Impedance, or positive sequence impedance, is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit.Quantitatively, the impedance of a two-terminal circuit element is the ratio of the complex representation of the sinusoidal voltage between its terminals, to the complex representation of the current flowing through it, it possesses both magnitude and phase, unlike resistance, which has only magnitude. The notion of impedance is useful for performing AC analysis of electrical networks, because it allows relating sinusoidal voltages and currents by a simple linear law. In multiple port networks, the complex voltages at the ports and the currents flowing through them are linearly related by the impedance matrix.
For cables, the negative sequence impedance is equal to the positive sequence impedance.
For subsea armoured three-core cables, the three screens are assumed to be solidly bonded to the armour at least at both ends.
The equations are based on the Cigré Guide 531 for symmetric systems without earth return path.
