ADSS Fiber Optic Cables in Overhead Transmission lines and the Corona phenomenon

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The Need for & Hi-Quality Telecommunication

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 are installed improperly on high-voltage transmission lines, unexpected failures can destroy these high-speed and high-capacity communication channels.

General Information on Fiber Optics

ADSS Fiver Optic
ADSS Fiber Optic Cable

An optical fiber is a thin, long, transparent material (glass or plastic) that propagates light waves in order to transmit information.

Advantages:

  • Light signals have low power consumption, convey signals over long distances without amplification
  • Large bandwidth – large amount of data
  • Easy installation – small dimension & low weight
  • No EMI effect (Electro Magnetic Interference)
  • Fit for digital signals
  • Have immunity to RF interference
  • Many fibers can be bundled in a single cable for a larger capacity
  • Cables can be mounted on existing grid

Disadvantages:

  • Optical to electrical conversion & vice versa  is complicated
  • Fragile, delicate and therefore relatively high cost
  • Welding broken fibers, if at all, is difficult and problematic

Cable Types

fiber optic cables locations on electrical pole

Three types of fiber-optic cables are commercially available for installation in high-voltage lines:

    • Optical Ground Wire (OPGW) is a cable encased within the ground wire. Used in transmission lines on ground conductors with the fiber-optic cable embedded inside. However, since the majority of HV and MV transmission lines have no lightning protection ground wires it is not frequently used. Moreover, OPGW besides being an expensive cable,  installation requires long term outages
    • Wrapped Round the Phase conductor or the ground wire (WRAP) is spiral wrapped cable around existing overhead ground conductors or in some cases, phase conductors. Winding the fiber-optic cable on the phase conductors of HV or MV is not possible because the conductor diameter is generally too small and of insufficient mechanical strength for the wind loading moreover, hot-line installation is very difficult
    • All Dielectric Self Supporting (ADSS) the most frequently used cable.  ADSS is  self-supporting, has no metal components and can be installed on live lines without outages. ADSS has multiple fibers channels per cable.  They have 2 design families: double jacket with a max of 576 single fibers for spans that are up to 6000 feet, or single jacket with a max of 144 single fibers for shorter spans.

ADSS INSTALLATION

ADSS 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:

  • Electrical
    • Corona effect
    • Dry-Band arcing
    • Space potential effect
  • Mechanical
    • Span length and sag
    • Tension on cables
  • Environmental
    • Wind velocity and Aeolian vibration
    • Sheath composition for UV resistance (UV from the sun)
    • Pollution & temperature

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 Effect

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.

Dry-band Arcing

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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