Many electric utilities install fiber optic cables on their high voltage lines to satisfy their own internal communication needs and to gain additional revenues by leasing excess capacity to telecommunication network providers. Electric utilities have been using fiber-optic cables to provide a communications link between neighboring substations or between substations and their control desks. However, ADSS turned out to be a industry wide problem where surface deterioration lead to their failure. Investigations found the failure mechanisms. Recommended methods were published, based on the geometry and voltage levels. Utilities should review these recommendations hence these can help them optimize stringing positions for various towers and power line configurations.
Fiber optics can offer a unique solution to the ever-increasing demand for bandwidth because of their remarkable high capacity for carrying data and their immunity to electromagnetic interference. However, integrating fiber optic cables into high-voltage corridors poses some technical and safety-related challenges that relate to corona and arcing. When ADSS fiber-optic cables (ADSS cables for short) are installed improperly on high-voltage transmission lines, unexpected failures can destroy these high-speed and high-capacity communication channels.
An optical fiber is a thin, long, transparent material (glass or plastic) that propagates light waves in order to transmit information.
Welding broken fibers, if at all, is difficult and problematic
Three types of fiber-optic cables are commercially available for installation in high-voltage lines:
ADSS cable is installed 10 ft to 20 ft (3m to 6m) below the phase conductors. Grounded armor rod assemblies support the fiber-optic cable at each supporting structure. Utilities have reported failures of ADSS installed on their high voltage lines. The high electric field on transmission lines generates continuous corona discharge at the end of supporting armor rods. This discharge leads to cable deterioration. In polluted environments, dry band arcing causes cable deterioration when fog or dew occasionally wet the cable. ADSS life expectancy on power lines depends on the following factors:
All Dielectric Self Supporting (ADSS) fiber optic cables are located in high electric fields. Their sheath can be subjected to dry band arcing and corona especially in highly polluted areas with little rainfall where there is regularly high conductivity precipitation on the cable and it is rarely cleaned by natural rainwater.cleaned by natural rainwater.
Corona or dry-band arcing puncture the cable sheath allowing water penetration leading to destruction of the aramide yarn and exposing the PBT tubes that contain the optical fibers. In more severe cases a massive reduction of mechanical strength can even lead to cable dropping.
Corona discharge is expected in locations with local high electrical fields caused by sharp edges, for example, at the tips of armor-rod assemblies of transmission lines conductors. Armor rod assemblies are attached to transmission line structures, support cables, provide mechanical strength protecting them from bending and also ground the cables. Depending on the placement of the cable with respect to the high voltage conductors, the conductor’s voltage and the pollution on the cable surface can induce current to flow along the cable. This gives rise to corona, micro-sparking and dry band arcing, especially near the tower where the e-field is greatest. The local partial discharge processes involves the emission of UV and the creation of ozone and other acids, reagents that erode the galvanized conductors and the ADSS sheath.
Even though ADSS cables are electrically non-conductive, contamination that accumulates on their outer sheath can turn them into being conductive. When the layer of pollution gets unevenly wet it becomes semi-conductive. Most cases of failing ADSS occurred in highly polluted areas or in coastal areas. The wind from the sea drives saltwater droplets onto the fiber-optic-cable surface, covering the cable with a thin layer of salt. Fog or dew wets the pollution layer and forms a conductive layer on the cable surface. Capacitive coupling between the phase conductors and fiber-optic cable induces current along the wet pollution layer. This current dries the layer and forms small dry bands. The dry band interrupts the current and generates high voltage across the band. This voltage produces arcs. The heating effect of the arc extends the dry-band length, which stops the arcing. However, condensation and wind-driven saltwater from the sea wet the cable and re-initiate the arcing. Dry-band arcing is a periodic phenomenon that occurs when the cable is simultaneously wet and polluted.
The higher the contamination and moisture, the higher the induced current thus, in subtropical areas, dry-band arcing as well as corona are especially harmful due to the specific climate conditions: an eight- to nine-month dry period followed by a shorter rainy period in winter. The cable surface pollution layer after the dry period is hard and adhesive, and particularly thick in the regions nearing clamps causing a serious corona problem.
Utilities should look for tests procedures for evaluating ADSS fiber optic cables installed on overhead power lines. Organizations such as IEEE, WAPA, EPRI, BPA, Washington State University and others keep on studying this issue.
Reference:
[1] “Fiber Optic Cables in Overhead Transmission Corridors: A State-of-the-Art Review Report Fast-Track Project Beats the Heat”, TR-108959, Nov 1997
[2] “Track Project Beats the Heatby George G. Karady, M. Tuominen and D. Torgerson”, Transmission & Distribution World, Dec 1, 2000
[3] “ADSS Cables Recognizing the Problems and Solutions” Felix G. Kaidanov, IEC, Oct 1, 2002 http://www.tdworld.com/mag/power_adss_cables_recognizing/index.html
[4] “Assessing Deterioration of ADSS Fiber Optic Cables Due to Corona Discharge” Final Project Report, George G. karady, Johnny Madrid, PSerc Publication 02-17, May 2002
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Ofil develops and manufactures UV Bi-spectral optical and digital inspection systems with embedded proprietary patented Solar Blind technology. Ofil aims at providing precise solutions for science, industry and environmental wellness. Ofil owns the brand name DayCor® that stands for Daytime Corona detection.
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