SOne thing I have seen from silicon rtv coatings is they may also deteriorate over time.  We ended up re-applying after about 10 years based on some ugly flaking and discoloration indications during visual inspection. We invited the vendor in and he told us we'd better take care of it. As I remember his preferred solution was corn-cob blast to remove the first coat before re-applying. We didn't have the time so we just sprayed over the existing stuff.

Sylgard and Powersil are examples of typical silicon rtv coatings.

In the Veracruz area of Mexico they have a pollution problem which occurs during their winter, when salt laden winds blow off the Gulf of Mexico. Severly  polluting insulators, they had many insulator flashovers during mornings of high humidity.
Spraying the insulators with SILIDRIEL a silicone based paint solved the problem. I have seen 132kv substations which have operated reliably after treatment for 5 years or so. Re-application is simply a matter of re-spraying.
They also developed a live spray and washing technique.
Contact the manufacturer via rmier@ciateq.mx ( GALADRIEL SA Cerro de las Campanas 14, Fracc. las Americas, Queretaro, Mexico .
Señor Mier will give you e.mail addresses from the Comision Federal Electricidad de Mexico to check his references.

GrahamP - Did the OEM agree with just spraying over the existing coating without removing the old?

Does the cotaing improve on the creepage.

Suggestion: There are websites that include different approaches, e.g.
http://www.levico.com/upgrading_insulator.htm
for: For insulators identified as under-dimensioned for the pollution severity of the site, the design and application of INSILSHED HTV silicon-rubber shed extenders to create profiles and creepage distances to ensure reliable, maintenance-free performance.
http://www.corocam.com/papers/3.1.pdf
http://energy.ee.unsw.edu.au/AP15/15_105E.PDF
for: Flashover forcasting
http://www.lappinsulator.com/faq/lappgmbh/tests.asp
for: How can the pollution severity on-site be determined on the silicone rubber insulator’s surface?
etc. for more info

The coating improves creepage and re-coating is sprayed over old

Thank you Graham

Seems a good group. Regarding this inquiry, there may be another alternative: resistance graded insulators.

Results of EPRI tests in "polluted" conditions strongly  suggest RG insulators may be worth considering in especially troublesome areas. Because the glaze is designed to continuously "leak" a milliamp or so, there's an evenly distributed voltage gradient across the insulator surface and the usual flashover mechanism doesn't exist. No washing or reapplications, either.

Probably more expensive, however (never priced any). I believe both Lapp and NGK offer these.
 

Thank you all for your comments and advice. I have looked into a number of avenues and am going to analyse our flashovers, looking for any trends to confirm a particular problem, and then tackle a costing exercise. Do they use similar methods to combat the carbon deposits on insulators in and around big cities, or does natural washing do the trick?

dkle40h,

Have you investigated using dry ice blasting?  There are a number of electric utilities in the U.S. that are using this system to remove contamination from energized switchgear up to 138 kV.

As the blast media is non-polluting, the environmental impact is near zero.

Finding solution for flashover could be very expensive since many people start guessing and implement none effective programs.

Cleaning and or coating certainly help but in general are a temporary remedy that varies from few moths to maybe more than a year depending mainly of overvoltage and pollution level in the area.  Besides temporarily coating such as silicon grease, there are other products that could last longer but the coating will be permanent.

In addition to the coating and cleaning, consider the following suggestions:
·    Explore is the use of conducting ring at the end of the insulators. That could help to reduce the electrical stresses in the insulator linearization the voltage distribution across the unit.
·    Investigate if your installation is experience overvoltage. Proper line surge arrester and substation arrester in the line entrance could help to mitigate any temporary power frequency overvoltage.
·    Possible in dry desert areas lightning activity may not be very high. However, double check the shielding insulation of the lines and substation and verify the substation grounding parameters as well the towers footage resistance.
·    Compare the performance characteristics of neighboring installation with the objective to determine a realistic creepage distance suitable to the site pollution level.
·    Check for animals specially birds that fly in the mornings or the end of the daylight. There is product in the market that could help to make pole and towers less attractive to animal that could pollute the insulators.
_{NOTE:
The mechanism of flashover on insulators is more critical during the drying process of the moisture on the insulator surface combined with conductive pollutant attached.
IEEE Std C57.19-100 provides some guidance to classify the line pollution level as light, medium, heavy or extra-heavy. Similarly, IEC Std. 815 classifies pollution level 1 to 4. These levels of pollution have associated normalized creep distance of 28, 35, 44 or 54 line-to-ground kV/mm respectively.
That probably could help to made a final decision whether coating/replacement partial critical sections or the entire installation}

good comments cuky.  Should it be mm/kv instead of kv/mm?

Suggestion to the previous cuky2000: There may also be considered a gradual replacement of existing insulators with better ones; especially, where insulators have to be accessed for cleaning or spraying. The better insulators include larger disks, better geometry and better materials.

Good catch Electripete. The creep distance is in mm/kV.
Dkl40h, what is the system voltage?

The enclose site might has relevant info for this post.
http://www.ims.uconn.edu/centers/eirc/15.PDF

For Voltage up to 69 kV there is creepage extender product relatively new in the market.
http://energy.tycoelectronics.com/rrg/raychem_rrg/564129.pdf

Does any one know if there is a pollution monitor device in the market with reasonable level of accuracy?

Thanks for your responses Cuky, I have found them very useful.  Our system voltage is 66kV.  

I am still of the opinion that I will have to do a thorough analysis of the faults in terms of climatic conditions at the time of flashover, which phase (left, centre, right), time of day, location, load, seasonal relationships, etc before jumping to any conclusions about a possible cause.  Thereafter the final decision will have to be made considering all available options and their associated advantages and disadvantages.