An optical ground wire (also known as an OPGW or, in the IEEE standard, an optical fiber composite overhead ground wire) is a type of cable that is used in

overhead power line An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy across large distances. It consists of one or more uninsulated electrical cables (commonly multiples of three for three-p ...

s. Such cable combines the functions of grounding and

communications Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...

. An OPGW cable contains a tubular structure with one or more

optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...

s in it, surrounded by layers of

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...

and

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...

wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is c ...

. The OPGW cable is run between the tops of high-voltage

electricity pylon A transmission tower, also known as an electricity pylon or simply a pylon in British English and as a hydro tower in Canadian English, is a tall structure, usually a steel lattice tower, used to support an overhead power line. In electrical ...

s. The

conductive In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gener ...

part of the cable serves to bond adjacent towers to

earth ground In electrical engineering, ground or earth is a reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth. Electrical circuits may be con ...

, and shields the

high-voltage High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and Electrical conductor, conductors that carry high ...

conductors from

lightning Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...

strikes. The optical fibers within the cable can be used for high-speed transmission of data, either for the electrical utility's own purposes of protection and control of the transmission line, for the utility's own voice and data communication, or may be leased or sold to third parties to serve as a high-speed fiber interconnection between cities.G. F. Moore, (ed) ''Electric Cables Handbook (3rd Edition) '', Blackwell Publishing ,1997 , chapter 50.32 ''Composite overhead conductors'' The

itself is an insulator and is immune to power transmission line and lightning

induction Induction, Inducible or Inductive may refer to: Biology and medicine * Labor induction (birth/pregnancy) * Induction chemotherapy, in medicine * Induced stem cells, stem cells derived from somatic, reproductive, pluripotent or other cell t ...

, external

electrical noise In electronics, noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies greatly as it is produced by several different effects. In particular, noise is inherent in physics, and central to the ...

and

crosstalk In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, induc ...

. Typically OPGW cables contain

single-mode optical fiber In fiber-optic communication, a single-mode optical fiber (SMF), also known as fundamental- or mono-mode, is an optical fiber designed to carry only a single mode of light - the transverse mode. Modes are the possible solutions of the Helmholtz ...

s with low transmission loss, allowing long distance transmission at high speeds. The outer appearance of OPGW is similar to aluminium-conductor steel-reinforced cable (ACSR) usually used for shield wires.

History

An OPGW cable was patented by BICC in 1977 and installation of optical ground wires became widespread starting in the 1980s. In the peak year of 2000, around 60,000 km of OPGW was installed worldwide. Asia, especially China, has become the largest regional market for OPGW used in transmission-line construction.

Construction

Several different styles of OPGW are made. In one type, between 8 and 48 glass optical fibers are placed in a plastic tube. The tube is inserted into a stainless steel, aluminum, or aluminum-coated steel tube, with some slack length of fiber allowed to prevent strain on the glass fibers. The buffer tubes are filled with grease to protect the fiber unit from water and to protect the steel tube from corrosion, the interstices of the cable are filled with grease. The tube is stranded into the cable with aluminum, aluminum alloy or steel strands, similar to an ACSR cable. The steel strands provide strength, and the aluminum strands provide electrical conductivity. For very large fiber counts, up to 144 fibers in one cable, multiple tubes are used. In other types, an aluminum rod has several spiral grooves around the outside, in which fibers in buffer tubes are laid. The fiber unit is covered with a plastic or steel tape, and the whole surrounded with aluminum and steel strands. Individual fibers may be in "loose buffer" tubes, where the inside diameter of the tube is greater than the fiber outside diameter, or may be "tight buffered" where the plastic buffer is coated directly on to the glass. Fibers for OPGW are

single-mode A transverse mode of electromagnetic radiation is a particular electromagnetic field pattern of the radiation in the plane perpendicular (i.e., transverse) to the radiation's propagation direction. Transverse modes occur in radio waves and microwav ...

type.

Comparison with other methods

Optical fibers are used by utilities as an alternative to private point to point microwave systems, power line carrier or communication circuits on metallic cables. OPGW as a communication medium has some advantages over buried

optical fiber cable A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable, but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with ...

. Installation cost per kilometre is lower than a buried cable. Effectively, the optical circuits are protected from accidental contact by the high voltage cables below (and by the elevation of the OPGW from ground). A communications circuit carried by an overhead OPGW cable is unlikely to be damaged by excavation work, road repairs or installation of buried pipelines. Since the overall dimensions and weight of an OPGW is similar to the regular grounding wire, the towers supporting the line do not experience extra loading due to cable weight, wind and ice loads. An alternative to OPGW is use of the power cables to support a separately-installed fiber bundle. Other alternatives include fiber-bearing composite power conductors (OPCC), wrapped fibre optic cable ( SkyWrap or OPAC), or using transmission towers to support a separate All-Dielectric Self-Supporting fiber cable with no conductive elements. Evsl1

Application

A utility may install many more fibers than it needs for its internal communications both to allow for future needs and also to lease or sell to telecommunications companies. Rental fees for these "

dark fiber A dark fibre or unlit fibre is an unused optical fibre, available for use in fibre-optic communication. Dark fibre may be leased from a network service provider. Dark fibre originally referred to the potential network capacity of telecommunic ...

s" (spares) can provide a valuable source of revenue for the electrical utility. However, when rights-of-way for a transmission line have been expropriated from landowners, occasionally utilities have been restricted from such leasing agreements on the basis that the original right of way was only granted for electric power transmission. Lower-voltage distribution lines may also carry OPGW wires for bonding and communications; however, utilities may also install all-dielectric self-supporting (ADSS) cables on distribution pole lines. These cables are somewhat similar to those used for telephone and cable television distribution. While OPGW is easily installed in new construction, electrical utilities find the increased capacity of fiber to be so useful that techniques have been worked out for replacement of ground wires with OPGW on energized lines.

Live-line working In electrical engineering, live-line working, also known as hotline maintenance, is the maintenance of electrical equipment, often operating at high voltage, while the equipment is energised. Although this is more hazardous for personnel than wo ...

techniques are used to re-strand the towers with OPGW replacing the all-metal type of overhead shield wires.

Installation

Installation of OPGW requires some additional planning because it is impractical to splice an OPGW cable in mid-span; the lengths of cable purchased must be coordinated with the spans between towers to prevent waste. Where fibers must be joined between lengths, a weatherproof splice box is installed on a tower; a similar box is used to transition from the OPGW to an outside plant fiber-only cable to connect the fibers to terminal equipment.''Emerging Trends in Power Systems, Vol. 1'' Allied Publishers, pp.401-402

References

External links

*{{cite web , url=http://electronics.ihs.com/document/abstract/WZRTZAAAAAAAAAAA , title=IEEE 1138 Standard Construction of Composite Fiber Optic Overhead Ground Wire (OPGW) for Use on Electric Utility Power Lines , date=17 March 1994 , accessdate=7 August 2009 Optical fiber Fiber-optic communications Overhead power lines