A summary of the presentation made by Project Sales Corp at a Conference in Hong Kong.
Good Morning Ladies and Gentlemen
I am Satish Agrawal representing Project Sales Corporation from the beautiful port city of Visakhapatnam in India.
Project Sales Corp has been a distributor for Dow Corning for its Power and Utilities Range of Maintenance Products, and we have been selling high voltage insulator greases and coatings, fire safe transformer fluids and various potting and encapsulating materials over the last 5 years.
We have chosen to speak on the experiences of Silicone RTV Coatings for insulators.
In this presentation we would review RTV Silicone Coating experience primarily in Asian Countries and we would do these by presenting case histories of certain field applications of the product. The focus countries would be China, Taiwan, Thailand, Bhutan, Korea and India…
Before we get into that, we shall spend a little time to look into the process of contamination induced flashovers, its damaging effects, and the evolution of RTV coatings as an approach to address this problem.
If the idea is to avoid flashovers, we will need to understand how this flashover actually occurs.
The Flashover Process:
The process begins with the accumulation of contaminants over an insulator surface. The source of this contaminant could be salt particles in coastal environments, or could be industrial pollution (cement dust, fly ash, carbon dust, fertilizers dust, etc).
When this contaminants are wetted by light rain, dew or heavy fog, an electrically conductive solution is formed. When this happens, small amount of electricity leak out from the conductor and along the body of the insulator.
This Leakage currents lead to formation of a dry band and dry band arcing and eventually a thru pass or flashover that could result in an extensive outage or damage to equipments.
Any approach that is designed to address this problem should be able to intervene in the process at an early stage to prevent costly flashovers.
Why is it important to avoid flashovers ?
Because flashovers can result in outages that can result in a loss of revenue to an utility. This could be a few thousand dollars, to in some cases, over a million dollars as well.
In an industry environment it could affect the production process and result in heavy losses before restarting operations.
For service providers, it can also affect their image detrimentally.
Its very important that unscheduled outages be avoided which are a result of contamination induced flashovers.
Approaches available to Prevent Flashovers
1 of the most common method is periodical cleaning of the insulators, either based on experience or certain extreme weather events. This could be manual wiping or the use of sophisticated equipments for energized cleaning with dm water. In any case this process needs to be repeated often to effectively prevent the contaminants from combining with the moisture to form an ionisable solution that can begin the process of a flashover. Must add that energized washing also requires trained manpower.
2nd approach is to use units designed for a higher voltage class with extended creepage. This is possibly a solution only when conditions are well known before the construction of a sub-station. I would also put the increasing use of silicone composite insulators as an approach to address the problem. As this would be extensively covered by other presenters, including my colleagues from Dow Corning, in the coming sessions, we will not talk on this now.
A very effective way to prevent flashovers as a result of contamination is by modifying the surface properties of the insulator. What we do here is we use silicones in the form of greases or coatings that will impart a hydrophobicity to the surface preventing the formation of a conductive solution that could result in leakage currents, and address this problem at a very early stage.
In this presentation, we would cover more on the silicone RTV (room temperature vulcanizing; that takes moisture from the atmosphere and cures into rubber) coating that’s been developed by companies like Dow Corning Corporation in the United States.
The Beginning of RTV Silicone Coatings:
Until the 1970s, the industry would use either regular cleaning or greasing insulators as a protection measure to avoid flashovers. During 1970 to 1974, Dow Corning was working on a product that would ideally be an one time application and would virtually be maintenance free. The product was developed and extensively tested at various utilities across the United States in between 1974 and 84.
In 1984, this product after having been successfully tested was launched as Sylgard HVIC (High Voltage Insulator Coating).
In the last 2 decades, the product has been applied across power generation companies, utilities, and industrial sub-stations in voltages ranging from 11kv to 500 kv DC. And this has been done globally across the United States, Europe, Asia and the Middle East.
How does a RTV Coating Work ?
This RTV coating is a dispersion of silicone with arc resistant filler additives and low molecular weight silicone oils.
When applied over an insulator surface, either by brush for smaller application or spray, the coating forms a hydrophobic film that prevents the wetting of the insulator.
The low molecular weight silicones which are dispersed in the material move onto the surface as a result of the diffusion process. That is they migrate from the body of the coating to the surface of the coating. This molecules encapsulate the contaminants sitting over the insulator and render them hydrophobic and non-conductive.
This does two things – the particles are now passive, and two the silicone on the surface do not let the wetting of the insulator thus interrupting the process that could result in a flashover.
As a last line of defence, alumina trihydrate or ATH as is commonly referred by coating manufacturers, is incorporated into coatings formulation meant for high voltage applications. When the dry band arcing takes place, this arc suppressant additive releases water to protect the coating as well as the insulator by absorbing the heat energy.
With this we end 1st phase of the presentation.
We started with the understanding of how flashovers occur, the damages that flashovers can cause, the common approaches available to prevent flashovers, the evolution of RTV Silicone Coatings, introduction of Sylgard by Dow Corning in the year 1984, and how RTV coating actually works to interrupt the process of a flashover at an early stage.
In the coming slides, we will look into certain case histories of field application (and not application at the OE or the insulator manufacturer level, in some cases insulator manufacturer coats this material and supplies the equipment) in the Asian countries and cover as many as case histories as time would permit.
Each of this case history would go to demonstrate that RTV silicone coatings are now being looked at as a long-term protection against flashovers due to contamination from various sources
The Din-Hu Sub-station is located in LinCo, Taipei County in the North West Region of Taiwan.
