High voltage transmission 4

[Herb765]

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

 

[Mbrooke]

Great post! Well earned star.

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).

 

[stevenal]

Reason for corona rings at 115kV

 

[waross]

corona ring actually causes corona to get rid of it on the sharp line hardware?

The Corona Ring encircles the hardware that has sharp edges with a Faraday Shield. The outer profile of the Corona Rings have a large effective radius (Thank you for the proof reading David) to lessen or avoid corona.
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.

http://tdworld.com/overhead-transmission/corona-rings-are-they-needed[/URL]]

Since 2006, some utilities have been experiencing an increased number of polymer insulator failures on their 115-kV and 138-kV transmission lines. Investigations have shown these failures can be attributed to high electric fields (E-fields) occurring close to, or on the high-voltage end fittings of, these insulators. 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.

A 2008 report by the Electric Power Research Institute (EPRI) was conclusive in its findings that there is an issue with polymer insulator degradation on 115-kV and 138-kV lines on certain configurations and specific insulator designs.

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
--------------------
"Why not the best?"
Jimmy Carter

 

[bacon4life]

Thanks for the link stevenal. Our standards department has been considering adding corona rings to the live end our 115 kV polymer insulators to avoid discharge induced mechanical failure of the insulators.

 

[Mbrooke]

Thanks for the links and explanations thus far

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?

 

[waross]

Corona Rings.

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
--------------------
"Why not the best?"
Jimmy Carter

 

[wroggent]

Which type do you think these are? Sort of seems like neither since it's only on one end of the insulators and does not encapsulate the hardware.

(500kV and 230kV btw)

 

[magoo2]

wroggent

- 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.

 

[Mbrooke]

Thanks! So my original pick was not cornoa, but rather grading rings? Fascinating stuff, I never knew they were called different things. I always assumed grading rings were corona rings and did a little of both.


Any idea why they used grading rings on lines built in the 1930s and earlier?

 

[bacon4life]

I had assumed the loops in this picture were for lighting discharge. Seems like this setup would be better than parallel rings for flashover protection.

http://files.engineering.com/getfil...1671-41de-96d1-5b8a35df8d89&file=IMG_1770.jpg

 

[crshears]

Now I know why most of the time I just end up lurking here instead of posting responses; maybe I should change my "handle" to not.an.engineer.at.all ...

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]

 

[waross]

I agree. That looks like flashover protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Can I argue thats lightning protection?

 

[waross]

I'm starting to share your feelings, crshears.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[Herb765]

Thank you gentlemen, they are vibration dampers. I have been wondering abut these devices for 15 years. Now I found out.

Thank everyone

 

[Mbrooke]

Just for the record if any future students stumble along, I found some up-close pics of the lines in my 138kv link. Believe it or not, it also looks like there are flash over protection "horns" incorporated as well.