2 Series Impedance 2.2 Impedance of an N Conductor Transmission Line 3 Shunt Admittance

2.3 Series Impedance Computations

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.

2.3.1 Computation of k in the Series Approximation to P and Q

The parameter k appears in the series expansion which approximates the P and Q terms of Carson’s equations (see Equation 10 and Equation 12 of Section 2.1.1 for details). It is of the form

k=4\pi d\sqrt{2\lambda f}

(20)

where

$\lambda$ 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

k=4\pi d(u_{CS}\to cm)\sqrt{\frac{2\lambda f(\lambda\to\rho)}{\rho}}

(21)

where

u_CS is conductor separation unit. In the US, conductor separation is usually measured in feet.

u_CS $\to$ cm is the number of centimeters per conductor separation unit.

$\lambda\to\rho$ 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

4\pi(u_{cs}\to cm)\sqrt{\frac{2\lambda f(\lambda\to\rho)}{\rho}}

(22)

is a constant which is computed once then stored for reuse.

2.3.2 Constants in the P and Q Terms of Carson’s Equations

After P and Q are computed, the terms 4 $\omega$ P and 4 $\omega$ Q in Equation 5 through Equation 8 of Section 2.1.1 produce impedances in units of abΩ/cm. If impedances are expressed in Ω/u_LL, these terms expand to

4\omega(u_{LL}\to cm)(ab\Omega\to\Omega)P

(23)

and

4\omega(u_{LL}\to cm)(ab\Omega\to\Omega)Q

(24)

where

u_LL is line length unit. In the US, line length is usually measured in miles.

u_CR $\to$ cm is the number of centimeters per line length unit.

ab $\Omega\to\Omega$ 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

4\cdot 2\pi f(u_{ll}\to cm)(ab\Omega\to\Omega)

(25)

The first terms of of the inductive reactance equations (Equation 7 and Equation 8 of Section 2.1) are also multiplied by half of this value, i.e.

2\cdot 2\pi f(u_{ll}\to cm)(ab\Omega\to\Omega)

(26)

Once again, both of these constants are calculated once then stored.

2.3.3 Unit Conversions Associated With GMR Terms

When the logarithmic term in Equation 7 of Section 2.1 is computed, the conductor’s GMR must be converted to conductor separation units, i.e.

ln\left(\frac{2h_{i}}{gmr_{j}}\right)

(27)

is actually evaluated as

ln\left(\frac{2h_{i}}{gmr_{j}(u_{CR}\to u_{CS})}\right)

(28)

where u_CR $\to$ u_CS converts conductor radius units to conductor separation units. Factoring out a constant in this expression yields

ln\left(\frac{ch_{i}}{gmr_{j}}\right)

(29)

where

c=\frac{2}{u_{CR}\to u_{CS}}

(30)

The factor c is also calculated once and stored.