The substation, which started in 1998 is situated close to the coast and as a result is exposed to saline environment, as well industrial and chemical pollution from industries near-by.
To prevent flashovers, Din-Hu started with weekly washing schedule in its first few years of operation. It then moved onto application of silicone greases in 1992, but figured that this would be a repeated exercise and was looking for a longer term maintenance free solution.
Sylgard HVIC was applied in 1993 on the 345 kV trafo bushings and continued to perform until 1998. In between, during a third year inspection schedule the bushings were washed, and it was found that hydrophobicity was still good.
In 1999, an additional layer of Sylgard of 0.25 mm (0.5 mm is the optimal thickness for initial application) was applied.
Information available from my colleagues during this visit here, the product continues to perform well, and there have been no instances of any flashover on these equipments.
Another Case Study from Taiwan – Hsinchu Industrial Park 68kv SS
This substation is located in the Hsinchu Industrial Park in Taiwan, and about 10 kms from the sea coast. The Substation serves industrial customers including large manufacturing plants.
The SS started with the application of silicone greases but due to high particulate contamination found the grease to getting saturated and this called for repetitive application.
Classic case of continuing to look for a better alternative continues. Starts energized washing weekly but experienced flashovers during washing and in between as well. Energized washing requires trained manpower, clear access for washing.
In 1993, the entire substation was coated with Sylgard HVIC, including the support and suspension insulators, bushings, etc. Took about 8 hours in each stage and it was completed in 2 stages over a week.
No flashovers since 1993.
Shanghai Extra High Voltage Power Transmission Company: 500 kV Nanqiao HVDC Converter Station.
In China, the 500 kv DC transmission line linking the central and eastern network takes power from the Gezouba Hydroelectric plant on the Yangtze river near Yichang city to Shanghai over 1000 kms away.
As is well known, HVDC systems are an economical and extremely reliable method for long distance power transmission and for linking electrical network of different frequencies.
But they also create special problems for electrical insulators which attract contamination from the atmosphere. Even though a substation may not be located in a particularly dirty environment, the DC field causes contaminated particles to become polarized. They become drawn into its area and a large portion of the surrounding pollution is attracted to the insulators. Should this contaminant become damp due to the presence of mist, fog or relatively high humidity as we mentioned earlier, the classic failure mechanism of increased leakage currents, dry band arcing and flashovers usually associated with HV AC insulators in more severe environments is likely to occur.
In the case of Gezouba- Nanqiao project, the Gezouba and Nanqiao converter stations are located at each end of the line. This was commissioned in 1989.
Light industrial and river pollution necessitated continual hand cleaning of the HVDC 500 kV wall bushings. Still, several flashovers occurred on wall bushings.
In 1991, Sylgard HVIC was applied on these equipments. After the application, from the details available with us, one flashover occurred on a wall bushing in 1997, almost 6 years after its application when the coating had lost part of its hydrophobicity.
Some of these details have been taken from a presentation made by an ABB Team on “Operating Experiences of Insulators in HVDC Converter Stations” at the NPSC Conference held at Bangalore India in December 2000. This study investigated 47 HVDC converter stations worldwide with regard to external flashovers.
Bhutan Power Station 220/132 kVSS
The Pashaka 220/132 kv Substation in Bhutan is located in a valley surrounded by mountains, carbide plants on one side and cement plant on one side.
The slurry from mud slides, carbide and cement dust created made it necessary for regular cleaning of the support insulators and isolator switches. For this the sub-station had to take a shut-down.
Take a look at the picture and you can see a thick layer of dust that’s been settled on the insulator surface.
Sylgard HVIC was applied in year 2000 and there have been no reported flashovers.
Trend towards Silicone RTV Coating
In India we have found that there is a gradual shift from regular water washing to silicone greasing and finally to Silicone RTV Coating application.
And Sylgard HVIC has proven as a cost-effective protection method.
In an internal study we have done on 132 kv SSs we found the following:
the cumulative revenue losses from outages over a period of 3 years is almost equal to the investment in coating an entire sub-station.
it also works to approx. 1% of annual revenue a sub-station. Given a life of 5 years + in extremely challenging environments, annualized costs of RTV coating application is about 0.2% of revenue.
It is more reliable method of protection vis-à-vis other methods, because it’s difficult to predict all extreme weather events, salinity levels, and establish a periodicity of cleaning; or have repeated applications of silicone greases.
Concluding Remarks
From 1984 till date over hundreds of installations across the United States, Europe, Asia and Middle East have been coated with Sylgard HVIC.
From the 1990’s Asian Utilities have also looked at Sylgard as a proven alternative to protect their equipments from flashovers.
Sylgard HVIC continues to be the most cost-effective as well as it is proven in most kinds of pollution and weather environments.
Thank you, ladies and gentlemen, for a patient hearing. I shall take questions if any during the QA session.
To obtain a .pdf file copy of this, please write to me at satish@projectsalescorp.com
To know more on Sylgard HVIC and other Dow Corning solutions for the electrical industry, please visit http://www.dowcorning.com/content/power/powercoating/default.asp or to see some application references of HVIC projects completed by Project Sales Corp in India see our blog www.sylgard.blogspot.com
Sylgard HVIC products are marketed in select parts of india by:
Project Sales Corp
28 Founta Plaza
Suryabagh
Visakhapatnam 530020
AP, India
+91891256493 phone
+918912590482 fax
www.projectsalescorp.com
