Advantages of DC system over AC system

AC is easier to manage and convert to higher or lower voltage using transformer which are normally very high efficiency machines. Nevertheless DC systems show at least a couple of advantages respect to AC:
1) There are not reactive losses (capacitive and inductive) which affect in particular the long distribution lines and this permits, if necessary, the energy distribution using underground cables. This would be impossible with AC because of the capacitive losses. This is the principle reason to use the usual open air distribution for AC.

2) In case wires with big diameter are needed (because high currents have to be distributed), it becomes relevant the skin effect due to the 50 or 60 Hz of the AC. This forces to complicate the distribution system splitting the single wire in more wires having a total section egual to the single one. Typical is not convenient distribute AC with single wires having a diameter bigger than 15 mm.

Actually there were advantages and disadvantages for each particularly in Industrial Applications. DC easy to control (speed, torque, etc.) due to DC characteristics for Electrical drives. Due to the new technology in industry pertaining to control and automation (VFD, soft starter, PLC, etc.) AC systems become more convenient, efficient and cheap for industrial purposes.