Transmission Lines transient behaviour discrepancy

[Inrushman]

Hi All,

While nominated for carrying out insulation coordination study for EHV system using PSCAD, I came to a point for transmission line transient that takes me back to some undergraduate courses, the point is the reflected wave.
I understand that there is a wave which is travelling along a line in a time T, reflecting and travelling back and so on, the resultant output will be oscillatory with a frquency of 2T, where T is dependent on line surge impedance as well as length, however, the oscillation isn't giving any relation to the actual capacitance and inductance of a line, where the osciallation frequency for a RLC network is w = 1/sqrt(LC).
Discrepancy of whether we will have two oscillations superimposed on each other ( similar to that of VFTO, two frequencies are there, one due to internal GIS reflections and the other due to solving for the total RLC circuit of the entire GIS), or the the concept of travelling waves is another form of travelling wave but on a longitudal format?.

 

[jghrist]

When you are dealing with insulation coordination, you are dealing with a fast rising front shaped surge, not oscillations as in an RLC circuit. The surge gets to a discontinuity and is partially or totally reflected, either positively or negatively, depending on the relative surge impedance at the discontinuity. The time T is the time it takes for a surge to travel a certain distance, not the period of an oscillatory wave.

 

[magoo2]

I agree with jghrist.

The oscillations you described are normally associated with switching surges where the disturbance results in a delta voltage and the constants of the circuit determine the frequency.

In general, you get a reflection at a discontinuity. At an open point, you get a voltage doubling. At a short to ground, you get a superposition of a negative voltage such that the total voltage is zero. If the reflection is terminated in the surge impedance of the circuit, you'll get no reflection.