Optical attached cable (OPAC) is a type of fibre-optic cable that is installed by being attached to a host conductor along overhead power lines. The attachment system varies and can include wrapping, lashing or clipping the fibre-optic cable to the host. Installation is typically performed using a specialised piece of equipment that travels along the host conductor from pole to pole or tower to tower, wrapping, clipping or lashing the fibre-optic cable in place. Different manufacturers have different systems and the installation equipment, cable designs and hardware are not interchangeable.
Although lashed cable systems and clipped cable systems have been investigated as a means of attaching optical fibre cables to overhead power lines, wrapped cables were the first type to be developed and are the only type in common use today.
Wrapped cable systems were developed independently in the UK (SkyWrap) and Japan (GWWOP) during the 1980s and have been widely used, with installations in every continent except Antarctica. Through licensing and through independent development, wrapped cable systems have also been supplied by French, Italian, German and Russian companies.
The installation process for wrapped cables involves passing a drum of cable around and around the host conductor as the carrying device moves across the span. For installation on hosts within 10 m of the ground (medium or low voltage overhead lines), it is possible to pull the wrapping machine by hand from the ground below the line. However, a radio-controlled power unit using batteries or a petrol engine is normally required when the host conductor is on a high voltage transmission line. Wrapped cables can be applied to earth wires (ground wires, shield wires) on power transmission lines and to phase conductors on transmission, sub-transmission or distribution lines.
SkyWrap is the most successful example of OPAC and is used together with more familiar optical fibre cables such as optical ground wire (OPGW) and all-dielectric self-supporting cable (ADSS) to build communications networks for power utilities.
Etymology The generic International Electrotechnical Commission (IEC) and IEEE designation for attached cable is OPAC. OPAC can be used in the same sense as the nomenclature OPGW and ADSS. OPAC refers specifically to fibre-optic cables and specifically to installations where the host conductor is part of the electricity supply system. Installations using metallic cables (coax or copper telephone cables) or messenger wires as the host are not covered by the term OPAC.
The term ground-wire wound optical cable (GWWOP) is sometimes used to describe wrapped fibre-optic cables for installation on power lines. and may be a brand name owned by Furukawa Electric of Japan. The Russian language description for wrapped cable technology is "ОКНН" (оптоволоконного кабеля неметаллического навивной) (in English: OCNN = Optical Cable, Non-metallic, coiling or wrapping (Navivnoj)). The French language term is "Câble Optique Enroulé" (COE)
The name SkyWrap is one of three related brand names introduced by FOCAS in 1990 to describe its fibre-optic cable products used in the construction of power utility communications networks, the others being SkyLite OPGW and SkySpan ADSS. When AFL acquired FOCAS in 2000, the SkyLite and SkySpan brand names were dropped but SkyWrap has continued to be used. Prior to 1992 the brand name was Rayfos. The Rayfos name is owned by Raychem Corporation, who developed the technology before selling it to FOCAS. Rayfos is a portmanteau name constructed from "Raychem" and "fibre optic system".
History
UK and Japan Wrapped optical fibre cable technology was developed independently in the UK and Japan in the early 1980s. In the UK, Raychem Ltd had a background in polymeric materials with resistance to high voltage environments; used for example in heat-shrinkable 33kV cable terminations and in polymer insulators. The initial development involved an all-dielectric fibre-optic cable with a sheath made from tracking resistant material. The first installation was carried out on a 33kV overhead distribution line between sub-stations at Hartley and Goudhurst in Kent, UK in December 1982.The choice of a 33kV line indicates the expected market for the technology – building communication networks over local distribution lines when transmission and sub-transmission lines were expected to be equipped with OPGW. Over the next five years, installations were carried out in Norway, New Zealand, USA, India, Germany, France and Finland totalling about 180 km in both ground-wire and phase-conductor installations, and on both distribution and transmission lines. Raychem sold the wrapped optical fibre cable technology to Cookson Group plc in 1987. A subsidiary of Cookson Group was FOCAS Limited who completed the development work and commercialised the product as SkyWrap, supplying more than 16,000 km to customers in over 30 countries. FOCAS was acquired by AFL in 2000
In Japan, Furukawa Electric Company developed a product explicitly for installation on the earth-wire of transmission lines, and this was reflected in the name: Ground-Wire Wrapped Optical cable. The first installation was carried out in 1985 on a 275kV transmission line owned by Chubu Electric Power and included a remote controlled pulling device and a self-compensating counterbalance on the wrapping machine. These are both necessary technologies for successful wrapped cable installation on the ground wire of long-span HV transmission lines.
