A transmission line is a distributed parameter circuit and distinguishing feature of such a circuit is its ability to support traveling waves of voltage and current. A circuit with distributed parameters has a finite velocity of electromagnetic field propagation. In such a circuit the changes in voltage and current, owing to switching and lightning do not occur simultaneously in all parts of the circuit but spread out in the form of traveling waves or surges. When a transmission line as shown in fig 1 is suddenly connected to a voltage source by closing of a switch, the whole of the line is not energized all at once (the voltage does not appear instantaneously at the other end). This is due to the presence of distributed constants (inductance and capacitance in a loss free line).

When switch S is closed, the inductance L1 acts as an open circuit and C1 as short circuit instantaneously. The same instant next section cannot be charged because the voltage across capacitor C1 is zero. So unless the capacitor C is charged to some value whatsoever, charging of the capacitor C2 through L2 is not possible which, of course, will take some finite time. The same argument applies to the third section, fourth section and so on. So we see that the voltage at the successive sections builds up gradually. This gradual build up of voltage over the transmission line conductors can be regarded as though a voltage wave is travelling from one end to the other end and the gradual charging of the capacitances is due to associated current-wave. The current wave, which is accompanied by a voltage wave, sets up a magnetic field in the surrounding space. At junctions and terminations these surges undergo reflections and refractions. In an extensive network with many lines and junctions, the number of travelling waves initiated by a single incident wave will mushroom at a considerable rate as the waves split and multiple reflections occur. It is true that the total energy of the resultant waves cannot exceed the energy of the incident wave. However, it is possible for the voltage to build up at certain junctions due to reinforcing action of several waves.

If you enjoyed this post, make sure you subscribe to my RSS feed!