A deeper insight into fault location on long submarine power cables

Abstract

This paper provides a deep insight into fault location on long Submarine Power Cables. Several field results on submarine power cable faults are provided. For land cables the main focus is on the electrical and thermal design of cable insulation and on the electro-thermal and thermo-mechanical design of cable accessories, in order to grant sufficient endurance performances and reliability to the whole cable systems (Mazzanti, G., Marzinotto, M., Extruded cables for high-voltage direct current transmission: advances in research and development. New York, Wiley-Blackwell, 2013). In submarine cables more issues arise, some related to the harsh laying environment, some others—even more troublesome—associated with man-made activities. Indeed, on the one hand submarine power cables are subject to strong mechanical stresses during the laying operations and critical service conditions in their working ambience. On the other hand, submarine cables are continuously exposed in all water depth to random mechanical damages caused by fishing gears, anchors and natural hazards (Bawart, M., Marzinotto, M., in Insulated Conductors Committee ICC/IEEE-PES, C11D submarine cables, St. Petersburg, FL, November, 2012; CIGRE, Third-party damage to underground and submarine cables, Brochure 398, WG B1.21, December, 2009). The longer the expected life and the longer the path of the submarine cable link, the higher is the probability of facing one or more faults due to human activities. Based on surveys about submarine cable failure data recorded worldwide over long periods, it can be concluded that the probability of experiencing at least one fault during lifetime is close to certainty for long submarine links. Statistically most damages to submarine cables are caused by human activities; only a low percentage is caused by natural hazards. Based on growing energy demand and dependency on offshore produced renewable energy, submarine power cables become essential for reliable electric power supply and often can be classified as critical infrastructure (International Cable Protection Committee—ICPC, About submarine power cables, 2011, www.iscpc.org ). Repair of damaged submarine power cables requires specialized ships as well as experts to recover the cable from the sea bed and replace the faulty cable section. Another critical aspect associated with long submarine cables is that, whenever a fault occurs, a fairly long time is spent for repair. For this reason, fast and efficient fault detection is essential in order to reduce the overall outage time as much as possible. All these aspects are discussed in this paper. The best practice commonly employed for classifying submarine power cable fault types are included in the paper, together with the results of measurements carried out in the field. The paper points out that fault location on submarine power cables differs by much from classical cable fault location on buried land cables as to both conditions and measuring methods, thereby illustrating the most efficient cable fault location methods. Some field results on submarine power cable faults are provided, measured on AC submarine cables as well as on HVDC submarine links. A unique case study of fault location on longest HVDC Submarine Link will illustrate TDR based measurements on cable lengths above 400 km. The case studies further focus on TDR diagram analysis in order to explain how to identify cable joints. The results prove that the overall outage time for repair activities can drop significantly if the fault location system is peculiarly designed for detecting faults in very long submarine cables with a good measuring accuracy. The hazards for operators and instruments connected to the huge amount of electrical energy that may be stored in very long links are also tackled in the paper, thereby addressing the particular safety issues involved by extra-long submarine cables.