ideas on wire feedthrough sealing of oil filled HV device?

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So I decided to use mineral oil once again for my HV transformer device  
after debating and even trying paraffin.  However, this time I want to  
be sure my wires passing through the PVC junction box are sealed  
correctly so no oil leaks through.

The transformers will be inside a 6x6x6 PVC junction box (which takes up  
about 50% less volume than the prior box I used) and four connections  
will be needed:

2 low voltage 600 V wires for driving the transformers, and
2 wires for high voltage output (around 100 KVDC)

So, these four wires will need to pass through the PVC box into the oil  
and to the transformers.

In the past, I used two part epoxy to seal the solid wires on the inside  
of the box with generous application of silicone around the wire outputs  
on the outside of the box (I added the silicone layers because the  
outside wire didn't quite have enough insulation and would jump to my  
hand sometimes... a bunch of layers of silicone around the exit points  
stopped this).

Wires are all thin 20 gauge solid.  I was going to use the same  
procedure, using the epoxy to seal around the inside of the container  
where the wire feedthroughs occur with silicone layers on the outside,  
but maybe there's a better way?  As I said before, when all is dry, the  
interior will be filled once again with mineral oil.  The entire volume  
of the container will be filled with oil, so all holes with wires  
passing through will need sealed correctly.  Thanks in advance for your  
ideas.

Re: ideas on wire feedthrough sealing of oil filled HV device?
wrote:

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Oil is always a mess. Why not use a plastic potting material?



--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: ideas on wire feedthrough sealing of oil filled HV device?
snipped-for-privacy@highlandsniptechnology.com wrote in

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  Plastic?  Do you mean an RTV type?

Oil filled containment is typically a vertical canister, by design,  
and wire ingress and egress would be in and out the top 'lid', or at  
least well above the oil fill line, and on a unit never meant to be  
tipped or aggitated.  Usually found in permanent, hard mounted  
installations.

  Yet another reason RTV potting media is a far better choice  
(especially if it is a small device/package volume, and even more  
serviceable inasmuch as it should not have been potted unless it  
worked, so it should not fail after potting unless it is a bad  
design.  However, potted is easier to work on as small excavations  
can be easily back filled in all instances *except* HV sections.

Re: ideas on wire feedthrough sealing of oil filled HV device?
On Sun, 20 Oct 2019 15:04:01 +0000 (UTC),
snipped-for-privacy@decadence.org wrote:

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No, polyurethane or some such. Tap Plastics and other hobby shops have
nice clear flexy casting material.

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--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: ideas on wire feedthrough sealing of oil filled HV device?
On 10/20/19 10:50 AM, snipped-for-privacy@highlandsniptechnology.com wrote:
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What I just described I actually had for over 8 years without leaking.  
However, the cause for the small amounts of oil that finally leaked into  
a larger container just for the purpose remained a mystery, so I decided  
to first try using paraffin to repot everything after first removing the  
oil.  While paraffin worked, it shrank a lot upon cooling and I didn't  
like that plus the fact that any heating of the transformers might  
compromise any insulating qualities he paraffin provided leading to  
possible arcovers and such.  Then, I had read about additives for  
paraffin that increased its strength and melting point, such as EVA glue  
sticks.  However, I couldn't find any mixing ratios on EVA/ paraffin  
mixes, so abandoned the idea.  Budget is limited and this is a hobby  
device, so trying to keep costs minimal.  So it was back to mineral oil  
like used originally, except this time in a smaller PVC junction box  
housing and better feedthroughs into and out of the mineral oil filled  
box so that I wouldn't have leaks in the future.  Unless I read anything  
more, I will probably use epoxied solid short wire lengths as  
feedthroughs and then either use alligator clips to attach the  
transformer wires to them, or just solder everything before applying  
epoxy.

BTW, another possible DIY potting material I came across was "machinable  
wax".  There actually is a recipe well known that uses a mix of HDPE and  
paraffin.  This ends up significantly raising the wax melting point (to  
240 F) and making it machinable.  However, the big drawback is that 240  
F is needed to melt it all together and I'm sure not going to try  
potting my transformer at this temp, let alone ever trying to melt it if  
there were problems.  So... back to mineral oil.


Re: ideas on wire feedthrough sealing of oil filled HV device?
Since a picture says a thousands words, here is what I had and now what  
I want to do.  I have included captions describing what you are seeing  
with the last indicating my uncertainty about proper HV feedthrough set up:

https://imgur.com/a/15c4FKK

Comments and suggestions welcome.  Thank you.

