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Just so it doesn't get lost..
Date: Sun, 28 Feb 2021 16:30:34 -0600
Subject: Re: [TCML] Maxwell .02uf/45kv cap
Hi Gary and all,
Bulging often occurs in fluid-filled high-voltage pulse or utility PFC
capacitors. Bulging indicates that partial discharges have previously
occurred, usually along the plate foil edges (the triple point where
foil-film and oil all meet). Partial discharges progressively chemically
"crack" the surrounding dielectric fluid and the resulting UV attacks
the polypropylene dielectric film as well, producing various gaseous
breakdown products. These form bubbles, typically consisting of mixtures
of hydrogen, acetylene (C2H2), ethylene (C2H4), and other volatile
hydrocarbons. Localized bulging implies that at least one of the series
chain of capacitor rolls has been subjected to abnormal voltages that
exceeded the Corona Inception Voltage (CIV) of that roll over a period
of time. Unfortunately, once these gas bubbles have formed, they're not
reabsorbed, The presence of these gases now further lowers the corona
inception voltage in that capacitor section. Further breakdown and gas
buildup lead to eventual failure of the capacitor, sometimes via a messy
case rupture/explosion. In practice, bulging capacitors are usually
removed from service and replaced since they are weakened and will
likely fail soon. Bulging can also occur on fluid-filled, and even
dry(!) self-healing capacitors. In this case, gas buildup is driven by
an excessive number of repetitive arc and fault clearing operations
(again, due to overvolting)..
From a hobbyist perspective, if the bulging is confined to a single
section of these older foil-film caps, it may still be possible to use
the capacitor at lower voltage stresses. This might be done by reducing
the voltage stress, including oscillatory voltage reversals, to a point
where corona no longer occurs, even in the weakened section of the cap.
and very carefully continuing to monitor for further case bulging. If
the size of the bulge is still increasing, the voltage stress must must
be further reduced. Remember that, although a rectangular welded
polypropylene Maxwell case can tolerate some overpressure, there IS a
level where it will fail...sometimes spectacularly. You may get a
leaking oily mini-mess or it may culminate in a spectacular (and very
messy) mini- explosion. One example, from coiler Joe Mastoianni, can be
found in the following link. If you look carefully, you can see the
significant bulging of topmost capacitor roll, which is likely seen in
the other rolls as well:
https://www.flickr.com/photos/iceowl/5437747654/in/photostream/<https://www.flickr.com/photos/iceowl/5437747654/in/photostream/>
With any typical pulse capacitor, history and remaining lifetime are
usually mysteries, so buyer beware. However, a bulged pulse cap tells us
that it has been overstressed at some point in its life, or that one
section is failing or has already failed. The capacitor's future life
and operating voltage capabilities are likely to be significantly lower
than a similar cap with no bulging, and the fair market price should be
lower due to the higher risk assumed by the buyer.