Insulators and Surface Degradation

Nothing can solve surface contamination from degrading an insulator. Allow me to make my point with a lot of hyperbole. Take any high quality high voltage transmission line insulator, this type of insulator is designed to have surface contaminations drip off of the lobes of the insulator in irregular patterns that will likely leave an irregular discontinuous film that will not complete the circuit from end to end. However, if one sputtered a layer of copper over all of the surface of the insulator then one would have a conductor instead of an insulator.

Thank you so much for that very logical explanation.

I am new to insulator field, but am tryng to cook up some very new idea and wanted confirmation for the logic - ULTIMATELY ANY INSDULATOR HOWEVER GOOD IT MAY BE does provide a surface / path for conduction due to collection of impurities. Hence the utilies have to do manitenance ad keep cleaning the surface of insulators. at regular intervals.

I learnt that now a days to cut costs, conductors are being operated at higher temperatures- where composite insulators have temperature limitations- is this true?

I do not directly work on power line distribution networks, but I often work on and around so absurdly high current devices.

Some composite insulators do have lower temperature limitations. But I suspect that most utility power line maintenance work is more focused on inspecting insulators for mechanical damages than current leakage from surface contamination.

ULTIMATELY ANY INSDULATOR HOWEVER GOOD IT MAY BE does provide a surface / path for conduction due to collection of impurities. Hence the utilies have to do manitenance ad keep cleaning the surface of insulators. at regular intervals.

Insulators for outdoor operation and overhead conductors are specially designed to be partially self-cleaning (low stick and tracking surfaces, cleaned by the rain, etc).

I learnt that now a days to cut costs, conductors are being operated at higher temperatures- where composite insulators have temperature limitations- is this true?

In some cases yes, for outdoor overhead HV cables it allows more power to be fed down the power line cables without the expense of having to replace the cables with larger ones (saving money). The problem is that the cable losses are greater and the cable joint life is reduced when you run cables hotter (which can lead to premature joint failure = very expensive repairs). A careful balance must be maintained.

Composite insulators have temperature limitations (as do glass insulators) but I didn't think overhead outdoor cable temperatures were ever that high (even when running special high temperature composite power cables designed to run at a few hundred degrees C).

Insulators vary from application to application (indoor/outdoor, voltage, location, pollution level, etc), what insulators are you specifically interested in (and at what voltage)?

At the outset, I wish to thank everyone of the responders fro very professional / constructive comments.

From the various discussions it is clear that the voltage breakdown is because of surface defects/ contaminations.

Can providing a break is surface continuity from high voltage conductor to the ground/ mounted surface provide higher / better insulation?

If YES, how to provide break in surface continuity could be looked into. Will this result in smaller insulators for the same voltage? Will it have longer life? Will it result in reduced maintenance?

I am a distributor of CoroCAM corona cameras ( www.specialcamera.com ) where I have many pictures of corona (caused by electrical field intensity breaking down the nitrogen in the air), the corona by products include a nitric acid deposit (white powder in a clean location) which may be washed off by rain every couple weeks, if in a contaminated environment, a plating action along with the acid may combine with other chemicals in the air to destroy insulator coatings and surfaces.

I also have the Corona Technology Course website www.corona-technology-course.com to sell a high voltage training course which explains corona and its effects on everything from conductors thru line insulators and bucket trucks (really long insulators). The course was assembled by a EE-PE who worked in an outdoor test lab for 15 years of his 30 year utility career and myself covering the types of test equipment use and applications.

There are instruments to determine the voltage drop across ceramic or glass insulators, NCI or composite does not have this across segments, except in the test lab.

