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Need Advice! Insulation thickness at HF HV AC

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Patrick

Thu Oct 22 2009, 06:22AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Hey all, ive look through the forum and google the ususual web pages, for insulation thickness for plastics.
and i find the same data tables pretty much everywhere. However, how does high frequency effect insuateing effectiveness?

For example i am vacuum potting my own tranformer design, 500 volts AC square wave in, 26,000 volts out at 30-40khz.
Sooooo, 26,000 volts / 500 volts per mil = 0.052 inches of polyethylene.

Am I Crazy? or does this seem way too thin for HV, AC, HF ? Is there a recomended derating factor for frequency?

Any help is apreciated.

-PatrickC.

LutzH

Thu Oct 22 2009, 08:10AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

I know this may not be the answer that you are looking for, but nothing is better than an experimental "test", and even better several "Tests". I once had 15KV HF arc right through the silicon insulation on 40KVDC wire, more than once!

There are a few things which seem to greatly enhance the ability of HF to punch through totally unreasonable thicknesses of insulation, the main one that I have noticed is sharp edges or points, capacitance some how plays a role to in this to. Some insulators seem to hold up to HF better than others. PTFE or Teflon works great, so s polystyrene. PVC is strange because the clear product does well but the filled product sucks for me. Kapton tape is good for transformers at HF, I prefer it to the normal polyethylene, or polyester.

As far as the "Poly Sisters" go I have had more luck with polypropylene, than with polyethylene with HF, this may however just be coincidence. Some ceramics are great like Mica. I know there are some flexible Mica filled products which may be good.

If all else fails good old transformer oil always works, maybe better you can substitute some organic oils here also, just look up the threads on this subject. With oils they have to be dry, gentle heating can drive off absorbed moisture. A friend used to heat the oil to about 50C or 120F, and then use an aquarium pump bubbler to pass air through it to drive out the moisture, he claimed a 50% improvement in voltage holding with this. The new fancy sealed "snap top" food containers have simplified oil projects with a little silicon sealant for the wire holes, or use copper foil strips to bring connections into the oil filled container as the seal still works when they are there.

So in a nutshell I would consider oil, if solid potting is a must, then I would go with Kapton tape interwinding insulation. Also a good vacuum is a must.

You could try self supporting coils suspended in the liquid potting epoxy, in which the coils are allowed to harden in the right position. This idea came up in a thread before but no one has tried it :) The wire for these comes pre-coated with an insulating adhesive which hardens on gentle heating. In other words once the coil is wound, it is heated and then it can be removed from the form and it will hold its shape.

Anyway good luck, hope this helps a little.....Lutz

Patrick

Thu Oct 22 2009, 04:43PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Dam it, i was afraid some one would suggest a destructive test, OK then, that is what i will do. it will also tell me if the
methyl-methacrylate epoxy is compatible with my vinyl-acetate mag wire. Stress crazing upon contact with aromatic hydrocarbons,
has been known.(Acetone, Xylene, Touluene, Benzene etc... napthalene too.) so maybe oil inslation is more worthy of consideration.

Hmmm, a thought occurs, if i use dried vacuum pulled oil in my transformer, what about thermal expansion or head space. my main concern is a pressure explosion from arc heating.

do i need a pressure relief valve ?

thanks LutzH, will consider and post when new info/data availiable.

Dr. Dark Current

Thu Oct 22 2009, 05:35PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Remember that if you put square wave in, there can be peaks of up to 3X of your nominal output voltage, because of "ringing" on the output. (the leakage L and secondary C form a series LC circuit)

Patrick

Thu Oct 22 2009, 06:20PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Yep, yep. Drkilovolt,

I am eventually going towards 50KVAC but i start at 26KVAC to measure those transient moments and stray effects like impulse and ringing. i am currently spec-ed out at 0.150 inches of solid PVC between the ferrite core to hv out wire, i am hoping thats good for
26 KV AC 30-50Khz square wave (power is ultimatley aimed for 500 to 1000 watts out.)

what i really need are ferrites the size of marco denicolai's 100+millimeters long, but i start small first for safety and co$t.

