THE EFFECT OF TRANSMISSION LINES ON A FIBRE-OPTICAL COMMUNICATION CABLE SUSPENDED ON OVERHEAD TRANSMISSION LINE TOWERS. A METHOD FOR DETERMINING FIBRE-OPTICAL CABLE IMMUNITY TO THE ELECTRIC FIELD INFLUENCE

摘要

At present, for the transmission of information cablesrnsuspended on HV overhead transmission line (OHL)rntowers are widely used. In particular, a promising tendencyrnis to use for these purposes self-supporting nonmetallicrncommunication cables of ADSS type.rnThe self-supporting non-metallic cable of ADSS typernusually consists of fibres located in a tube, a carryingrnpart of glass fibre plastic rods or aramide filaments andrna weather-proof sheath.rnIn real service conditions a fibre-optical cable of ADSSrntype, suspended on OHL towers, is in the stressed staternunder the action of tensile mechanical loads in the electricrnfield produced by the high potential of OHL phasernconductors. The ADSS cable is also subjected to pollutionrnand wetting.rnIn wetting on the cable contaminated protective sheathrnsurface leakage currents occur. They can produce electricrndischarges on the cable sheath, which result in thernformation of conductive paths (tracks). In dry weatherrndischarges in the form of corona near the ends of tensionrnand suspension clamps can occur.rnTracking resistance of insulating material of an opticalrncable sheath under the action of electrical field at thernstage of its development is determined, based on testsrnaccording to IEC Publication 587.rnThe permissible potential of the electric field, in whichrna fibre-optical cable of ADSS type can be placed, Up, isrnestablished in specifications by a manufacturer in accordancernwith a condition of this cable long-term servicernon OHL.rnValues of electric field strength near an OHL dependrnon its design and operating voltage. The area of fibreopticalrncables possible suspension on OHL towers isrnlimited by equipotential lines, the potential of whichrnmust not exceed the permissible one, Up, for a particularrnfibre-optical cable.rnTo evaluate fibre-optical cable resistance to OHL electricrnfield influence in dry weather a method of laboratoryrntests of non-metallic fibre-optical cables underrnpollution with wetting has been developed.rnFor determing long-term optical cable resistance tornOHL electric field influence an optical cable sample isrnproposed to test at a potential which is three timesrnmore than Up. By regulating voltage on the potentialrnelectrode, the given potential in the tested cable suspensionrnpoint is achieved. The duration of such testsrnshould be no less than 500 hours.rnAfter the tests the sheath of the tested cable sample isrnexamined for leak-proofness at 100 – 250 kPa.rnAfter completion of the forced tests the fibre-opticalrncable can be recommended for application on OHLrnwith the arrangement of this cable in the electric fieldrnarea with a potential up to Up.rnThe laboratory tests simulating simultaneous influencernof mechanical loads and electric field have shown thatrnin dry condition, corresponding to fair weather electricrnfield concentration occurred near the earthed metalrnfittings by means of which an a fibre-optical cable wasrnattached to an OHL tower. Electric discharges arisingrnat fitting ends can result in erosion of a fibre-opticalrncable protective sheath. For limiting these dischargesrnspecial screens, equalizing electric field strength distribution,rncan be used.