As computer modeling of overhead conductor systems becomes common, customers are raising even more questions about overhead conductor impedance and admittance. The increased use of modeling software has developed a need for additional conductor characteristics, some of which are dependent on the system design and therefore can’t be supplied by the conductor manufacturer. “Calculation data must take the design into consideration to get accurate results,” says Mark Lancaster, Director of Overhead Transmission Engineering for Southwire’s Energy Division. Since the necessary information is not provided by conductor manufacturers, it is necessary for the utilities to understand the various conductor characteristics and how to determine them.
Many computer modeling software programs require that you know the admittance of the conductor system. “To calculate admittance, the measure of how easily a current flows through a conductor, you first have to calculate the impedance of the system,” says Lancaster. Presented as a formula, admittance (Y) is calculated as the inverse of impedance (Z): Y = Z-1 = 1/Z
(Revisit part 1 of this series covering impedance in the October issue of T&D Update for details on impedance calculations.)
Admittance and its components, conductance and susceptance, are simply parameters based on further manipulation of impedance-related values. While they add no new mathematical information, they are are required by some computer modeling programs and are convenient for certain design evaluations.
Using the following formulas and information provided by the cable manufacturer, you can calculate the values for admittance, conductance and susceptance based on your specific line design parameters.
Let’s take a look at the math. Like impedance, admittance is a complex number with real (conductance) and imaginary (susceptance) components. Our formula and values are:
Y = G + jB
Y = Admittance, in siemens
G = Conductance, in siemens (real component)
B = Susceptance, in siemens (imaginary component)
j = Square Root of -1
While some line design software handles these calculations for you, it is helpful to understand the theory behind the calculations. As we mentioned in part 1 of our series, to calculate G (conductance) and B (susceptance), we need to have calculated the values of R (resistance) and X (reactance) from the impedance formula.
With those figures available we can now solve for Admittance as follows:
Understanding these definitions and calculations provides context for more accurate and efficient line design. More information on calculating components of impedance and admittance can also be found in the Southwire Overhead Conductor Manual available at: https://southwire.phase3media.com/CTOOrder/ProductList.asp?authID=1081539804