Introduction
The OPGW cable ( short for optical
ground wire or, in the IEEE standard, an optical fiber composite overhead
ground wire) is a type of cable that is used in the construction of electric
power transmission and distribution lines. Such cable combines the functions of
grounding and communications. An OPGW cable contains a tubular structure with
one or more optical fibers in it, surrounded by layers of steel and aluminum
wire. The OPGW cable is run between the tops of high-voltage electricity
pylons. The conductive part of the cable serves to bond adjacent towers to
earth ground, and shields the high-voltage conductors from lightning 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.
The optical fiber itself is an insulator
and is immune to power transmission line and lightning induction, external
electrical noise and cross-talk. Typically OPGW cables contain single-mode
optical fibers with low transmission loss, allowing long distance transmission
at high speeds. The outer appearance of OPGW is similar to ACSR cable usually
used for shield wires
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 the construction of electric power
transmission and distribution lines. Such cable combines the functions of
grounding and communications. An OPGW cable contains a tubular structure with
one or more optical fibers in it, surrounded by layers of steel and aluminum
wire. The OPGW cable is run between the tops of high-voltage electricity
pylons. The conductive part of the cable serves to bond adjacent towers to
earth ground, and shields the high-voltage conductors from lightning 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.[1]
The optical fiber itself is an insulator
and is immune to power transmission line and lightning induction, external
electrical noise and cross-talk. Typically OPGW cables contain single-mode
optical fibers with low transmission loss, allowing long distance transmission
at high speeds. The outer appearance of OPGW is similar to ACSR cable 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
world-wide. 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 ingress 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, aluminium 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
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. 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.
Application
A utility may install 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 fibers" (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 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.
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