/images/buttons/swirl_60.jpg/images/southwire_100x70.jpgSolving Overhead Conductor Impedance and Admittancesolving-overhead-conductor-impedance-and-admittance.htmPart 1 of 2: Characteristics of conductor system impedance
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Solving Overhead Conductor Impedance and Admittance

As computer modeling of overhead conductor systems becomes common, customers are raising more questions about overhead conductor impedance and admittance. “It’s important to remember that impedance and admittance are characteristics of the conductor system as installed, not just the conductor itself,” says Mark Lancaster, director of overhead transmission engineering for Southwire’s Energy Division.

Conductor impedance refers to the conductor’s opposition to the flow of an alternating current where as conductor admittance is a measure of how easily a current flows through a conductor.

Real and Imaginary Components of Conductor Impedance

Conductor impedance (Z) is a complex number, with real (Resistance) and imaginary (Reactance) components:

Z = R + jX, where 
      Z = Impedance, in ohms
      R = Resistance (ac), in ohms
      X = Reactance, in ohms

The real component of impedance, Resistance (R), is dependent on the conductor material, temperature and operating frequency. Manufacturers generally provide resistance for common conductors in units of ohms/mile.

The imaginary component of impedance, Reactance (X), is a measure of a conductor’s opposition to a change in current due to the buildup of electro-magnetic fields. Reactance has two components: inductive reactance (Xa) and capacitive reactance (Xc).

      f = Frequency, in hertz
      L = Inductance, in henries per mile
      C = Capacitance, in picofarads (10-12 farads) per mile

  • Xa is usually given in units of ohms/mile; Xc given in units of MOhm-mile
  • Xa increases with line length; Xc decreases with line length

Both inductive (Xa) and capacitive (Xc) reactance vary with the line design. Conductor manufacturers, like Southwire, do not provide a value for total reactance per mile. Inductive and capacitive reactance are calculated based on phase spacing and line length. Sum the two to determine the total reactance of the conductor. Southwire engineers can help you understand and compute the calculations.

Overhead Conductor Manual discusses phase spacing

Manufacturer’s tables normally give inductive reactance (Xa) and capacitive reactance (Xc) values at “1 foot spacing.” These values need to be modified to reflect the actual conductor spacing. “Transmission line designs vary widely,” Lancaster says. “To get good results from your modeling software, it’s important that your data reflect your individual design.”

The Southwire Overhead Conductor Manual discusses conductor reactance beginning on page 1-20. To adjust the “1 foot spacing” reactance numbers for the line design reference, Table 1-10.

Some transmission line modeling software uses conductor admittance as a key parameter. Learn more about calculating conductor admittance in Part 2 of this article, to be featured in the December issue of T&D Update.