By 1995, several other companies had entered the wrapped cable market: in Europe, British Insulated Callender's Cables (BICC) (in UK) introduced a product called Fibwrap' and Alcatel subsidiary IKO Kabel of Sweden marketed GWWOP under license from Furukawa. In Japan, GWWOP development had become a joint effort involving Sumitomo Electric Industries, Hitachi Cables, Furukawa Electric and Fujikura companies and each of the 4 companies supplied their own slightly different wrapped cable systems to power utilities in the domestic Japanese market. BICC also had a technology licence from Furukawa, but carried out extensive development work to make the product suitable for the European market: BICC re-designed the cable to use loose-tube technology and so provide a zero-strain environment for the optical fibres at all operating conditions of the overhead line; the tug and wrapping machine were re-designed to reduce the overall load on the conductor during the installation process. This included the use of a light-weight (but very costly) carbon-fibre cable drum in order to maximise the length of cable in a fixed payload weight. Being derived from GWWOP, both the BICC and Alcatel systems were limited to ground-wire installations and no phase-conductor installations were carried out.
Russia A novel wrapping technology was developed by Russian company Teralink during the early 2000s. The wrapping machine is self-balancing with the fibre-optic cable payload held on a pair of drums arranged one on either side of the host conductor and therefore on either side of the axis of the machine. The cable pays off one drum for 100-150m then the other drum and so on, alternately switching between drums. The drums are never more than 2 kg out of balance and the machine does not require an external counterbalance system. This keeps the machine smaller than other designs of wrapping equipment and the payload constitutes a higher proportion of the all-up weight. Two constructors are able to place by hand the machine with full drum of cable on the wire.
A second Russian company, Scientific Innovations, introduced a more conventional design of wrapping machine in the mid-2000s, with a single drum of fibre-optic cable and a counterbalance arm. There are several records of successful installations using this equipment.
France The final commercial developments of wrapped cable systems took place in France in about 2005 when RTE, the national electricity transmission utility began to install substantial quantities of optical fibre cables including OPGW and wrapped cables. Two French contractors, Transel (part of the Bouygues group) and Omexom (a Vinci SA company), independently developed their own wrapped cable systems (known as câble optique enroulé (COE) in French) to participate in this program. Neither system had a product name other than COE or was marketed outside France. Both systems were withdrawn in about 2010 after a total of about 1000 km of wrapped cable installations for RTE.
Technology There are three basic technology requirements for a wrapped cable system – a fibre optic cable with suitable performance for installation on an overhead power-line; a device for carrying out the wrapping operation (wrapping equipment) and the appropriate hardware to stabilise and complete the installation.
Cable Wrapped fibre-optic cable must provide the following characteristics: Small diameter to have as little as little impact on the wind load of the host conductor as possible; Light weight in order to keep the total weight of wrapping machine and cable payload as small as possible. Tough outer sheath to provide protection against the types of damage mechanism encountered on overhead lines: sunlight, rain, atmospheric pollution, birds’ claws and beaks, shotgun pellets (hunters shoot at birds which perch on or fly close to overhead lines) and wind-induced vibration. Sufficient strain-margin to protect the optical fibres at the extremes of the overhead line operating window: low night-time or winter temperatures cause the host conductor to shrink, high day-time or summer temperatures cause expansion. The optical cable design must allow the fibres to continue to transmit data across the whole daily and seasonal temperature range. Strong winds and built up layers of ice will cause conductors to stretch even more, requiring high levels of strain margin in the fibre-optic cable design. These characteristics are unique to OPAC and mean that wrapped cables are designed and manufactured specifically for the application: generic fibre-optic cables cannot be used for wrapped cable installations.
The strain margin requirements tend to favour cable designs using multiple loose tubes. Tight-buffer cable designs do not provide sufficient strain margin and the optical fibre transmission performance is compromised under strong winds, heavy ice accretions and at high temperatures. In cables based on a single loose tube design, the optical fibres have too much freedom of movement along the cable axis. Consequently, mechanical energy in the form of wind-induced vibration of the host conductor causes the optical fibres to move gradually ‘downhill’ and allowing the excess fibre length to collect in the low point of the span. This uneven distribution of excess fibre length compromises the optical performance of the cable with increased optical attenuation at low temperatures in the relatively crowded sections of tube in the middle of spans and at high temperatures where the fibres are under strain at the high sections close to the towers. These problems are worse on longer spans.