Distributed temperature sensing (DTS) is known to be a valuable technology for power cable monitoring since the optical fibre sensor is immune against electromagnetic interference. Single-mode fibres would be most promising for long range temperature monitoring since they exhibit the lowest attenuation, are widely present for telecommunication use and are available at low cost. However, application of pulsed laser sensing systems is limited by the stimulated Raman-scattering in those fibres. We applied a Raman-OFDR system for this investigation using peak powers of about 0.1 W only and without duty cycle requirements. Long range was easily measurable using low modulation frequencies.
Full temperature and spatial resolution was achieved even for very long single-mode fibres. Measurements were taken at the Burbo Bank offshore wind farm owned by DONG Energy. The wind farm comprises 25 wind turbines and is situated on the Burbo Flats in Liverpool Bay at the entrance to the River Mersey, approximately 6.4 km from the Sefton coastline and 7.2 km from North Wirral. DONG Energy collaborates with manufacturers on the development and optimisation of monitoring concepts that enable analysis of operational conditions before an emergency occurs.
The transmission of electrical power in the power cable leads to an electrical heating of the cable conductor. The conductor temperature is limited to approximately 90°C. Continuous distributed monitoring of the conductor’s temperature along the entire cable length enables the real-time detecting of hot spot locations and allows preventing life time reduction by avoiding a thermal overload of the cable.
The Burbo Bank offshore wind farm is designed for generating up to 90MW. The expected annual output is 315,000 kWh. Three submarine cables with an expected lifetime of 40 years are used for the power transmission of 25 wind turbines. Each installed wind turbine is designed to run for approximately 6000 hours each year over 20 years. Up to 9 wind turbines are connected to each power cable string. Each cable section and the subsections between the wind turbines have individual cable designs including different conductor cross sections. The conductor cross sections decrease for the outer wind turbines taking into account the expected lower current load at those subsections.
Raman-OFDR DTS provide valuable and accurate data on local temperatures along whole long distance submarine power cables. Hot spots can be accurately localized and monitored for emergency situations. Analysis of temperature data in dependence on location, time and current load provides evidence of a proper installation and allows conclusions about the expandability of existing installations. Furthermore the analysis of the temperature survey supports gaining knowledge about thermal conditions of subsea cables for optimisation of future long distance power cable design and laying.