sources, storage systems and dc loads, arechanging the circumstances gradually. Considering newlyemerged conditions, it seems necessary to study again the possible roles of dc system in now and future electrical system,i.e. Smart Grid. In this paper, determining the true reasons of the transition, firstly brief history of the transition from ac to dcis mentioned. Also it is investigated whether the reasons are still important and reliable. Then, considering the recent modification in electrical industry, the necessity of new challenge of ac system by dc system is described. Finally, theeffects of the transition from ac to dc, on the two main sectionsof electrical system, i.e. load and generation, have been studied and derived quantitatively.

dc system, ac system, load, generation, dc micro grid, smart grid