Unintended tripping of healthy parallel lines during single-line-to-ground faults: field evidence and mitigation

Distance relays may maloperate under various conditions such as improper settings, measurement distortion, or abnormal network operating states. This article investigates an overlooked mechanism leading to distance relay maloperation in parallel sub-transmission circuits containing underground cable sections. During a single-phase-to-ground fault on one circuit, a portion of the fault current may return through the neutrals of receiving-end transformers and flow back toward the source through the parallel healthy circuit. This reverse fault current component can become aligned with the load current of a healthy phase, resulting in a significant reduction of the apparent impedance measured by the distance relay. Consequently, the impedance trajectory may enter the relay operating characteristic from the load region, causing undesired zone operation and tripping of the healthy line. Such a misoperation may lead to simultaneous disconnection of both parallel circuits and severe downstream outages, despite the fault being confined to only one line. The mechanism is analytically explained and validated using disturbance records from a real 63 kV network. In addition, DIgSILENT PowerFactory simulations are conducted to reproduce the observed behavior and confirm the current amplification. Key factors influencing the severity of the phenomenon, including cable shield grounding conditions and fault proximity to the receiving-end substation, are also discussed. Finally, practical countermeasures are proposed to improve protection security and prevent unnecessary tripping.