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[TCML] Bert's post on the Capacitor

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.

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