Cross Country Faults - Protection Challenges and Improvements

Abstract

Cross Country faults (simultaneous/evolving faults at different locations) in transmission and distribution systems create challenges to distance, directional and phase-selection functions. Relay can miss to operate for in-zone faults or incorrectly operate for out-of-zone faults. This is because zerosequence and negative-sequence components distribution through the protected line, used by above mentioned functions is not easy to predict. This affects both solidly-grounded and isolated/impedance grounded systems.Single-pole switching is used in transmission network as it improves power system stability, compared to three-pole switching and also ensures system availability. In-order to correctly isolate the faulted phase, phase selector is used to supervise distance element when single-pole switching is enabled. However, certain fault scenarios such as high resistance faults, cross country faults pose a challenge to phase selector.In this paper, we start with the basic principles of phase selection for single-location faults and then we provide insights into the limitations of detecting the faulted phase during cross country faults. Finally, we introduce the improved phase selection logic to handle cross country fault scenarios, which provides correct detection of faulted phase using local information.