Does stacking mylar dielectric film reduce it's breakdown voltage?

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Summary:

Does stacking mylar dielectric film reduce it's breakdown voltage?
According to the attached image, the dielectric strength of Mylar film decreases with increasing film thickness. My question is if this trend is also present for when films are stacked or if the film can be stacked arbitrarily high with the same breakdown voltage as a single layer.
 

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  • #2
Baluncore
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There is an assumption that the surfaces of the insulation are equipotential surfaces. When the insulation is doubled in thickness there is no equipotential conductor in the middle to balance the voltage gradient through the material. Natural variation in local conductivity will result in higher voltage gradients than expected, so the breakdown voltage of the thicker film will be less than expected.

mil V/mil = Vbreakdown
0.25 * 18,000. = 4,500.
0.5 * 10,200. = 5,100.
1.0 * 7,400. = 7,400.
2.0 * 5,200. = 10,400.
4.0 * 3,700. = 14,800.
6.0 * 3,000. = 18,000.
8.0 * 2,700. = 21,600
10. * 2,300. = 23,000.
14. * 2,000. = 28,000.

Oil saturated paper used for HV insulation will have layers of foil, called chroming, between the layers of paper. Natural leakage provides the voltage divider that controls the voltage gradient. Mylar has less leakage than paper so chroming will not provide the same protection. Mylar would need an external voltage divider connected to the equipotential foils to maintain the dielectric strength of the thin mylar film.
 
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  • #3
DaveE
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Any void in an insulation system creates a reliability problem, particularly if there is air (esp. O2) present. These molecules can break down or create undesirable chemicals that can attack the insulation, especially in the presence of strong electric fields. For example, you could research corona discharge, which can make ozone. These breakdown processes are typically a positive feedback sort of thing; once it goes bad it gets worse more quickly. For this reason layers are problematic. Techniques to eliminate voids, like assembly in a vacuum often don't work with plastic films because air can be trapped.

The end goal in a good system is a uniform "block" of insulator, typically a plastic of some sort. The best systems use permeable fabric (like cotton, fiberglass, nomex, etc.) which is then vacuum impregnated with an epoxy based insulator that is applied as a liquid under vacuum, then the vacuum is removed so the air pressure will force the liquid into the voids, after this the plastic hardens to form a block, ideally with no voids.

In large components like distribution transformers you can use liquids (oils like PCBs and flourinated compounds). These have mostly fallen out of favor for environmental and failure effects reasons. But I believe it is the only way to make very high voltage and high power transformers. Others will know better than I about these, I only did small stuff like lasers and satellites.

Sometimes film insulators are applied in layers as you describe for redundancy. The idea being that in the film manufacture, or in handling, the film may have holes. With multiple layers the idea is the holes won't line up. Many safety standards favor redundant systems. However, these aren't necessarily the most reliable.

I recall the old rule of thumb for insulators was that any good insulator could handle 500V/mil and not much more, and it didn't really matter which one. Of course that isn't true, but it does convey the need for lots of margin in practice compared to the results of material property testing.
There can be a lot of engineering in these insulation systems, although once you have a system you like you can copy it in new applications. The key factor isn't does the prototype work; it's really all about reliability and lifetime.
 
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