High voltage transmission
High voltage transmission
(OP)
Will someone please explain what are the small coned shape devices on high voltage transmission lines? They are located near the insulators on the lines. I have seen them on 138 KV transmission lines and less.
Sometimes they are every tower and sometimes not, maybe every other tower.
Thanks,
Herb765
Sometimes they are every tower and sometimes not, maybe every other tower.
Thanks,
Herb765






RE: High voltage transmission
RE: High voltage transmission
RE: High voltage transmission
HamburgerHelper; Kidding aside, thanks for the pictures. Those are interesting bird-cones. I haven't seen either of those styles before.
Keith Cress
kcress - http://www.flaminsystems.com
RE: High voltage transmission
https://upload.wikimedia.org/wikipedia/commons/thu...
Edit, guess not lol. Do you have any pics?
RE: High voltage transmission
RE: High voltage transmission
Bill
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"Why not the best?"
Jimmy Carter
RE: High voltage transmission
RE: High voltage transmission
http://www.ipernity.com/doc/321461/42869048/sizes/...
This is indeed 138kv
http://www.ipernity.com/doc/321461/42869048
RE: High voltage transmission
Link
Bill
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"Why not the best?"
Jimmy Carter
RE: High voltage transmission
I guess someone working for the utility had a friend who sold corona rings. ;)
RE: High voltage transmission
RE: High voltage transmission
Link
Some of the illustrations for Corona Rings show rings on both ends of insulator strings. There is no corona whatever at the grounded end of the insulator. Those are grading Rings.
From Wiki:
Grading rings
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. Although they may also serve to suppress corona, their main purpose is to reduce the potential gradient along the insulator, preventing premature electrical breakdown.
The potential gradient (electric field) across an insulator is not uniform, but is highest at the end next to the high voltage electrode. If subjected to a high enough voltage, the insulator will break down and become conductive at that end first. Once a section of insulator at the end has electrically broken down and become conductive, the full voltage is applied across the remaining length, so the breakdown will quickly progress from the high voltage end to the other, and a flashover arc will start. Therefore, insulators can stand significantly higher voltages if the potential gradient at the high voltage end is reduced.
The grading ring surrounds the end of the insulator next to the high voltage conductor. It reduces the gradient at the end, resulting in a more even voltage gradient along the insulator, allowing a shorter, cheaper insulator to be used for a given voltage. Grading rings also reduce aging and deterioration of the insulator that can occur at the HV end due to the high electric field there.
Bill
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"Why not the best?"
Jimmy Carter
RE: High voltage transmission
https://en.wikipedia.org/wiki/Corona_ring#/media/F...
https://en.wikipedia.org/wiki/Corona_ring
RE: High voltage transmission
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:
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
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"Why not the best?"
Jimmy Carter
RE: High voltage transmission
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: High voltage transmission
That is probably the correct explanation for the original post.
However the discussion somehow expanded to include other devices that may be attached to or near high voltage insulators.
I note that the OP has not been back.
Bill
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"Why not the best?"
Jimmy Carter
RE: High voltage transmission
RE: High voltage transmission
Corona is a result of a steep electrical field gradients.
Corona is more pronounced from sharp points or edges which tend to produce higher field gradients than flat surfaces.
Corona may be reduced by reducing the effective radius of a surface.
A rectangular shape may have a very small radius corner, but the fields from each surface will interact at the corners so as to increase the effective radius of the corner.
The outside radius of a two conductor bundle will have a greater effective radius than one of the conductors by itself.
A four conductor bundle will have a still greater effective radius than the two conductor bundle or a single conductor.
Corona losses along the length of a transmission line may be reduced by using conductors of greater diameter and further reduced by using bundles rather than single conductors.
Another source of corona is the sharp edges (Small effective radius) of the corners of hardware and bolts, nuts and cotter pins.
Corona rings are generally placed on each side of the conductor ends of the insulators. The rings are made of fairly large diameter tubing And are in the shape of ovals or rectangles with semi-circular ends. The voltage field gradients develop outside the profile of the rings eliminating corona discharge from sharp edges within the space encircled by the rings.
Now with that background, let's consider the grading rings. Although the purpose of the grading ring is to reduce non-linear stresses in the insulator string, the grading ring itself should not cause corona.
To that end, the grading ring on the conductor end of the insulator should be constructed much in the same manner as a corona ring.
A guard ring would probably function as a guard ring if it were to be constructed of a ring of 1/8" x 2" flat strip bent in a circle.
Such a construction would function as a guard ring but would possibly introduce unwanted corona discharges.
How to tell the difference?
