Real-time condition monitoring has become a valued tool for thermal assessment and accurate rating of transmission and distribution assets.
However, the use of fiber optics as sensors for distributed temperature sensing (DTS) utilizes more complex technology. Before installing a DTS system, consider the following:
- Selecting the right fiber
For measurement ranges of up to some 20 km the GI 50/125 multimode fiber typically provides the best distributed temperature measurement performance and allows deployments of less expensive multimode DTS systems. For longer ranges of more than 20 km single-mode fibers compliant to ITU-T G.652.C or D enable distributed temperature sensing even for applications like subsea cables.
- Long term performing fiber
Especially multimode fibers are available in different qualities. Besides attenuation limits the bandwidth has to be specified to > 500 or even >1000 MHz*km @ 1300 nm as it is important to achieve measurements with good spatial resolution also from remote distances. In order to ensure the long-term service life of the fiber, it is recommended to select phosphorous-free qualities only. A securely embedded fiber within the high voltage power cable in the form of a FIMT – fibre in metal tube – prevents damages in course of cable laying and delivers superior temperature information due to its proximity to the conductor.
- Spatial resolution at the most remote distances
The spatial resolution along the entire fiber length shall be 1.5 m or better. As the spatial resolution degrades with a function of distance with some DTS technologies the actual spatial resolution at the most remote distance shall be indicated.
- 24/7 operation
DTS equipment shall be powered directly by a secure substation DC supply, so even critical moments during mains outages are covered by condition monitoring. The DTS system shall operate independently from any kind of PC-based system; Windows operating systems shall be solely used for data visualization purposes. Equipment based on Windows operating systems drops statistically to MTBF ratings below 12 months.
- Direct link to SCADA
Condition monitoring data shall be available in real-time, so the DTS shall be capable to send its measurements and derived events directly to the operator’s SCADA or overall management system using protocols meeting substation standards based on IEC or DNP .
- Reliable DTS equipment with a proven track record
Reliability and a long lifetime are essential for investment goods like condition monitoring systems. The DTS laser diode and other key components shall be tested according to the Telecordia standard GR-468 or similar with medium lifetimes >25 years. The mean-time-between-failures (MTBF) rating of the DTS shall be higher than 25 years and based on a proven track record. Manufactures with ISO 9001 and ISO 14001 certification are recommended.
- Preference for single end
Known from the early days of DTS systems the double-ended or loop fiber set up is still sometimes requested today. However, the latest DTS systems achieve measurements with sufficient temperature and spatial resolution at fast cycle times even at single-ended fibers. The single-ended measurement makes more economic use of the DTS (longer range coverage, fewer channels occupied) and is the recommended operation mode for typical power cable monitoring installations.
- Ampacity predictions by RTTR
Real-Time Thermal Rating (RTTR) removes all uncertainty left by the DTS measurement. The power cable’s sheath temperature measured by the DTS gives a good idea of the temperature of the conductor, but unless an accurate model for the conductor is provided there will be some uncertainty left. Sophisticated RTTR systems cover apart from steady-state also the more critical transient operations based on IEC 60853 for this calculation and provide trustable ampacity predictions.
- The software speaks your mother tongue
The DTS operation software shall be user-friendly and available in the native language of the operator. This helps to prevent misunderstandings and encourages staff to use the system with pleasure.
- More than temperature
Real-time thermal assessment of the T & D grid is a powerful tool that allows accurate rating and ensures safe and stable grid operation. By adding other real-time diagnostics (i.e. partial discharge), failure uncertainty is reduced and maintenance procedures can be optimized. Aim for an integrated Global Condition Monitoring solution.