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THE PROCESS
Dry it, then drown it
Watch an assembled capacitor go through vacuum drying and oil filling.
Impregnation replaces every pocket of air with oil, the single biggest defence against partial discharge.
WHY AIR IS THE ENEMY
Voids light up first
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Lower permittivity
Air has ฮตr = 1, lower than film or oil, so the field crowds into any void
Lower strength
Air breaks down at a far lower field than oil or film
Partial discharge
Tiny sparks inside the void, every cycle or every shot
Erosion
The sparks slowly eat the film until it punctures
A void is a small spark gap built into your dielectric.
DIELECTRIC FLUIDS
Which oil?
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Mineral oil
The classic, from transformers and older capacitors
Castor oil
A natural oil long used with paper in pulse capacitors
Silicone oil
Stable over a wide temperature range
Synthetic aromatics
Engineered fluids that can absorb gas produced by discharges
Processing
Every oil is filtered, degassed and dried before it goes in
The oil is part of the dielectric, so it gets the same care as the film.
THE DRY ALTERNATIVE
Potting instead of oil
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Dry types
Some parts are filled with polyurethane or epoxy instead of oil
Upside
No oil to leak, and often lighter
Watch for
Voids and incomplete cure in the potting
Common for
Many metallized parts, such as defibrillator, DC link and power-factor capacitors
Oil and dry potting solve the same problem, filling the gaps, with different risks.
PROCESS CONTROLS
What gets watched
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Vacuum level
Deep enough, held long enough, to pull water out of film and paper
Temperature and time
Heat speeds drying; schedules run from hours to days
Oil moisture
Measured in parts per million
Fill under vacuum
Oil enters with no air to trap, then soaks into every layer
Seal
The fill port is soldered, welded or plugged, with room left for the oil to expand
Drying and impregnation are slow, invisible steps, and they decide whether a part passes partial-discharge testing.
UNIT 9 STUDY COMPLETE
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Ready for the Fab Challenge?
You've seen why air and water must come out, which fluids go in, and what a good impregnation process controls.