24 8 Power System Transients
3.2.4 Theoretical Formula of Sequence Currents
The sequence impedance/current calculation of overhead lines is well-
known and introduced in textbooks [2]. For underground cables, theoret-
ical formulas are proposed for the cable itself [6–9]. However, in order to
derive accurate theoretical formulas, it is necessary to consider the whole
cable system, including sheath bonding, because the return current of an
underground cable ows through both the metallic sheath and the ground.
Until now, no formula existed for sequence impedances or currents that con-
sider sheath bonding and sheath-grounding resistances at substations and
earthing joints. As a result, it became a common practice to measure these
sequence impeda ...