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Donuts > Fried Eggs
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isn't these systems old (using power lines as communication cables) i though today's method was the OPGW's (optical ground wire) for communications between stations
I know they often include fiber in the static lines now, but I don't claim to know much about it or the old line trap systems; I've never worked on either.

I think Celtic has run a bunch of OPGW, he'd probably be more help.
 

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Donuts > Fried Eggs
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17,042 Posts
So I ran across a bit about line traps the other day that explained one of their uses:

In medium-voltage generation and distribution you have a type of protection called "differential relaying." Y'all that have been heavily involved in arc-flash studies are likely familiar with it, because it's a really common and effective method of fault detection. It works on the same principal as a GFCI: It knows how much and what type of current should be flowing in different parts of a system, and it compares those parts to make sure they add up mathematically. If there's current missing or flowing the wrong direction, it knows there's a fault.

For short distances, you just place a bunch of CTs in different locations. For long distances you can't run CTs that far, so you hook them to a relay on each end, and you let the relays talk to each other. This is called a "pilot wire" setup. For short pilot wires of a couple miles or less, we either run a copper pair or used a leased BANA phone line between the relays.

But apparently in transmission this is really cost prohibitive because a transmission line runs for hundreds of miles, so what they're doing is actually using the line itself as the pilot-wire to communicate from relay-to-relay and the line-traps are how they tie that signal in. And the cost of the line-trap itself is less than it would be to run a separate copper pair all that distance.
 
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