Mixed circuit protection using Refase™
Protection of mixed overhead-underground circuits
- There is an industry trend to underground cables due to societal pressure
- Overhead lines cost significantly less, but undergrounding improves reliability and visual impact
- These factors are balanced by mixed overhead-underground circuits (hybrid circuits)
The challenge: detect immediately if the fault occurred underground or overhead
- Mixed lines pose challenges for the protection logic due to the nature of faults overhead and underground
- In overhead lines, the majority of faults are transient. Auto-reclose is widely applied.
- In underground cables, faults are often permanent. Auto-reclose should be blocked.
- Operators’ desired solution is auto-reclose blocking for all cable faults based on a robust current differential algorithm
- Faulted section identification using current differential protection algorithm
- Passive array of current transducers
- Uses standard optical fibers (e.g. OPGW or wrap)
- Simple installation & maintenance-free
- Precision manufactured
- Proven interoperability: onboard differential protection algorithm and full support for AR block and IEC 61850
In hybrid circuits containing both overhead lines and underground cable sections, it is advantageous to determine in which section an electrical fault has occurred. This enables an operator to auto-reclose (AR) in the event of a transient OHL fault, or to block auto-reclose in the event of a permanent cable fault. This is not typically possible if simple distance protection schemes are deployed, which will typically result in some ambiguity in identification of the faulted section, rendering auto-reclose prohibitively risky. Ideally, the circuit would be instrumented at every transition point such that robust differential current algorithms may be used to immediately and definitively identify the faulted section in the event of a fault, and so permit or block auto-reclose accordingly.
However, to deploy conventional current measurements at remote transition points is typically very expensive, requiring significant local supporting infrastructure including power supplies, active digital telecoms, and GPS access to obtain reliable measurements. Optical NCITs have fundamental limits on range and environmental withstand and consume multiple fibres per instrumented location, fundamentally limiting their deployment at scale.
A single Refase™ system is uniquely capable of instrumenting multiple hybrid circuit sections with passive, highly-multiplexable current sensors over unprecedented range. This system enables discrete, robust, and scalable differential current monitoring over long-range, with measurements maintaining consistent high accuracy over wide operating temperature ranges when compared to these prior technologies. Discrete AR block signals are output on protection timescales (milliseconds), allowing an operator to cost-effectively manage hybrid circuits in the most efficient manner possible.