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Repairing a severed high voltage secondary winding

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ramuna

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Hi folks,
I was winding a high voltage (20 kV 120 kHz) secondary for a push-pull transformer, using 33 swg (0.25mm diameter) copper wire when the inevitable happened - the wire broke. I've completed 41% of the winding. I would like to know if I can repair the break by scraping off the enamel along 1 cm nearest the end of the broken winding and do the same with the free end of the spool wire, then lay these scraped ends parallel to each other:
_________________
_______________________
then twist these together and solder the joint. Finally, wrap the soldered connection with plumber's PTFE threading tape a few times. I believe the breakdown voltage of a single layer of the tape, even though its very thin, is 4 kV DC. For interlayer separation I'm using a turn of kapton tape (the golden coloured stuff) on top of which I'm wrapping a turn to 1 1/2 turns of 0.5mm thick acetate film. The ends of the acetate being stuck to the winding with more kapton tape. The completed transformer will be immersed in mineral oil (medicinal paraffin, aka Nujol).

If such a repair is safe, is there a limit to how many breaks can be fixed in the same winding in this manner ?

(The red parts of the wires in the drawing above, should be aligned. For some reason, even though I put in the required spaces, the text editor refuses to keep them aligned :(!)
 
I have repaired a broken wire with Kapton. Make sure that there are no sharp points that might poke through the insulation.

You should be layer winding so there is never more that 100s of volts between any two wires. If there many volts per turn then I add a layer of tape per layer of wire.

Do you have pictures?
 
No pictures...its late at night now here, but as soon as I have daylight, I take a couple of snaps & post 'em.

BTW Thank you Ron for your superfast reply!

I estimate that the total winding will have 15 layers, ie 1.33 kV interlayer voltage.
 
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So you have about 100V/layer so depending on how you layer you could have 200 volts from one layer to another.
 
Nearly Ron ;) ! BTW I love the fact that the ETO text editor has these dinky emoticons...makes posting such a breeze ! Now, where was I...yes, the total output voltage is 20,000 Volts. Which is how I calculated the 1.33 kV interlayer voltage in post #3.

The 0.5mm thick acetate film, which I'm using as interlayer wrapping, is quite stiff. That, the additional 1 to 2 layer Kapton film wrap and the oil immersion are intended to mitigate arcing between layers of winding.

I just checked and confirmed my suspicion that the Kapton adhesive is oil-soluble. This will mean that soon after oil immersion, the only restraint keeping an interlayer wrapping in place will be the copper winding wound above it. The windings are tight enough to hold them securely though.
 
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Oh, I did not understand.
1.33kv/layer. Your wire insulation is not good for 1kv. You need tape over every layer.
We potted transformers. Any air in the transformer can cause sparks.
Have you done this before?
 
No Ron I have not. But someone else has:

https://uzzors2k.4hv.org/index.php?page=hvxfrmrwinding

I am following in Uzzors2k's footsteps. He used overhead transparencies as interlayer insulation. These are acetate films which are 4mil (ie 0.11mm) thick. [here's one of the two sites where I got the thickness from: **broken link removed** ]. His website gives the breakdown voltage for the film as 14kV. I reckon that he is overestimating by a factor of 10, based on Fig 5 here:

https://www.m-ep.co.jp/en/pdf/product/iupi_nova/physicality_05.pdf

which gives the impulse breakdown voltage of Cellulose acetate at 15 MV/metre. The 0.25mm film (I just measured the thickness. My earlier post stating thickness as 0.5mm was wrong) I'm using should therefore withstand 3.75 kV. The additional layer of 0.025mm Kapton tape I'm using has a breakdown voltage of 7kV [https://www.par.gb.com/files/0602 7010 Kapton Tape.pdf]. That adds up to 10.75kV for interlayer strength, or 8 times my application interlayer voltage. The oil immersion is to take care of ionization conduction of air trapped in the winding assembly. Granted that without vacuuming out the air before the immersion, there will still be some air bubbles trapped inside. But the oil should take out the larger voids, and it is self-healing.
 
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