Real time condition monitoring has become a well accepted tool for thermal assessment and accurate rating of transmission and distribution assets. However, the use of fibre optics as sensors for distributed temperature sensing (DTS) is a rather complex technology with many parameters to consider.
- Selecting the Right Fibre
For measurement ranges of up to some 18 km the GI 50/125 multimode fibre 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 singlemode fibres compliant to ITU-T G.652.C or D enable distributed temperature sensing even for applications like subsea cables.
- Long Term Performing Fibre
Especially multimode fibres 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 long term service life of the fibre it is recommended to select phosphorous free qualities only. A securely embedded fibre within the high voltage power cable in form 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 Most Remote Distances
The spatial resolution along the entire fibre 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 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 visualisation purposes. Equipment based on Windows operating systems drop statistically to MTBF ratings below 12 months.
- Direct Link to SCADA
Condition monitoring data shall be available at 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 Proven Track Record
Reliability and 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 fibre set up is still sometimes requested today. However, latest DTS systems achieve measurements with sufficient temperature and spatial resolution at fast cycle times even at single ended fibres. The single ended measurement makes a more economic use of the DTS (longer range coverage, less 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 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 save 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.