Trafo bushing arcing horn specs?

[HandiAndi]

Looking for any info on arcing horns used on trafo MV bushings. Had too many flashovers causing damage to the bushings and now looking for a solution to prevent further/future damage to other installations. Thanx.

Making the smoke come out is easy... getting it back in... isn't!

 

[cuky2000]

Arching horns is used in disconnect switch. Please clarify

Probably you are referring to grading ring or rop-gap device. Beware that rop-gap is presently substituted by Metal Oxide Surge Arrester (MOSA). Rob-gap (arching horn) may not be effective to prevent flashover in the transformer bushing.

Flashover occurs more often by improper surge arrester selection, high level of contamination that exceeds the minimum creepage distance or combination of two phenomena.

 

[HandiAndi]

Firstly there are no surge arrestors installed on any of the transformers, roughly 30 of them outdoors, and due to the climatic conditions contamination on the bushings is a major problem. I have noticed that on some of the older trafos fitted with arching horns there are no flash marks on the bushing itself but evidence of arcing on the tips of the horns. Many bushings without the horns have distinct flash marks on them. From my findings I was under the impression that horns may help to keep the arc that is generated in a flashover away from the porcelein surface and so prevent damage to the bushing. I have tried using silicone compounds like "sylgard" on the bushings to improve BIL. This has helped in the few cases where it has been applied. But the short shelf life of the product makes it a very expensive avenue as the environmental conditions are seldom favourable for application aswell as getting the trafo out of service long enough to apply the product.
With reference to MOSA's, is the selection dependant on the spec of the trafo itself or does its location on the network play a role, as some of these trafos are moved around on the network a couple of times a year?

Making the smoke come out is easy... getting it back in... isn't!

 

[cuky2000]

What kind of transformer are installed in your system?

In the US the rod-gap is no popular for overvoltage protection mainly because of the weather dependence performance made this device unreliable. In addition, the cost of metal oxide surge arrester is affordable

Simple spark gaps are used sometimes instead of lightning arresters, because they are less expensive. [...] The simple spark gap cannot stand on its own feet, and is likely to promote outages rather than to prevent them. The attribute that has from time to time attracted users is its relatively low cost.

Here is a product that combine rod-gap and zinc-oxide surge arrester