HV Insulator Discharge 2

[Mbrooke]

Can't say I've seen this before. Then again I tend to be aloof. My question is this normal, and what would cause it?

https://www.youtube.com/watch?v=ZPe0Ucy0wMY

 

[marks1080]

I'd guess a combination of dirty insulators, very damp air and an overvoltage condition.

The camera man is pretty brave. Lots of shrapnel when an insulator stack explodes.

 

[Mbrooke]

Even if they are silicon (exploding)?

 

[qman5]

Silicon insulators will not explode causing fragments.

Just an observation: Are the shed profiles not reversed? Looks like this would cause water to accumulate and possibly contribute to the tracking.

​

 

[edison123]

Insufficient vertical spacing between the discs?

Muthu

http://www.edison.co.in

 

[waross]

I had some issues with contactor based transfer switches. There would be a voltage dip that would cause the line contactor in the transfer switches to drop out and pull back in with a "thump".
I spoke to an engineer at the power company and he gave me this explanation.
During the dry season, the wind off of the ocean would bring in salt spray and coat the 13.8 kV distribution insulators with a film of salt.
At the start of the rainy season the salt would become conductive and the insulators would flash over randomly. As the rainy season progressed, the rain would wash the insulators clean and the flash overs would stop until the next year.
Mitigation; The poco was in the slow process of relocating the distribution lines that ran near and parallel to the coast.
Loads near the coast were fed by radial lines and the last transformer on each line was located as far away from the coast as possible.
The little island utility that I worked for had 25 kV class insulators on the 13.2 kV lines.
The higher voltage insulators seemed to work well avoiding salt spray flash overs.
The issue only lasted for a week or so.
The onset of the rainy season soon washed the insulators clean.


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

 

[prc]

It is caused by dust, pollution, salty deposits. When the first rains come, conductive path is developed that results in corona and arcing. When the rains wash out the pollutants it disappears. In areas where salt is used in winter to clear snow in the roads, such arcing can be seen on insulators in substations on the side of road- effect of salt deposits.

This problem is very severe in certain regions near to sea eg. Mediterranean sea-countries like Greece. In my state in southern part of India, we never have this problem though on sea coast. But as we move north, in Konkan coast such pollutants create havoc.

In areas with high pollution level, with frequent vetting from mist, rain or fog, RTV (Room Temperature Vulcanising) silicone coating is applied over porcelain insulators to avoid flash overs from pollution. This came in to market around 1985.
Principle of RTV coating is that a thin layer of RTV material adhering to ceramic porcelain and glass insulators ,vastly increases their pollution withstand as the silicone imparts hydrophobicity to what would otherwise be a hydrophilic surface. Moreover since the silicone contains low molecular weight (LMW) chains that continually migrate to the surface, the ceramic insulator will remain hydrophobic, even when covered by a layer of pollution..
RTV coating can be applied at factory or at site and recoating will be required only once in its useful life of 30 years or more. (IEEE Std 1523-2002 Guide for application, maintenance and evaluation of RTV silicone rubber coatings for outdoor ceramic insulators, CIGRE Technical Brochures 442 &478) In such wet locations, composite polymeric insulators will lose their hydrophobicity quickly thorough repeated discharges and hence the recommendation of RTV coating over ceramic porcelain insulators. At certain locations composite insulators may not be feasible, like areas where bird- pecking is a problem. But composite silicon insulators are effective against pollution in dry regions with less humidity levels.

Close spacing between discs can also create flash overs as Edison mentioned. But chances are rare for such mistakes nowadays. When I was designing HV transformer condenser bushings in early 1970s, we thought increased creepage length esp protected creepage (protected from rain through deep corrugated profile under the shed- it was termed antifog shed ) will help in polluted areas. Hence we provided minimum flash over height and provided as many anti fog sheds as possible to get increased protected creepage.Later research by NGK Japan and Nagoya university professors showed that this is wrong as the wet pollutants in protected area created more corona and arcing. They suggested easy clean sheds ie long and short alternating sheds( to break water path) and this is now universally followed after these studies were incorporated in 1986 as IEC 60815-3 parts Insulators for polluted areas. Rules for insulator design are given in this standard. So earlier utility specifications called for more than 50 % protected creepage length for insulators of CT, bushings and LA. Today it is less than 50 % !!

 

[HamburgerHelper]

That happened in houston, dirty insulator flashover. There was a dry spell that allowed salt and pollutants to build up on the insulators. It flashed over at a refinery. The option was put out to wash down the insulators with pure water, which is a good insulator. It might have been from a hellicopter, I can't remember. Everyone took an outage to wash them down and didn't try to clean them while energized.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[Mbrooke]

@Gman5: Its possible I look at it from here...

To everyone else: Great replies. Other than the sheding being reversed, I think dirt/dust/ect is the culprit.


@Waross: Yes, I have seen stuff near the coast over insulated. 35kv on 13.8kv lines, transformers with some serious long bushings. At first I assumed it was over insulated 34.5kv, LOL.


@PRC: Do silicon insulators require any type of washing? Or is it simply less needed like porcelain?


Good video on the subject for anyone curious about silicon insulators:

https://www.youtube.com/watch?v=Oc6f8n-be3Y

 

[crshears]

In northern climes where salt is used on freeways/tollways to control ice formation, and HV lines are built along the same corridor because of NIMBY thinking, guess what happens...

My utility routinely monitors insulator contamination levels at numerous locations along such corridors, and when there has been a lack of rain for long enough in the winter and the Insulator Contamination Monitoring Protocol determines it is warranted, "power washing" of the insulators using high-pressure demineralized water takes place, either with the equipment off potential or with automatic reclosure blocked, depending on circumstances.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[prc]

Mbrooke- One of the advantages of silicon insulators is that it does not require washing in dry polluted conditions. But for wet pollution situations, I understand, RTV coating over porcelain insulators is a better option.
Crshears- In such situations, why RTV coating is not used instead of live water washing? I read some time back, in Mediterranean countries, they could avoid live washing after putting in RTV coating.
Suppliers for RTV silicone rubber insulator coatings - www. si-coat.com,

http://www.midsungroup.com,www.wacker.com.Application

of RTV coating shall be done by specialists. Cleaning of surface, surface preparation and uniform coating are some of the precautions to be taken during application.
During service, normally the RTV coating need not be cleaned. If pollution is excessive washing shall be done with low pressure water. End of life may be found out by monitoring the leakage current and indirectly via thermal ageing, best taken during periods of light surface wetting >since the goal of the coating is to limit leakage current,indirect thermal imaging gives an indication of the presence of surface currents. This can also be checked by hydrophobicity measurements conducted during routine station maintenance shutdowns.