Creepage Distance for 500kv Post Insulators

This is interesting to me because I've always wondered why high-tension lines don't just go to ground during rainstorms!!!

If overhead power lines and substations could be built in clean, dry, pristine environments, the insulators would not need the petticoats (or weathersheds) that provide creepage. In fact, the conductors could be insulated with wooden broomsticks; provided the broomsticks had sufficient mechanical strength to support the conductors and maintain the required phase to ground and phase to phase electrical clearances.

As previously noted, the primary insulation is air and insulator creepage or leakage distance is only important for insulator performance under wet and contaminated conditions. Under all dry conditions, insulator strike distance (shortest distance between line and ground over the surface of the insulator) determines the insulator electrical characteristics. (Strike distance can be defined as the shortest "tight string" distance between the energized and grounded metal ends).

A good indication of the insulator performance under contamination conditions is the ratio of creep to strike because the creep is required only to maintain the electrical characteristics provided by the strike under wet and contamination conditions.

You do need to consider both air and surface creep to avoid arc-over.

My company produces rugged insulators for the transportation industry among others. I will forward the question to one of my product managers.

There are some very simple "rules of thumb" that you can use to base your engineering. You will need to tweek your actual results to match your end use, but thats why you engineers get paid the big bucks!

So is the ribbing on insulators there to increase the distance?

Still, it would seem to me that during a heavy rainstorm, an uninsulated wire carrying multi-kilovolts would simply arc down to the metal tower and head straight for ground. Much like the pictures you see of very high-voltage Tesla coils leaping sparks all over the place! I mean, it's AC, it's 500 kV, and God knows how many Amps!!!

I R confused.

No reason to be confused.

The "ribbing" on insulators is there to provide the required creepage distance, break up the water stream to prevent cascading water, and provide protected dry creepage distance in rainfall (the underside of the "ribs" and the adjacent inner surface).

There is no flashover from the bare conductor to the steel tower because of the phase to ground clearance (air gap) that is maintained by the insulator string. Transmission lines are designed to prevent phase to ground flashovers under all normal and abnormal system voltages and overvoltages except lightning and, if properly shielded or otherwise protected, lightning flashovers will generally occur only in case of shielding failure (when the lightning stroke bypasses the overhead shield wire [OHSW] and terminates on the phase conductor). That's the purpose of the shield or sky wire that one normally sees located above the towers and phase conductors. That wire is normally solidly grounded but may be insulated from the steel towers by special ground wire insulators, if the OHSW is also used for communications.

Bluestone will also probably know about some insulators that instead of ribbing rings had a corkscrew rib. I recently saw one and I think it was going to be used, hopefully indoors or in the desert.

Dan

I'm familiar with the corkscrew (helical) ribbed insulators (screwalators) as well as many other weathershed shapes and features that have been proposed or tried over the years in attempts to make pintype, multi-part, suspension, post and so-called longrod insulator shapes more efficient. Bet you've never seen oil bath insulators. You may be too young to remember oil bath air cleaners that were used years ago on automobile engines to trap contaminants but the same principle was applied (for a short time) to some fog and suspension insulators in an attempt to improve contamination performance. The insulators were shaped to include an open cavity or chamber that could be filled with mineral oil when the insulators were installed in service. That idea didn't work too well for lots of reasons.

Oil bath air cleaners, my folks restore Model A & T Fords, but the modified (22" shorter wheelbase) 1935 Ford 2-door had a oil bath air cleaner, I put an alternator on it and rewired it for 12 volt. My Dad is still restoring Model A bodies I sand-blasted in the 1960's, as well as trophy winners he has.

I did see what looked like a brand new "screwalator" within the past couple months.

On the bottom of the "links" page I have some insulator collectors, etc. and would appreciate and will add any additional links you may know of.

www.corona-technology-course.com

Yes, thank you for noticing my youth and bringing it to everyones attention.

Your link is one of those "small world" things. I knew Jim Booker very well. We worked on many projects together over the years, dating back to his AEP days. I am a EE who retired after 44 years with the company that held the original patents on the modern, one-piece, cap and pin porcelain suspension insulator (which was developed for the 110 kV Niagara Falls-Toronto transmission line in 1909). That design remains essentially unchanged today.

I have a modest collection of significant insulators and associated items; including a couple of strings of early Hewlett's (porcelain discs chained together), a couple from the 287 kV Boulder Dam line, and a few other interesting ones. I'll probably donate the stuff I have to a museum at some point.

I figured you might know Jim. I would write more but not here.

Do you have some pictures (insulators / linemen at work / ? ) you might want me to put up on the Corona website? I can label pictures with your copyright info (for what it is worth on the internet).

From my website you have my email address cameras@maqs.net

It's good to know that there are some insulators that people didn't change just for the sake of change.

I was first across Boulder Dam in 1968 at 9pm on the way to Vegas and never since, Jim and I at the Phoenix corona course had a student from the Boulder Dam site, I was also to Grand Coulee on a camera sales trip, knew the A-C reps who did the expansion there. I started hydro's for Allis-Chalmers in the first half of the 80's and then to Hydra-Co Enterprises a NIMO sub to build cogeneration plants in 1985 and by '89 when they had 1,000MW and were hiring 100 people into plant operations where I started and then I went back to WI to develop hydros and start my own sales rep company, where I am nearly 20 years later selling corona and SF6 gas leak cameras across North America, I leave next week until probably February, into Canada and then down the west coast to Phoenix doing camera demo's.

Dear Mr.inayathussain

Pl.open the link below and you will find the answer.

http://electricalnotes.wordpress.com/2011/04/06/electrical-clearance-in-substation/

• Electrical Q&A Part-1
• Electrical Q&A Part-2
• Electrical Q&A Part-3
• Electrical Q&A Part-4

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