In genera,l the cable impedance can be calculated in accordance with IEC 60909-2 'Short-circuit currents in three-phase a.c. systems - Part 2: data of electrical equipment for short-circuit current calculations. This standard give gives appropriate formulae for a variety of single and multi-core cables with or without metallic sheaths or shields. For situations not covered by IEC 60909, we can use the fundamental equations to derive suitable formula.
For a single conductor, the internal (self) inductance due to its' own magnetic field is given by:
$L=\frac{{\mu}_{0}}{8\pi}$
with the reactance given by: $X=\omega L=\frac{\omega {\mu}_{0}}{8\pi}$
and: μ_{0} = permeability of free space, 4π 10^{-7} N.A^{-2} L = self-inductance in H.m^{-1} X = reactance in Ω.m^{-1} ω = angular frequency = 2πf
For a second external conductor, the inductance due to the field produced by this other conductor is given by:
${L}_{e}=\frac{{\mu}_{0}}{2\pi}\mathrm{ln}\left(\frac{d}{r}\right)$
and: L_{e} = inductance due to external conductor , H.m^{-1} d = distance to external conductor, m r = radius of the conductor, m
For two parallel conductors, the total inductance of one conductor is given by:
${L}_{t}=L+{L}_{e}=\frac{{\mu}_{0}}{8\pi}+\frac{{\mu}_{0}}{2\pi}\mathrm{ln}\left(\frac{d}{r}\right)=\frac{{\mu}_{0}}{2\pi}\left(\frac{1}{4}+\mathrm{ln}\frac{d}{r}\right)$
whit the reactance given by:
${X}_{t}=\omega \frac{{\mu}_{0}}{2\pi}\left(\frac{1}{4}+\mathrm{ln}\frac{d}{r}\right)$
and:
L_{t} = total inductance of one conductor (cable core), H.m^{-1} X_{t} = total reactance of one conductor (cable core),
For multicore cables, it is often the case that the distance between cores varies. For example in three cables in flat formation, L1 to L2 and L2 to L3, will be difference than L1 to L3. To cater for these differences, we use a concept of geometric mean spacing (also see Geometric Mean Distance):
$d=\sqrt[3]{{d}_{L1L2}\times {d}_{L2L3}\times {d}_{L1L3}}$
The above can be extended for cables with more cores, or to find the average phase to neutral spacing for example.
The above are valid as positive sequence impedance for a cable. Unfortunately, the calculation of zero sequence is more difficult. Several papers have been published on this and the use of Carson's equations as a means to calculate the zero sequence impedance is an accepted approach. Deriving equations utilising Carsons' equations is fairly complex and not strictly necessary here. The end results of such derivates are presented in IEC 60909 and we can use these directly.
In some instances, it is also necessary to consider the soil penetration depth ẟb in the calculation of zero sequence impedance (see IEC 60909 part 3):
$\delta =\frac{1.851}{\sqrt{\omega {\displaystyle \frac{{\mu}_{0}}{\rho}}}}$
where: δ - equivalent soil penetration depth, m μ_{0} - permeability of free space (= 4π × 10−7), H.m^{−1} ρ - soil resistivity, Ωm
For three single core cables, in either trefoil or flat formation, the positive and negative sequence impedance is given by:
${Z}_{1}=R+j\omega \frac{{\mu}_{0}}{2\pi}\left(\frac{1}{4}+\mathrm{ln}\frac{d}{r}\right)$
* equation can be applied to single core and multicore cables * equation can be applied to three three-phase (with and without neutral), and single phase loads
Single Core Cables:
Multicore cables:
${Z}_{0}=R+3\omega \frac{{\mu}_{0}}{8}+j\omega \frac{{\mu}_{0}}{2\pi}\left(\frac{1}{4}+3\mathrm{ln}\frac{\delta}{\sqrt[3]{r{d}^{2}}}\right)$
* for current return through the earth
(11)
${Z}_{0}=4R+j4\omega \frac{{\mu}_{0}}{2\pi}\left(\frac{1}{4}+\mathrm{ln}\frac{\sqrt{{d}_{LN}^{3}}}{r\sqrt{d}}\right)$
* current return through fourth conductor * equation applied to single core and multicore cables
${Z}_{0}=R+3\omega \frac{{\mu}_{0}}{8}+j\omega \frac{{\mu}_{0}}{2\pi}\left(\frac{1}{4}+3\mathrm{ln}\frac{\delta}{\sqrt[3]{r{d}^{2}}}\right)-3\frac{{\left(\omega \frac{{\mu}_{0}}{8}+j\omega \frac{{\mu}_{0}}{2\pi}\mathrm{ln}\frac{\delta}{{d}_{LN}}\right)}^{2}}{R+\omega \frac{{\mu}_{0}}{8}+j\omega \frac{{\mu}_{0}}{2\pi}\left(\frac{1}{4}+\mathrm{ln}\frac{\delta}{{r}_{L}}\right)}$
* for current return through fourth conductor and earth
For three single core cables, with a metallic sheath or shield, in either trefoil or flat formation, the positive and negative sequence impedance is given by:
${Z}_{1}={Z}_{1\left(10\right)}+\frac{{\left(\omega \frac{{\mu}_{0}}{2\pi}\mathrm{ln}\frac{d}{{r}_{Sm}}\right)}^{2}}{{R}_{s}+j\omega \frac{{\mu}_{0}}{2\pi}\mathrm{ln}\frac{d}{{r}_{Sm}}}$
* equation can be applied to three three-phase (with and without neutral), and single phase loads * Z_{1(10)} from equation (10)
Multicore Cables:
${Z}_{0}={Z}_{0\left(11\right)}-\frac{{\left(3\omega \frac{{\mu}_{0}}{8}+j3\omega \frac{{\mu}_{0}}{2\pi}\mathrm{ln}\frac{\delta}{\sqrt[3]{{r}_{Sm}{d}^{2}}}\right)}^{2}}{{R}_{s}+3\omega \frac{{\mu}_{0}}{8}+j3\omega \frac{{\mu}_{0}}{2\pi}\mathrm{ln}\frac{\delta}{\sqrt[3]{{r}_{Sm}{d}^{2}}}}$
* current return through shield and earth * Z_{0(11)} from equation (11)
where: R_{s} - metallic sheath or screen resistances, Ωm r_{Sm} - mean radius of the sheath or shield, m Z_{1(1)} and Z_{0(2)} are the impedances given by equations (1) and (2)