Question

How does a capacitance affect Long Transmission lines? If the receiving end of the long transmission is open circuited, is there a current flow at the sending end?

Answer 1

Capacitance of long transmission line increases the charging current as the length of line increases.This cause to increase in power loss in transmission line.

Injection of reactive current by capacitors(formed because of stray capacitance between transmission line and ground) into the transmission lines.As a result of this the phasor sum of currents from the sending end side and this reactive current would make the receiving end current higher than sending end.

If you have heard of a thing called Ferranti Effect, thats the result of Capacitive nature of transmission lines. If we consider a unit length of conductor as a capacitor plate and Ground as another plate, separated by the air medium, it forms a small capacitor. And upon traversing the entire length of the line, we can see that its nothing but many such small capacitors connected in parallel. Since we know, capacitance increases when connected in parallel, there is a voltage build up due the capacitance and voltage at receiving end becomes more than that of sending end. This is Ferranti Effect. This effect is commonly seen in long transmission lines.

The receiving current in long transmission lines would be greater than sending end current only under the condition of no load or light loading conditions