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Internal breakdown of a charged high-voltage capacitor

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LAN_403

Physikfan

Tue Mar 21 2017, 07:38PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi all high voltage friends

I have the following question:

Please, who in this forum has already experienced an internal electrical breakdown of a charged high-voltage capacitor?
For example, if 3000 Ws of energy are released inside the capacitor in a short moment?

I have some experiments in planning and would like to benefit from your experience.

Thank you for your comments.

Physikfan

klugesmith

Wed Mar 22 2017, 01:30AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Not me. Got one 52 uF 20 kV capacitor, which I have never charged beyond 6 kV (9% of rated energy). It's been gathering dust for about 10 years. I fear it may have lost a small part of its capacitance, or gained a little ESR, from high-current insults in the last days before it was set aside.

I bet the internal-fault behavior would depend a lot on your capacitor's construction. Metallized-film capacitors are designed around the self-healing property of their plates. When the dielectric film suffers a breakdown at one point, the conductive metal film electrode burns away at that spot, acting as a fuse to isolate the bad-dielectric spot. Capacitors designed for very high current pulses have metal foil plates. But they are rated for a certain number of shots, under certain conditions, with the end of life being a certain departure from the original capacitance.

[edit] Just found this paper about failure mechanisms in pulse-power capacitors, as well as power-factor correction capacitors.

[\edit]

Suppose your big capacitor has a volume of 10 liters, and that its materials have an average specific heat density (rho * Cp) value of around 2e6 J / m^3 K. See

. Then its average temperature change from dissipating 3000 watt-seconds will be 0.15 K.

Not to dismiss the hazard from something that might yield 3000 J of kinetic energy, as in your falling-10kg-weight example in another thread. Around 1992, I used to launch a 10 kg weight to heights of 15 or 20 m (1500 or 2000 J). Too much energy to be safe in a suburban neighborhood. The apparatus was a mortar, or cold gas gun, made for measuring the energy converted when 2-liter soda bottles were pressurized to the bursting point. I still have the old analog pressure gauge, with its pointer extended to go with a 20" diameter dial.

We've seen pictures on this forum of at least one rectangular HV capacitor whose sheet metal casing has ruptured. I think it had been part of a bank of similar capacitors. Maybe the weak one received all the energy from its partners.

Finn Hammer

Wed Mar 22 2017, 07:03AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

I seem to recall having seen pictures, some 10-12 years ago, posted on the Pupman list, of a big Maxwell cap. in the 20kv 30uF range, that failed due to dielectric breakdown.
The poster ran by the nicname "The Arcstarter".

Cheers, Finn Hammer

Mads Barnkob

Wed Mar 22 2017, 07:50AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I can not find it right now, but I remember there was a guy that built a PIO capacitor bank in a acryllic casing on wheels, one of the capacitors ruptured through the side.

Kizmo had something as small as microwave oven capacitor explode at very high voltage, he described it as a hand grenade, shrapnel in the ceiling, burning oil everywhere etc.

Sulaiman

Wed Mar 22 2017, 10:15AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have seen small (few nF) home-rolled capacitors (for one of my early sgtc) arc over internally,
the stored energy was small so most of the damage was done by the eht power supply.

3000 Ws (3 kJ) is about the energy of a 7.62mm rifle round, inside your capacitor !
I have seen the after-effects of such a capacitor explosion when I worked on railway train maintenance, from one (large) polypropylene capacitor ... bent steel and sheered bolts !
High voltage caps can release all of their energy in an instant, hence the destructiveness.

Flachzange

Fri May 19 2017, 04:37PM

Registered Member #61569 Joined: Sat Apr 15 2017, 05:12PM
Location: Germany
Posts: 28

Hi,
i work on a daly basis for the high voltage metrology. My main area is impulse voltage and DC voltage calibrations. We perform up to 1500 kV lightning impulse voltage calibrations with a Marx generator. It has an impulse energy of 60 kJ at the maximum voltage von 2000 kV. One day when i measured one of our own dividers at 1200 kV i heard a sound that i was not familiar with. It was some sort of a "blobb" sound. I have inspected all the 40 capacitors (each 1,2 ÂµF at 50kV) but nothing was wrong with them.
So we continued with the measurements. About all 10 impulses we heard the sound again. At almost the end of the day however the capacitor exploded and a big fireball was visible.
The side of the capacitor ripped apart where the oil, paper and metal foil was seeping out. It was a hell of a mess and of course it happened in about 7m height and all the mess dripped down the impulse generator.
So this was a good reason to clean the entire generator

During this explosion the capacitor was not connected directly to the others, only by the high omic charging resistors, so only the capacitors internal charge lead to this damage.

Bert

Fri Jun 16 2017, 04:20PM

Registered Member #118 Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72

Over the last 18 years, I've had three large metal-cased capacitors catastrophically fail while being partially charged. One failure resulted in the case rupturing along a weld (a GE 54uF @ 15 kV). However, this was a DC filter capacitor, not an energy discharge cap. so its construction was not as robust as a true energy discharge capacitor.

The cases on the other two caps (Maxwell 70uF@12 kV true energy discharge) held with no leakage. These caps used considerably thicker sheet steel than the GE cap. As I recall, cases for Maxwell type C energy discharge caps are designed to contain the energy released during a fully-charged internal fault PLUS back-feeding from at least one other identical capacitor. When each Maxwell failed, the total bank energy was about 4.5 kJ. These failed because the dielectric fluid (castor oil) partially "froze" during an extended cold wave - my fault for not providing heaters to keep them warmer.

You may want to check with the capacitor engineering folks for capacitor(s) you plan to use to confirm how well they handle a catastrophic internal fault, especially if you're using them in a multi-capacitor bank.

macona

Mon Jun 19 2017, 02:58AM

Registered Member #3272 Joined: Mon Oct 04 2010, 11:40PM
Location: Beaverton, OR
Posts: 101

Many, many years ago there was a group buy in one of the tesla coil list servs for custom made caps for tesla coils made by Plastic Capacitors. Huge thing, about 4" in diameter and a foot long with 1/2" brass studs sticking out each side. Never got a chance to use it myself, lent it to a friend for their coil and something happened internally and the thing exploded throwing oil everywhere.

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