Earthing wire at transmision line

I believe that is because there are 2 circuits going in opposite ways on that individual corridor (towers). One earth or ground per circuit at ~137kv a side. You need the two sky wires in this case because of the opposing directions of current flow. Should some kind of a short or the lines cross (god forbid) You could get an effect similar to what occurred with the 2003 blackout on a single "neutral" line. Those high tension lines are also in delta connection so the separate earth lines are a result of that.

These are for Lightning protection and Often called "SKYWIRES"

Most of these transmission lines are in ungrounded Delta configurations and do not need a ground circuit for power flow.

Also note that often on newer systems (or re-conductored systems) many of these skywires are manufactured with fibre optic cables imbedded in the skywire core center. These fibre cables are used for communication between the stations and are a part of the protection system.

Power companies use this comunication link for remote tripping or trip blocking of the feeder breakers. The line fault relays are able to determine the direction and distance to line faults and then use a logic system with communication to trip only the required breakers for fault clearing. There is also a breaker backup system that uses these fibre links between stations for tripping the next breaker back should a breaker become stuck.

They are also used for telemetering and switching control. (there are called SCADA sytems (Supervisory control and data aquistition).

Power companies can also lease the extra fibre links to other communication services.

The primary reason is overhead shielding. The OHSW (also called skywires, ground wires, etc.) are positioned above the phase conductors to intercept lightning strokes. Double circuit construction normally requires two OHSW but two may also be required on single circuit towers (depending upon the phase conductor arrangement) to adequately shield the phase conductors. The objective is to position them such that the outermost phase conductors are within the shielding envelope, or cone, provided by the OHSW to minimize the possibility of shielding failure which results when lightning strokes bypass the OHSW and strike the phase conductors.

As others have mentioned, OHSW can also be used for other purposes.

Good Answer.. I got of the track.

Just to add no of shielding wires depends on the tower construction & the system voltage. towers of Y type configuration have conductors arranged horizontally and the clearance between the 1st and 3rd is too wide to fall in the shielding range of a single earth wire. Also with the increase in system voltage the clearances between phases increases which requires additonal earth wires. In our system for standard towers up to 220 kV single earth wires are used. If it's a Y type configuration then 2 are used. 400 kV and above have two earth wires for both single circuit and double circuits. Also for HVDC transmission +- 500 kV 2 earth wires are used.