And my transformer secondary is pie wound, to reduce inter-winding capacitence.(26 turns pri, 2300 turns sec)

TY,
-Patrick

Herr Zapp

Thu Oct 22 2009, 06:35PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Patrick C. -

You mentioned "methyl-methacrylate epoxy".

Can you provide some additional information on this material?

Is this an epoxy adhesive specifically compounded for use bonding PMMA (acrylic, plexiglass, etc)?

Or something else entirely?

Regards,
Herr Zapp

LutzH

Thu Oct 22 2009, 06:55PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

With transformer oil and other "heavy" oils explosion hazard is nil, you do however want to maintain an air space to compensate for expansion of the oil and internal parts on heating. If an air space is a problem you can for example seal in a metal bellows, or a rubber diaphram like they do in X-ray tube housings. Sorry not to push oil to much but the best thing about a liquid insulation is self-healing. So if you do get a minor breakdown the oil will fill the gap and restore the insulation, with solid insulation once a carbonized path is formed then its over and you have a permanent arc channel.

One interesting note is to insulate high voltage connectors they use silicon grease because when an arc happens the silicon grease breaks down partialy into silicon dioxide which is an insulator, instead of carbon which is a conductor like with normal grease.

One mostly overlooked method just for trivia is insulating gas, Sulfur Hexafluoride (SF6) gas at room temp and pressure has 3-4 times the insulation of air. It is non-toxic and inert under most conditions. If you sweet talk your local big electric utility they may let you fill a few ballons :) Even mixed with air at a 15-25% concentration it still insulates at 2-3 times the air values. It is used as an insulating and quenching gas in 120- 500KV switches. In particle accellerator tanks they add about 5% to boost the values up to 50% under pressure. I inquired into the cost a while back and it costs about $550 for a 40# cylinder from Conchord Specialty gasses. In the future I will have to buy a bottle because nothing else will work for what I eventualy want to do, but for now its out of the budjet. One last idea on this subject was to use an old style pressure cooker and fill it to about 45PSI with CO2, this would also give 2-3 time the insulation of air, but then how to do the feedthrough connections?

One question which I do not know the answer to is if Silicon HV potting material is better than epoxy? I would think yes because of the carbon track vs Silicon Dioxide thing? On this subject I would be willing to split say a gallon of HV potting material with someone. I know flyback HV epoxy comes in 1 and 5 gal sizes who knows maybe a few of us could even split up a 5 gallon pail to bring down the cost. The hardeware store sells new empty metal cans for paint and solvents so breaking up the order should not be to big of an issue. Then is we ship in a flat rate box we could send a gallon for $12.50 in the US. I figured this stuff would be perfect for potting our own HV rectifiers, resistors etc also.

The comments made by Dr. Kilovolt are appreciated by me also, I completely missed considering what he pointed out in my own design, there are some real sharp minds here :)

Patrick

Thu Oct 22 2009, 07:52PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Herr Zapp,

yes methyl-methacrylate is plastix bonder (epoxy) and others from henkel and loctite corporation. and others. ive use it since 1995 with good results for HV.

Crap ! i gotta go late for class. be back later to say more.

MinorityCarrier

Thu Oct 22 2009, 08:23PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

Two-part RTV Silicone potting compound is the preferred potting material for high voltage for HV military components (CRT's DVST's, TWT's etc).

Herr Zapp

Thu Oct 22 2009, 08:34PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Patrick C. -

Sorry, I'm still confused.

Methyl methacrylate (or polymethyl methacrylate, PMMA) is acrylic plastic, also known as Plexiglass, Perspex, etc.

Methyl methacrylate in not an epoxy; it's a totally different polymer family.

Are you talking about an epoxy-based adhesive for acrylic?

If you are using Henkel or Loctite products, can you provide a product name or part number?

Regards,
Herr Zapp

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