This discussion of overhead transmission line series impedance concludes with a brief dicussion of computing contant factors associated with the impedance matrix and reconciling units of measure while evaluating these constants.
is the earth conductivity in ab℧/cm3.
d is a distance in centimeters.
This can be rewritten in terms of readily available quantities (i.e. commonly published units) by substituting earth resistivity (Ω/m3) for conductivity and distance in conductor separation units for distance in centimeters as follows
uCS is conductor separation unit. In the US, conductor separation is usually measured in feet.
uCS cm is the number of centimeters per conductor separation unit.
is a constant converting ab℧/cm3 to Ω/m3.
Assuming that the frequency and resitivity are constant for any set of impedance computations the bulk of the expression
is a constant which is computed once then stored for reuse.
uLL is line length unit. In the US, line length is usually measured in miles.
uCR cm is the number of centimeters per line length unit.
ab is a constant converting abΩ to Ω, i.e. 1×10-9.
Assuming that the frequency is constant, both P and Q are multiplied by the same factor
Once again, both of these constants are calculated once then stored.
is actually evaluated as
where uCR uCS converts conductor radius units to conductor separation units. Factoring out a constant in this expression yields
The factor c is also calculated once and stored.