every electrical equipment is rated in VA or KW(motor) why transmission line rated in Volts though it carry power.
Primarily because motors and appliances are users and power lines are suppliers and have not fixed KVA value. The KVA value of a power line can vary from 0 all the way up to the full generating capacity of the feeder transformer. Although motors and appliances are KW/KVA rated, this rating means nothing without the voltage rating.
Transmission lines are rated in MVA. They do have a current carrying capacity and limits. However the voltage of these lines is so high and the current so relatively low, that it is the voltage which predominates in determining the heights, clearances, spacings and visual impact of the lines and therefore ultimately also the cost. In consequence common usage very often refers to the rating in terms of voltage only.
Because voltage is the only thing that remains fairly constant. Even though industry and residential pull off that transmission line, the voltage is and Hertz is maintained for the sake of equipment that depends on a constant voltage. The current in the line fluctuates highly , depending on how many people are drawing on it at any instant
