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keywords: Ofil Solar Blind product UV systems ADSS Cables OHTL Fiber Optics drybands ARCING Failure degradation ARCING DayCor® corona

ARTICLES

ADSS Cables - Aging Processes

Power utilities deploy optical fibers on their networks for telecommunication applications for their own use and for third parties. The All Dielectric Self Supporting (ADSS) cable with pultruded, glass-reinforced plastic or stranded aramid yarns is used for that purpose. At low voltages in benign geographical locations the cables present no problems, but at high voltages or in contaminated locations failures occur mostly adjacent to the clamp arrangements. Dry band and corona have been identified as the main aging agents of the sheath and the grounding metal clamps. Both phenomena are well incepted by a corona camera, and can be treated on time.

Failures of installed cables are commercially sensitive, so problems and their resolutions are not generally published. Several solutions to withstand degradation by dry-band arc activity are being used yet, unfortunately, the issue of predicting cable lifetime remains unsolved. Seeing arcing and corona discharges with Ofil's UV cameras is, in this case, a great advantage because it enables monitoring decaying processes and management of the valuable cable systems, mainly in areas with low rainfall, high contamination and close to coasts.

The use of ADSS – All dielectric self-supporting cables is widespread worldwide. Ever since its implementation the most widely improvement was in the "arc-resistant" sheath. The sheath material improves the performance of cables and extends their life. Nevertheless, it is impossible to forecast the cables' life expectance not in any given environment, even in cases where some degree of risk can be defined. Furthermore, some of the large volume of cables that are already installed are at risk, but due to commercial sensitivity this kind of problem tends not to be reported [1].

Electrical currents along the ADSS cables act as aging factors of the sheath material. These currents along the cables are expected due to potential gradient that originates in 3 sources: by the capacitive coupling between the conductors, the ADSS cable and the ground; by the voltage difference between the clamps and midspan locations; and by the conductivity of the cable surface. Normally, the potential would be greatest at midspan, where the position of the ADSS cable is higher than that of the conductors (sagging of cables), and forced to earth potential by the earthed metallic clamps at the tower, where the conductors and ADSS cable are leveled. Conductivity is affected by the state of contamination and humidity on the cable. When the sheath material ages it becomes hydrophilic and its resistance per unit length can get reduced to very low values of only few hundred kilo-ohms/m and current raised to several miliamps. For lines of 150kV laboratory measurements results suggest for polyethylene limits of 1 mA, and 1.5 mA.

Surface currents and discharges can give rise to Joule heating of the moisture and to a rise in the cable surface temperature. Following this heating effect a dryband may consequently occur on the surface on the cable. The dryband is most likely to occur close to the towers where the current is normally greatest. Drybands possess higher linear impedance than other parts of the cable surface. This high-impedance characteristic leads to a large voltage drop across the short section of dry cable and to arcing. When the current is high enough to sustain an arc, unstable discharge will occur across a dryband. Un-uniform conductivity on cable surfaces that results from drybands increases local e-fields and leads to corona and arcing discharges that generate UV and ozone, both of which damage the cable surface.

ADSS cables can perform well if they are located in temperate and benign locations. Such a case was examined in the UK, where an ADSS cable on a 132 kV line near Nottingham was taken out of service after 15 years and examined. The line was located remote from any coast, in a temperate climate and a benign situation. The aging of the polyethylene sheath material was identified by a loss of its "shiny" aspect, and exhibited only low levels of aging manifested by relatively lower hydrophobicity. It was concluded that this ADSS cable was within its design specifications, installed in suitable locations on the towers with its sheath not even close to the end of life [2].

