Calculation of the impedances of underground lines seen by distance protections

Meshed underground networks pose particular problems for network protection. If distance protection is used to protect cable networks, a good knowledge of the impedances is important. It is not possible to give a simple relation for the sequence impedances for all types of single core (single-phase) cables if the fault is on the underground line itself. This is due to the fact that, for this case, 6 or 7 independent conductors [3 phases+3 screens+(optional) 1 ground wire] have to be taken into account. This paper presents a model able to determine the impedances seen by distance protections for faults on the cable and discusses the findings.