Distance protection of multiple-circuit shared tower transmission lines with different voltages and underground cable sections

Multiple-circuit transmission lines combining different voltage levels in one tower present extra challenges when setting a protection philosophy, as faults between voltage levels are possible. This study analyses faults in multiple-circuit transmission lines that are partly underground cable, by investigating the impact of the cable section on the fault current magnitude and operation of distance protection relays, without pilot scheme. The study shows that a cable's bonding configuration and layout have a small impact on the short-circuit magnitude, whereas the screen's grounding impedance and the presence of an earth continuity conductor impact the fault current magnitude. It was also demonstrated that distance protection relays settings used to protect the higher voltage line against single-phase-to-ground faults are capable of protecting the line against combined faults, for typical line configurations and short-circuit powers.