ADSS Fiber Optic Cables in Overhead Transmission lines and the Corona phenomenon
Many electric utilities are installing 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 used fiber-optic cables for several years to provide a communications link between neighboring substations or between substations and the control desk.
Fiber optics can offer a unique solution to the ever increasing demand for bandwidth because of its remarkably high capacity for carrying data; The immunity of fiber optics to electromagnetic interference is another advantage. However, integrating fiber optic cables into high-voltage corridors also poses some technical and safety-related challenges such as corona and dry-band arcing phenomena. When ADSS fiber-optic cables are installed improperly on high-voltage transmission lines, unexpected failures can destroy high-speed and high-capacity communication channels.
The objective of this article is to give some general information on fiber optics and focus on ADSS (type of fiber optic cable) and its disadvantages to the electrical factors influencing on its life time such as corona and dry-band arcing. In this article an explanation of the process' failure is given and the prevention methods to ensure a fault-free fiber-optic-cable network.
General Information on Fiber Optics
An optical fiber is a thin, long, transparent material (glass or plastic) that propagates light waves in order to transmit information.
- Uses light signals: Consumes low power - Long distances without amplification
- Large bandwidth – large amount of data
- Easy installation – small dimension & low weight
- Does not cause EMI effect (Electro Magnetic Interference)
- Carries digital signals
- Many more fibers and channels per cable - Easier and faster to repair and ideal for using in existing network
- Immunity to interferences
- Difficult 'optical to electrical' conversion (& vice versa)
- Relatively high cost - delicate and expensive equipment
- Fiber welding is difficult and problematic
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. OPGW is not usable in many cases since the majority of HV and MV transmission lines have no lightning protection ground wires. It is also an expensive cable and its installation requires a long term outage
- 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) is a cable which generally electric utilities often select it as the best solution for several of reasons; ADSS is a self-supporting optical fiber cables. It has no metal component and can be installed on live lines with no outages. Since it is the most economical of all 3 types and has many more fibers channels per cable its easier and faster to repair in the event of damage or breakage moreover, its installation is easy and cheap and is suitable for hot line installation without deenergization of line
- Space potential effect
- Corona effect
- Dry-Band arcing
- Span length and sag
- Tension on cables
- Wind velocity and Aeolian vibration
- Sheath composition for UV resistance (UV from the sun)
- Temperature and pollution level
- Dry band arcing (short circuits over the dry band)
- Corona discharge
- Micro sparking
Installation of ADSS upon overhead transmission line
The lifetime of ADSS on power lines will depend on the following factors:
ADSS is installed 10 ft to 20 ft (3 m to 6 m) below the phase conductors. Grounded armor rod assemblies support the fiber-optic cable at each supporting structure.
The conductor sag is more sensitive to the change of temperature than the optic cable sag. If the cable is installed under the conductor, it is likely that the distance between the cable and conductor decreases when the temperature is high, because the conductor sag changes more than the cable sag. This implies that the danger of dry-band arcing-caused failure is higher in the summer if the pollution level remains the same.
The span is divided into 250-500 sections and the circuits representing each section are connected in series. The span length affects the dry-band arcing. The current increases with increasing span length. In transmission lines, the long spans are more likely to fail due to dry-band arcing. In most cases - even at high pollution levels - the current is less than 1mA if the span length is less than 200 ft (61 m).
Since all-dielectric self-supporting (ADSS) fiber optic cables are located in high electric fields, there is the threat of sheath damage and cable failure. Two mechanisms, dry band arcing and corona discharges near hardware are known to cause ADSS cable failures.
Whether from corona or dry-band arcing, the cable jacket is eventually punctured. Ensuring water penetration and internal electrical activity while destroying the fibers and cable-support material. In more severe cases a massive reduction of mechanical strength can even lead to cable dropping.
Several years of operating experience have indicated that fiber optic cables are not immune to the electric field produced by the transmission line. Whenever the intensity of the electrical field is more than 30 kV/cm corona or dry band arcing may appear causing damage such as degradation of the polymeric insulation along the fiber jacket and causing erosion close to the end fitting.
Corona and dry-band arcing triggers erosion on the optic cable deteriorating the fiber optic cables' sheath and cause it to fail exposing the aramide to the environment and degrade its tensile strength characteristics. The erosion process can cause flow of continuous surface current, surface contamination, change in hydrophobicity (moisture penetration & adhesion of water droplets) and drying segments of cable coating (dry bands) producing:
It is fair to say that the degradation processes of the polymeric insulating materials is similar to the aging of non ceramic insulators since corona discharge on ADSS cable generally occurs due to high electrical field generated by the conductors on the tower and in the long run causes puncture and failures on the cable jacket.
Corona discharge on ADSS cable generally occurs due to high electrical field generated by the conductors on the tower and in the long run causes puncture and failures of the cable jacket. Corona discharge at the tip of the armor-rod assemblies is a known problem due to sharp edges which intensify the electrical field.
Investigations have revealed that the phase conductors generate high electric fields, which can cause hardware corona as well as surface currents and subsequent discharges on a polluted jacket of the fiber-optic cable causing damage to the polyethylene (PE) cable jacket
When ADSS cable is wet and/or polluted, it becomes semi-conductive, dry-band arcing occurs when the fiber-optic cable is polluted and wet therefore, the majority of dry-band arcing-caused failures occur especially in highly polluted areas or in a coastal area. The wind from the sea drives saltwater droplets onto the fiber-optic-cable surface, which covers 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 a high voltage across the band. This voltage produces a flashover across the band and forms an arc. 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 reinitiate 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.
Since the electric field level upon the fiber optic cable depend several manageable parameters such as the voltage of the transmission line, the phase conductor parameters such as diameter, number of conductors per phase and conductor spacing all those can be changed and vary so that the local electric field will stay lower than 30 kV/cm.
Relocate the ADSS on the tower to points of less electrical field intensity will work. Position of ADSS cable (distance from clamps and fiber-optic cable to phase conductors and tower) will also change the electrical field and decrease the leakage current along it so, it is necessary to determine its optimal position on the tower to ensure safe distance from ADSS cable to phase conductors and to ground in order to avoid corona and partial discharges on its surface.
There are other methods of corona prevention as retrofitting the cable with corona coils and even to use track-resistant jacket materials but these methods are not always practical. In some cases corona rings on the clamp's extremities or polymer insulator can also minimize the electrical field intensity thus, eliminating the corona and arcing but they are not very typical.
Predictive maintenance with daytime corona camera is the best way to detect and pinpoint dry-band arcing and corona sources during daytime. It gives the ability to analyze findings and create a data trending for cost effectiveness. With a corona camera it is possible to detect loose hardware, bad groundings, broken strands, degradation and erosion of the cable jacket causing corona and most importunately dry band arcing caused by contamination and pollution.