steel is used

steel. that's why it otherwise known as acsr(al conductor 
steel reinforced)

Aluminium cored ateel reinforced.In a cable steel 
reinforcement is there in between anuminium conductor.

i dint ask the name of conductor i know it is a acsr 
conductor but in the middle a single wire is there that 
looks to be very hard i required that name

as i already said, steel is used in between al conductor.
because of the gravity and less strength of al the wire
tends to sag. to avoid as well as minimize thw sag steel is
reinforced with in the conductor. that's it.

if you are taling about overhead line then al uminium is 
not in center because of skin effect
if you are takling about expanded wire then there is 
fiborus or paper type insulation is used
if in udergrond there is dffreent type pvc & valcunized 
rubber is used depend open the voltage stress & working 
temprature