Fault location on mixed overhead line and cable transmission networks

In future, underground cables will be an essential part of transmission systems and many feeders will consist of multiple overhead line and cable sections, especially in environmentally constrained countries where planning constraints may restrict the construction of new overhead lines. Most of the faults that occur on overhead lines are transient, but those on cables are generally permanent. Therefore, a combined overhead and underground transmission feeder brings a significant challenge to an auto-reclosure system which normally recloses a circuit breaker immediately after it was tripped by a protection relay responding to a fault. This paper presents a new hybrid fault locator making full use of the robustness of a distance relay and the accuracy of a travelling wave fault locator. The proposed scheme can precisely distinguish if the fault is on an overhead line or underground cable, and then send the correct tripping and reclosure sequence to a circuit breaker.