I was being chauffered around Illinois this weekend, and was able to get a better look at something I've seen while driving. Now, I figured out a bit more of what I was looking at.
So, on HV transmission lines, they have 3 (or 6, if it is a double feeder) phase wires, and typically 2 shield wires on top. Generally, the shield wires are grounded at each pylon. Well, I noticed that every mile or so, there is a loop of wire mounted flat to the pole. Now, while not being the driver, I was able to see that the shield wire is interrupted and the loop is in series with the shield wire. Driving along the same feeder for a while, I was also able to see that the loops always seemed to be on the same shieled wire, they did not alternate from one wire to the other.
These loops appeared to be about 2-3 feet in diameter and consist of a number of turns, with a silvery round object in the center. It was not clear at a distance, but the wires may NOT have been insulated, they appeared to be a silver color.
So, I have been wondering what these are. Are these to add inductance to the shield wires, so as to not add a shorted turn to what is, in effect, a huge air-core transformer? But, doesn't grounding the shield wire at each pylon defeat that? Or, is the resistance of the earth so much greater than the wire resistance that it doesn't matter?
Or, is this to prevent shorting out the power line data transmission signals?