# Inductance of a three-phase transmission line

I was studying the difference between distribution and transmission lines and read that, because of the larger separation among the conductors, the transmission lines have comparatively higher flux linkage and hence, higher inductance. I have looked at the equations and I can see that that is indeed true but it seems counter-intuitive to me because I always thought flux linkage should decrease with distance.

Could somebody please help me understand what is physically happening there that causes this phenomenon?

## 1 Answer

A transmission line of this type can be modelled as a single pair of wires carrying current to a load and, at any one moment in time, there is a forward current on one wire and a return current on the other wire. Given that both wires are partially magnetically coupled, the magnetic fields produced cancel at the mid point between the two.

Now, if the wires are some greater distance apart, there is still cancellation at the midpoint but, each individual wire has more “spatial freedom” to generate its own localized magnetic field. The result is that there is more flux per amp produced and, this translates into more inductance.

It’s not the flux linkages that create more inductance (because forward and return currents are always opposing); it’s the self inductance of each wire that increases as the distance increases.

• Thank you for such a comprehensive explanation, Andy! Feb 2, 2020 at 15:11