In Israel, a subtropical area with eight to nine months dry season followed by a short rainy winter, dew on more than 100 nights during the dry season, proximity to desert and to the mediterenean coast [3] ADSS fiber optic cables were installed on 110 kV and 161kV OHTL. These cables demonstrated varying amounts of tracking and erosion after 1 to 3 years of operation. Damages were classified as erosion and tracking. The compromised sheath was located closer to the holding clamps with the following given explanations:

  1. The electric field intensity on the edge of the clamp wires exceeded the corona onset resulting in corona discharge that eroded the sheath material
  2. Induced currents at the end of the clamp reached their maximum values producing joule heat and forming dry rings zones and partial arcs

Simon M. Rowland, Member, IEEE, Osvaldo de la Cerda, and Neil R. Haigh [1] question the claim confining the use of ADSS to voltage levels below 275 kV because of dry-band arcing threat on the cable sheath and call it somewhat arbitrary. They state that "in polluted conditions, cables can fail at much lower system voltages, and in some higher voltage installations no problems have been witnessed". In their study, a solution to the problem of dryband arcing on ADSS cables is suggested and examined. Their solution involves attaching 50-m resistive rods to installed self-supporting cable. It is claimed that after six years of operations such systems proved successful endurance and operation, in spite of the fact that the installation was in a particularly aggressive situation and a high level of maintenance has been found to be necessary. The solution, known as the arc arrest system (AAS) has been deployed at an exceptional location where sheath damage has occurred only six months after the ADSS cable was originally installed. It is claimed [1] that this was the only solution that has been successfully applied to a situation where the problem of dry-band arcing has occurred with any severity.

Application of the rod prevents damage to the cable in two ways: it prevents buildup of high e-fields over the ADSS cable, thus preventing corona and arcing discharge in the area that is closest to the tower; second, the current diminishes away from the tower, and if the rod is long enough, the current beyond its length is too small to sustain a damaging arc. The idea of using the rod is to let the current flow along the rod even when the cable is dry. It is a challenging to design a rod that is not too conductive or too resistive, because if too conductive insufficient voltage will be dropped over their length to stop aggressive dry-band arcing at their ends. If the rods are too resistive, they will not prevent dry-band arcing within their length. Moreover, if a small fraction of a rod length is very resistive, thermal aging or runaway may occur within the rod at this point, creating a bigger problem than existed before. A regular inspection and maintenance schedule must be implemented on the AAS installed system to ensure no evidence of corona or arcing.

Contamination is a major concern in locations adjacent to the sea and to cement factories and mines, especially in areas where rainfall does not wash the cables off the solid deposits. In such areas insulators are being washed periodically. When washing the insulators part of the ADSS cable that is closest to the support structures gets sprayed and washed as well, leaving clean regions with a high resistance that will dry out faster than the rest of the cable. This encourages dry-band arc activity by creating small regions of high resistance close to the tower. Thus, the very act of washing insulators may make the situation significantly worse for the cable, increasing the periods of dry-band arcing and also increasing the arc current. Indeed, when designing AAS such scenarios must be taken into account. The maintenance efforts of this system that clearly provides a method of protecting existing and new ADSS cables in onerous environments are understood and acceptable. To simplify the involved costs Ofil's DayCor cameras come handy and effective because they indicate on time existing discharges and failure process even at their earlier stages.

Refrences:

  1. Simon M. Rowland, Member, IEEE, Osvaldo de la Cerda, and Neil R. Haigh, " Implementation of a Solution to the Problem of Dry-Band Arcing on ADSS Cables" IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 1, JANUARY 2007
  2. Simon M. Rowland, Senior Member, IEEE, Konstantinos Kopsidas, and Xin Zhang, "Aging of Polyethylene ADSS Sheath by Low Currents", IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 1, JANUARY 2007
  3. F. Kaidanov, R. Munteanu, and G. Sheinfain, "Damages and destruction of fiber optic cables on 161 kV overhead transmission lines," IEEE Elect. Insul. Mag., vol. 16, no. 4, pp. 16–23, Jul./Aug. 2000
  4. https://www.escholar.manchester.ac.uk/api/datastream?publicationPid=uk-ac-man-scw:100815&datastreamId=FULL-TEXT.PDF

Typical location of ADSS cable on the lower cross arm of a twin circuit tower
Scheme showing the relationship between the induced voltage, current and dry-band area on ADSS cable [4]
Complete failure of the sheath
Damage on a polyethylene sheathed cable on a 100kV line in Chile in the form of rings
Water cannon washing the insulators and the portions of ADSS that are close to the pole