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Monday, 10 November 2014

Optical Ground Wire for Transmission System Protection

An Optical Ground Wire (OPGW) or an Optical Fiber Composite Overhead Ground Wire, as known in the IEEE standards, is a type of cable mainly used by electric utilities to facilitate the function of grounding and communications in a transmission system. These cables are run at the top of a transmission line parallel to the power conductors. The conducting part of the cable shields the power conductors from direct lightning strokes whereas the inner fiber optics are used for high speed data transmission for the purpose of protection and control of the transmission system, communication etc. These cables look the same as an ACSR conductor normally used as ground wire and have the same dimension and weight. The OPGW are suitable for high loads and longer spans. From 1985 onwards OPGW has been extensively used as ground wires in transmission lines particularly in China.  
The OPGW contains one or more optical fibers surrounded by a layer of steel and aluminum wire. A typical cable may have 8 and 48 optical fibers placed in a plastic tube which is again inserted in a tube made up of stainless steel or aluminum. Buffer tubes filled with jelly are provided to protect the fibers and steel tubes from water and corrosion respectively. The number of fibers may go up to 144 fibers in a cable. Several other types of cable construction are also available in the market which may employ aluminum rods with spiral grooves for fibers. These cables are also custom made to suit the requirement of customers. figure shows a optical fiber ground wire.
Optical Fiber Ground Wire

Optical fiber ground wire is seen as an alternative to power line carrier system used for data transfer and communications. Optical fiber cables when used as ground wire on transmission lines has lesser installation cost as compared to buried optical fiber cables. As they are provided on towers hence are unlikely to get damaged by excavation and other repair works. Since the optical fiber is an insulator it prevents the induction effect of power line and the lightning stroke, external noise and cross talks. Vibration dampers are also provided on OPGW cables to reduce Aeolian vibrations.

Optical fiber ground wires have been so successful that power utilities are replacing the existing steel or ACSR ground wires of their transmission system. In India, Sterlite Industries and several other manufacturers have the technical expertise to manufacture optical fiber composite ground wire and its related hardware. These OPGW technologies have been implemented in many Indian transmission projects commissioned by Adani Power, UPPTCL, Areva & Vedanta Aluminum, etc.