Line differential protection for long-distance remote assets using passive optical sensing

Distinguishing between faults on mixed conductor circuits can be challenging when relying only on measurements from within the substation. Auto reclose must be controlled differently for faults on cable sections compared to overhead line (OHL) sections. Conventional approaches are impractical, inaccurate, or expensive to deploy. This paper describes a solution using passive sensing to acquire remote measurements from current transformers (CTs) at the cable/OHL transition points. In particular, this paper focuses on a challenging circuit requiring CTs located over 60 km away from the nearest substation. The solution passively collects the CT measurements by converting their secondaries to an optical signal at the CT locations, but without requiring any active electronics, control power, or conventional telecommunications. This represents the first time that line differential protection, using entirely passive sensing, has been delivered over such a long distance. The results in the paper demonstrate that sub-cycle differential protection operation can be achieved for the remote cable sections. This provides a convenient and cost-effective approach to resolving the issue of allowing or blocking auto-reclose on these complex circuits. Although this application focuses only on auto-reclose control, the approach can be generalised to performing multi-zone differential protection on any complex circuit topology.