Case Study of Long-Distance Optical Cable Line Damage

Long-distance optical cable failures often result from mechanical stress, installation errors, or environmental factors, and can be effectively monitored using distributed sensing technologies.

Submarine HVDC Interconnector Case

A ±320 kV, 700 MW HVDC submarine interconnector spanning over 300 km between two European countries experienced potential risks typical of long-distance optical cables. To ensure continuous operation and rapid fault detection, Distributed Acoustic Sensing (DAS) was deployed along the cable route.

Two DA

S instruments monitored 150 km and 25 km sections, respectively, using low-loss fiber and range extenders to maintain signal quality over extreme distances. This setup allowed real-time detection of mechanical disturbances, water ingress, or other anomalies, safeguarding uninterrupted power transmission and critical infrastructure reliability.

Hybrid Overhead and Underground Cable Circuit

In a 132 kV AC double circuit with four cable sections, the most challenging section started 44. 9 km from the nearest substation. Passive optical sensing was used to acquire current transformer (CT) measurements at cable/OHL transition points without active electronics.

Twelve three-phase CTs were installed to enable sub-cycle differential protection. The system successfully discriminated between faults on cable sections and overhead lines, allowing auto-reclose for OHL faults while blocking it for cable faults. Optical amplifiers and careful sensor design ensured signal integrity over long distances.

Field Damage Case: Optical Drop CableA practical field example involved an optical drop cable damaged during the installation of a new metallic cable in a conduit. The damage included severe bends, split jackets, and trench marks, causing service interruption. Investigation revealed that improper handling during rod and string installation led to mechanical stress on the optical fibers.

This case highlights the importance of careful installation practices and monitoring at transition points.

Common Causes of Long-

Distance Cable Failures

Mechanical Stress: Microbends and macrobends from tight routing, friction, or crushing can cause signal loss. Adhering to bend-radius guidelines and using bend-insensitive fibers mitigates this risk. Water Ingress: Moisture penetration in loose-tube or slotted-core cables can freeze and damage fibers.

Water-blocking features and sealed closures are essential for underground or submarine cables. Connector and Splice Failures: Poorly cleaned or misaligned connectors and splices can introduce reflection and attenuation. Fusion splicing and protective enclosures are recommended.

Environmental and Operational Hazards: Rodent attacks, temperature extremes, and accidental impacts during construction can compromise cable integrity.

Monitoring and Mitigation Strategies

Distributed Acoustic Sensing (DAS): Provides continuous, real-time monitoring over hundreds of kilometers, detecting vibrations, bends, or intrusion events.

Optical Time-

Domain Reflectometry (OTDR): Baseline and periodic testing detect attenuation spikes or reflective events indicating damage.

Optical Amplifiers and Low-

Loss Fiber: Maintain signal quality over long distances, especially in submarine or remote installations. Redundant Paths and Optical Line Protection (OLP): Backup paths and 1+1 protection switches ensure continuity in case of fiber failure. Preventive Installation Practices: Proper conduit handling, bend-radius adherence, and protective enclosures reduce mechanical damage risk.

ConclusionLong-distance optical cable failures can arise from mechanical, environmental, or installation-related factors. Case studies demonstrate that advanced monitoring technologies like DAS, OTDR, and passive optical sensing, combined with careful installation and protective measures, are critical for early fault detection, rapid response, and maintaining reliable communication or power transmission over extensive distances.