due to unbalance in line

BECAUSE TRANSMISSION LINE ITSELF HAS ITS OWN IMPEDANCE 
(JUST LIKE A SERIES CONNECTED LOAD)

Due to corrona effect

due to Phase to phase faults 
            or
Line to line faults

skin efect and corona effect r the most common effect that 
causes the line voltage drop.
apart from that drops the very major and very technical 
effect that cause loss in poer and freq. is reactive poer 
loss.
than can be compenseted only by using Capacitor Bank.

Because any line has an impedance, and acc to a simple law 
known as ohm's law, voltage drop, V=IR.. so wen ther is 
resistance/impedance, any line, whether it be 3-phase, 1-
phase, d.c. or any new invention...... it WILL have a 
voltage drop!!

Voltage drop especially in 3 phase system is due to low 
power factor.Low power factor leads to increase in I2r 
losses which in turn increses the regulation.As regulation 
increases voltage drop occurs which directly effects the 
efficiency of generation and increase in stress of 
transmission line.Thats why KEB people who are responsible 
for maintaining required voltage put fine if we maintain 
low power factor.Corona and skin effect are negligeble .