Corona Rings will be fastened in pairs at each side of the lower end of the insulator string to form a Faraday Cage around the fastening hardware.
Guard Rings will be installed around each end of the insulator. The guard ring on the lower insulator will be constructed in such a way so as to not introduce corona discharge.
With new designs the connecting hardware may be constructed with smooth surfaces and be encircled by a four conductor bundle, so corona rings may not be needed. In the old days of single conductor lines and rough and ready fastening hardware with lots of sharp edges and exposed threads, corona rings were needed at much lower voltages than today.
Bill
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"Why not the best?"
Jimmy Carter
RE: High voltage transmission
Typo.
RE: High voltage transmission
So wait, the corona ring actually causes corona to get rid of it on the sharp line hardware? But the grading ring is such that the goal is not to have corona? Can a line ever have both? Ive seen lines with corona ring appearance (oval with semi circle ends), but at the same time they are at both ends (like a grading ring).
RE: High voltage transmission
RE: High voltage transmission
Rings on insulator strings are grading rings.
Grading Rings should not cause corona. That would be replacing one unwanted effect (Non-linear voltage stresses on the insulator) with another unwanted effect (Excess corona).
A guarding ring or grading rind will generally be round. A corona ring will often be oval or oblong. Other than that, the same considerations for a corona ring such as increased effective radius and smooth surfaces apply to a grading ring.
For example and for visualization: Consider a single conductor suspended from an insulator string employing both Guard Rings and Corona Rings..
The Guard Rings will be horizontal, centred on the ends of the insulator string.
The Corona Rings will be oriented at right angles to the guard rings or vertical and one on each side of the hardware connecting the conductor to the insulator string.
I have been searching with various search engines for several days to find a good picture of Corona Rings.
I came across this which causes me to modify my original explanation somewhat.
Looking at the results of these fairly recent studies, it seems as though there are instances where the stresses on the ends of the insulators are great enough to cause corona, and the rings serve the dual purpose of both guarding and corona protection.
I apologize for my partly inaccurate information.
I do note the comment in the article:
"The findings of the investigations suggest that, contrary to conventional wisdom, it might be necessary to consider the application of corona protection on polymer insulators applied below 161 kV."
This article seems also to support Mbrooke's comment re: Corona Rings on 138 kV lines.
Never too old to learn. grin
Bill
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"Why not the best?"
Jimmy Carter
RE: High voltage transmission
RE: High voltage transmission
If anyone finds pics of the two side by side post them. I am still a tad confused here. Is it possible that in the 20s the idea was to do both corona and grading?
RE: High voltage transmission
Grading Rings.
Here is an interesting installation. Even though all three rings appear to be identical, the ring on the far left is providing grading for the insulator string.
The center and rightmost ring are providing corona protection for the connection hardware.
Bill
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"Why not the best?"
Jimmy Carter
RE: High voltage transmission
(500kV and 230kV btw)
RE: High voltage transmission
- the tower end is at ground potential. The line end of the insulator string is at line potential. That is why corona rings are just used at the line end.
RE: High voltage transmission
Any idea why they used grading rings on lines built in the 1930s and earlier?
RE: High voltage transmission
http://files.engineering.com/getfile.aspx?folder=a...
RE: High voltage transmission
I do have a clarification question, though; are the names grading ring and guarding ring just two different names for the same device, or is there a subtle distinction in there somewhere that I'm missing?
Edit/addition: I ask because some of our 230 kV sulphur hexafluoride breakers are equipped with a series stack of grading capacitors in parallel with the circuit breaker's [series?] contacts; these by logical extension would appear to serve a similar purpose as grading rings on insulators by assuring an equitable distribution of voltage drop across the various open breaker contacts, optimizing their function in keeping an off potential piece of equipment off potential. Note however that they are called grading capacitors and not guarding capacitors...
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: High voltage transmission
Bill
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"Why not the best?"
Jimmy Carter
RE: High voltage transmission
RE: High voltage transmission
A suggestion as to function;
A ring on the end of an insulator string serves the first purpose of grading or reducing voltage stress on the end of an insulator string.
Some insulator types are prone to corona discharge on the hot end of an insulator string. In this case the ring also serves to avoid corona on the end of the insulator string.
When rings are attached to both ends of an insulator string, flashovers will tend to be from ring to ring instead of across the surface of the insulator string. Now the ring is guarding or providing flashover protection.
Function"
a> Grading
b> Corona
c> Guarding
Name ???????
Bill
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"Why not the best?"
Jimmy Carter
RE: High voltage transmission
Thank everyone
RE: High voltage transmission