Good Evening to you in Bangalore. I suspect that the need for reliable power is growing by leaps and bounds in the BBMP and power interruptions caused by insulator failure is not a good thing. Yes, air is the best insulator for electricity no matter how polluted the air. No matter the particulate count in the air it is still cleaner than whatever insulator that is gathering all that particulate matter from the air on to the surface of the insulator. As stated previously in this thread the contamination on the insulator builds up and provides a path to ground for the electricity to follow. Here in the US we call that "tracking" or "spill-over" and no there is not a fool proof solution to it. Your problem is even more acute due to the fact that you don't get a "rain every couple of weeks" in your part of the world to wash the insulators. Some of the US electrical companies, Southern California Edison being one, has aerial basket trucks equipped with high pressure washers that SPRAY the insulators on their transmission lines on a regularly scheduled basis. I emphasized SPRAY as opposed to HOSE as the high pressure spray is composed of a stream of water droplets separated by air that does not allow a continuous path of conductive water. BTW, pure water (and pure ice) is not conductive, you just can't find the stuff outside of a lab or a bottle. Using a stream directed from a low pressure hose would allow for a continuous stream of conductive water and cause major problems for the guys on the hose. One thing that has been tried, with some success, in the coastal areas of the Southeastern US (heavy contamination caused by salty sea air) is just over insulating the line. It costs more on the initial construction but it does improve the reliability of the line. However; over a period of time you will still have failures of your insulators. Good luck in your efforts.

I think you understand issues in India well. We do not get rains every 15 days as some people have said I understand that Railways insulators keep failing regularly and need replacement.

May be SF6 kind of breakers / insulators are good - but prohibitively expensive fro say T&D lines - for every tower !!

Stage has come that I have gathered enough knowledge now. I thank everyone for sharing information / experience.

Natural air is not the best insulator. It just happens to be the environment in which overhead transmission and distribution lines are built. All gasses have a dielectric constant of 1 but electro-negative gasses such as SF6 (sulfur hexafluoride) have much higher dielectric strengths than air.

Air is the primary insulation. The insulators that support the conductors are mechanical supports. Their function is to provide mechanical support, maintain required electrical clearances, and not reduce the dielectric strength of the surrounding air gaps under practically all service conditions (lightning is the exception).

If overhead lines were built in clean, dry, pristine environments, the lines could be insulated with wood broom sticks (providing those broom sticks had sufficient mechanical strength).

Insulator surface contamination has no effect on the performance of an insulator, regardless of whether it is porcelain, glass, or polymer as long as it remains dry. Water soluble salt contaminants only become a problem under conditions of light wetting (heavy wetting will usually clean the surfaces, except for certain heavy, crusty contaminants).

Thus, the problem of minimizing the deposition and effects of contaminants. One solution is routine washing (high or low pressure water, abrasive, soda, etc.). In the US, those procedures are defined in IEEE P957 (Guide for Cleaning Insulators) and ANSI C2 (National Electric Safety Code).

Another step is to impart or maintain insulator surface hydrophobicity (repel water). Most polymer insulators with silicone rubber weathersheds have inherent hydrophobicity with excellent contamination performance as long as that characteristic is maintained. Porcelain and glass insulator surfaces are hydroscopic but their surfaces can be made hydrophobic by coating with a silicone film or, in some cases, a heavier coating of silicone grease to trap airborne contamination particles and prevent contamination of the entire surface. It's messy and has to be routinely cleaned and replenished.

The comments regarding high conductor temperatures are valid and all insulators do have temperature limitations. However, high conductor temperatures do not necessarily cause high insulator temperatures because there are significant temperature drops across all hardware connections between the conductor and the insulator.

Insulator contamination exists, from cement plants to mining coating's which can be conductive when dry.

Another example is cooling water towers, and road salt which are typically a significant outage problem when wet as the chemicals do everything the opposite of desired on an insulator. And are a flash-over risk as the moisture source stops and random heating from the coatings causes drying of the insulator surface. This creates corona spots that help to provide stepping stones for the flash-over to occur.

Punctured insulators create excess voltage stress on other insulators in the string. The punctured insulators usually are a result of lightening or cement growth.