At higher voltages, electrons will be forced off of conductors. On a long high voltage line, the cumulative effect of these losses may become a significant revenue loss. That is, enough revenue loss to justify the cost of mitigation.
The loss of electrons or "Corona" loss is more severe from sharp points. A smooth "Corona Ring" creates a larger radius surface to reduce Corona. This is related to the Faraday Cage effect.
Typically corona Rings will be placed on each side of the hardware connecting the conductors or conductor bundles to the insulator string.
This effectively shields the sharp corners and edges of the bolts and other hardware used in the fastening.
Guard Rings may, in some cases, be similar in appearance and construction to Corona Rings but the function is different.
One function of Guard Rings is to avoid damage to insulators strings caused by lightning induced flashovers.
The flash over will tend to be from ring to ring rather than directly across the surface of the insulator string.
Grading Rings may be employed at lower voltages than Corona Rings as lightning strikes do not respect the operational voltage of the targets.
Or, if you prefer, from the Wiki link:
Wiki said:
Grading rings
Grading rings along linear accelerator beam tube at University of Pennsylvania in 1940.
A very similar related device, called a grading ring is also used on high voltage equipment. Grading rings are similar to corona rings, but they encircle insulators rather than conductors
Looking at the title of the Wiki article and then jumping to the illustrations is very confusing. Most of the illustrations are of Grading Rings rather than Corona Rings.
But this thread is questioning the purpose of unknown devices attached to transmission lines, so it's all good.
Bill
--------------------
"Why not the best?"
Jimmy Carter