Lightning study on a transformer substation. Tower examples

[Anonymous]

I'm doing a lightning study on a transformer substation. I would very much appreciate any examples on towers (flat design) and tower footings.

Thank you.

Submitted on behalf of:
Kristin, Iceland.

[Anonymous]

I'd say for modeling faults outside the station, the corona model for surge propagation into the station is more important than the tower / footing impedance and would recommend for PSCAD-level studies:

Tower inductance

Z = 60 ln cotangnt (theta/2) where theta is the cone angle that best approximates the tower
t1 = travel time of path from overhead groundwire to phase conductor crossarm (along, down, along)
t2 = travel time of path from phase conductor crossarm to ground (along, down)
Put two inductors in series, each inductor has travel time t x impedance Z

Footing resistance:

use g=sqrt(Rx ^2 + Ry ^2 + Rz ^2)
A = area of footing (typically for four legs, 4xleg separation x foundation depth + (leg separation)^2

R = rho / (2 pi g) ln (11.8 g^2/A)

rho for Iceland is fairly complicated, about 1000 ohm-m in the north and a central part of the south, with 33 to 100 ohm-m in the southeast and southwest according to the world map of conductivity for VLF (30 kHz).

I'd need a field trip to Iceland to say more ...

Bll Chisholm

[Anonymous]

If I am reading this correctly, I'd interpret this post as saying that the surge impedance of the cross arms is assumed to equal the surge impedance of the tower. Is this interpretation correct?