A number of unit systems are involved in overhead transmission line impedance calculations. The current section is intended to make their distinctions clear. An engineer provides data for the calculations in what we will refer to as the “user” unit system (also referred to as “generally available units” or “commonly published units” in other sections of this document). User units may vary along the following lines:
Conductor separation units, u_{CS}, are associated with conductor-to-conductor and conductor-to-image distances. In US applications, conductor separation is usually measured in feet.
Conductor radius units, u_{CR}, are associated with effective radius (GMR) measurements. In American reference materials, conductor GMR is usually reported in feet.
Conductor diameter units, u_{CD}, are associated with outside diameter measurements. In American reference materials, conductor diameter is usually reported in inches.
Line length units, u_{LL}, are associated with the length of a line section. In US utility applications, line length is usually measured in miles.
The units in which equations are expressed in this document are called problem formulation units. The units in which impedance calculations are actually implemented are referred to as computation units.
Table 2 describes these unit systems in detail.
Units | |||
---|---|---|---|
Quantity | Formulation | Computational | User |
Frequency | Hz | Hz | Hz |
Earth Resitivity | ab℧/cm^{3} | Ω/m^{3} | Ω/m^{3} |
k | abΩ/cm | Ω /u_{LL} | n/a |
Resistance | abΩ/cm | Ω / u_{LL} | Ω / u_{LL} |
Inductive Reactance | abΩ/cm | Ω / u_{LL} | Ω / u_{LL} |
Potential Coefficients | F^{-1} · m | F^{-1} · u_{LL} | n/a |
Maxwell’s Coefficients | F/m | F / u_{LL} | n/a |
Capacitive Susceptance | ℧/m | ℧ / u_{LL} | n/a |
Capacitive Reactance | Ω · m | Ω · u_{LL} | (M)Ω · u_{LL} |
Conductor Diameter | m | u_{CS} | u_{CD} |
Conductor GMR | cm | u_{CS} | u_{CR} |
Conductor Separation | cm | u_{CS} | u_{CS} |
u_{CD} - conductor diameter unit
u_{CR} - conductor radius unit
u_{CS} - conductor separation unit
u_{LL} - line length unit