surge impedance of ground leads

[magoo2]

In considering lightning surges on overhead conductors, we frequently rely on the lossless line model in which the distributed nature of capacitance and inductance results in the characteristic or surge impedance, equal to the square root of (L/C). It is assumed that the line is infinitely long and at the same height above ground.

When you consider the downleads of shield wires, how to you determine their surge impedance? After all, the capacitance to ground in this case is changing with height above ground.

Any ideas other than treating it like it's a horizontal conductor?

 

[cuky2000]

General Comment: The surge impedance defined above (SQRT(L/C) is the natural or noncorona surge impedance Zo. The surge impedance above corona could be estimated as follow:

Zc=Zo.SQRT[1/(1+DeltaC/Cn)]

The Zc impedance is applicable on the front surge, where the voltage is increasing and returning to Zo on the tail of the travelling surge wave, where the voltage is decreasing Cn is the cable capacitance and Delta C is the additional capacitance originated by the surge wave

1. When you consider the downleads of shield wires, how to you determine their surge impedance? This could be account using the tower surge impedance. For a monopole tower the Zt=60ln(1.41*2h/r-1). Beware that Zt is not a sensitive parameter to determine the backflash rate

2. After all, the capacitance to ground in this case is changing with height above ground. The Delta C increases with the voltage. Consider average height


3. Any ideas other than treating it like it's a horizontal conductor? The above equation consider h as the average height of the conductor

 

[magoo2]

Many thanks, cuky2000