Re: ideas on wire feedthrough sealing of oil filled HV device?
Hysol white epoxy, its ceramic loaded.
Ive used it in lab applications in excess of 45 Kv.

Steve

Re: ideas on wire feedthrough sealing of oil filled HV device?
Hysol 1c off white is ceramic loaded. I've used it in lab applications at 45 KV.

CENTER your feed wire in a pvc tube, fuse or chemically weld it to your box and fill the space around the wire with some sequential layers of 1c.

It doesnt outgas either, once drt.

Steve



Re: ideas on wire feedthrough sealing of oil filled HV device?
snipped-for-privacy@gmail.com wrote in

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  Good advice.  Make a hole and place a short PVC tube in it to keep  
distances.  Fashion and center the wire in the tube and fill with RTV  
caulk or epoxy or other FIRM type potting with good adhesion. Once  
that cures, the tube/wire assy can be placed in the breech in the  
tank and fixtured in place by whatever means (more firm epoxy).

Re: ideas on wire feedthrough sealing of oil filled HV device?
On Sunday, October 20, 2019 at 2:29:50 PM UTC-4, Jim Horton wrote:
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p:
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I still say you can mitigate the shrinkage of the paraffin by coating the d
evices in layers.  You can still fill in the entire container if you wish,  
but by coating the devices in layers each one only shrinks a bit, then more
 is added after the shrinkage.  

Or you can fill the enclosure with paraffin only an inch or so at a time an
d deal with the shrinkage that way.  Since the molten paraffin will melt th
e previous hardened paraffin somewhat, the layers should not cause any issu
es with gaps between layers.  

Oil does seem like a terrible way to do this unless your container is seale
d.  After seeing the RTV added as a layered insulator I think you should no
t try that again.  Bring the leads out the top of the container above the p
araffin line.  

--  

  Rick C.

  - Get 2,000 miles of free Supercharging
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Re: ideas on wire feedthrough sealing of oil filled HV device?
On 10/21/19 1:21 AM, Rick C wrote:

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My initial try with paraffin did work.  There was some shrinkage in the  
center, but it didn't seem to be enough to expose anything.  I had two  
concerns about using paraffin over the long term: 1) the device is going  
to be in an area with poor climate control, so temps in the winter might  
dip into the 40's, but be in the high 80's during summer so, would this  
cause cracking over time, and (2) the transformers might, over a period  
of time, generate enough heat to start melting the paraffin.  This was a  
very remote possibility, but I still addressed it.

In the end, I went with mineral oil once again, but I had a lot more  
trouble fitting the transformers into the smaller junction box.  
However, the project is now completed with the small junction box housed  
within the larger older one.  This time, for leaks, the second box will  
catch the oil plus I custom trimmed an absorbent disposable bed pad  
underneath it to help absorb any oil.  Believe it or not, I have already  
had leakage, but it is from the junction box top as that doesn't make a  
good seal and I don't think I want it to either since someone else  
pointed out how mineral oil can expand and contract quite a lot.  Come  
to think of it, I did all of this rehousing and reoiling, but I think  
the problem with the original's leakage was simply from oil expanding  
out of the top cover during hot weather.  That's the only way it would  
have ended up on the bottom of the tub in which the device was housed.  
I didn't realize this at the time and thought the leaks came from within  
the junction box somehow.  Don't think so.  I could have simply placed  
an absorbent pad underneath in that case too and probably would have  
gone on for many more years.  Live and learn.

This revamping started to become costly because I had to figure out how  
to keep the transformers stationary within the new smaller junction box,  
and yet they had to be easily removable too if the time ever came.  I  
figured it out but it wasn't cheap.

Re: ideas on wire feedthrough sealing of oil filled HV device?

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The traditional approach is simply to provide some expansion space above the  
top of the normal oil level.

If it must be hermetic, a metal bellows can be used. Or a long bit of  
vertical pipe with a porous vent plug at the top so air pressure equalizes  
with weather and thermal expansion. The maximum observed range of air  
pressure variation is 26% peak to valley.

One can make hermetic seals using epoxy as the insulator, but one must match  
the thermal coefficients of the materials used in the assembly, or it will  
disassemble itself under thermal cycling.

Joe Gwinn



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