Catastrophic Current Limiting in HV Supplies using resistor - Why?

[w4bfs]

Yup .... big stored energy and arc over can mean big BOOM and shrapnel ....duck and cover...beefus

[K1JJ]

Yeah, I often wondered if these caps can actually blow up like a grenade and what the damage wud be.  1750 Joules sounds big, but I have no practical reference to gauge it. How many HP = a joule or is there some other real whirl comparison analogy? 

I'm using one 70 ufd @ 10KV cap that is about 24" tall and weighs maybe 90 pounds. In parallel is an additional pair of  35 ufd @ 10kv. The 35uf caps are about half the size of the 70uf ones.  = 140uf at 10kv.


I wonder what could happen to make them explode? Perhaps a typical internal arc over failure - or maybe a direct short that caused it to overheat inside in less than a second?

I've not heard of it happening much in big BC transmitters.  Of course, these are high quality HV caps vs: electrolytic types that are always puking out their guts...

T

[WA1GFZ]

Another term for joule is Watt Seconds so 1750 watts for 1 second, 17,500 watts for 1. seconds then there is 175,000 watts for 10 ms. etc It all depends on the ESL / ESR of the cap and series reisitance. So you can quickly see how 10 ohms can help limit current.
When we do lightning testing a Waveform 5 pulse can be as high as 2000 volts at 2000 amps. Storing that kind of energy in a bank of caps will put fear into you. 

[WB2YGF]

Frank, I think you meant 17,500 watts for 0.1 seconds 

One calorie is equivalent to exactly 4.184 J.  A calorie is the amount of heat required to raise the temperature of 1 g of water at 14.5°C to 15.5°C.

My college chemistry professor used to say:  "We use calories here...Joules are too expensive."

[WA1GFZ]

yup

[K1JJ]

Interesting.

After doing some reading, I see the quantity of watts is directly related to the speed at which it is delivered. It's like the force= mass X acceleration formula. Here's a link showing some every day examples:

http://home.earthlink.net/~jimlux/energies.htm

Stated in watts, notice that 1 pound of high explosives can put out 240 gigawatts of power cuz some explode in 4 microseconds. Whereas, a cup of gasoline explodes in milliseconds, thus spreading it over time and much lower watts delivered.

A nuclear bomb explodes in 1 microsecond and goes off scale in the example... :-)

It's also interesting that a 9V battery has a total capacity of 16,000 joules.

 

T

[Steve - WB3HUZ]

Hey yo! Watts are the product of energy and time (i.e. one watt is one joule per second) not what your Dosey meter sez on your Texas Star.   

[Opcom]

Tom,
I would leave the resistor in and add the fuse. AH yes the time the cat shorted the PDM supply....Fuses blow slower than grid wires

How is the cat?

[KD6VXI]

Hey yo! Watts are the product of energy and time (i.e. one watt is one joule per second) not what your Dosey meter sez on your Texas Star.   

He sed Dosy....

Don'tcha know us big strappuhz measure in pidgeons now?

--Shane

[John K5PRO]

Used to be that a standard size capacitor for high energy research was a 3 kJ can. These were about 2 1/2 feet high, 8 inches wide, and 15-16 inches deep. I am at home, so these are from memory. They are 45 kV DC rating, and about 5 uF. Thats about 5 kJ. We put 50 in parallel. Failure mode is an internal pack shorting, as GE didn't make 45 kV individual caps, but stacked series of 10-15 kV capacitors inside the can. When they failed, (we lost quite a few in 2001) the metal can would peel back like a banana, and the can buldged. The insulator was broken off, and flames lasted for about 30 minutes. Not to mention the mess of all the oil that is squirted all over the vault.
Shrapnel for sure. We have steel capacitor rooms to contain  it.

You don't want this to happen at home!

[w1vtp]

Ken W2DTC uses a "glitch" HV fuse.

http://w2dtc.com/w2dtc-hv-power-supply/2005-0304-ps-hv-fuse.jpg

The idea sounds good to me.

Al VTP

[John K5PRO]

Photos attached of what happens when a 5 kJ capacitor ruptures...Note that the photo of the bank of caps, is only half of them. Identical wall full on the right of the camera. You can see the series wirewound resistor beside one cap. The resistor has no wire (the emperor has no clothes!). The mechanical force of the magnetic field caused the wire to bunch up and rip off the core, despite being epoxied there.

[John K5PRO]

One more capacitor rupture. This one on top - where flames lept out.

[KD6VXI]

One more capacitor rupture. This one on top - where flames lept out.

Just beautiful!  Makes me wonder why we do what we do

--Shane

[K1JJ]

Those are pretty revealing pics, John!  First I've seen of blown HV caps like that.

My biggest cap looks close to that size.

Do you know what caused the flameout?  ie, What happened in the circuit to produce that failure?  - or did the cap fail as a normal bell curve percentage of using a large quantity of caps? (random)

T

[w4bfs]

I'd vote for Lentz's law inside the caps ... rapid energy discharge from a capacitor can form large mechanical moments and internal stress ... somewhat similar to Derb's recent oriental food experience... caps used in this form of testing are generally large for their rated capacity / voltage ... I have seen large thick cables 'jump' with this ...73 ...John

[KA1ZGC]

140 uF @ 10 kV? Why so much? Do you really need all that?

[K1JJ]

140 uF @ 10 kV? Why so much? Do you really need all that?

Hi Thom,

I did have the caps lying around doing nothin...

Well, first, the voltage rating (5kv extra overhead with a 5kv supply) is always a good thang.

The capacity was needed cuz my power transformer doesn't have the greatest regulation. The core size is probably too small for the job. If I throw a full carrier, the HV sags too much when I had only 35 ufd.  But now it sags a lot less with 140uf -  but the real important thing is now I see NO fluctuation on big AM voice peaks. The dynamic regulation has improved markedly with the X4 capacitance increase.  Same thing on ssb, so less distortion.

Some day I'll get a better HV transformer, but for now this will do.

BTW, Chuck has about 200 ufd in his HV supply, using a quality Peter Dahl transformer, so I'm in good company, caw mawn.

T

[KD6VXI]

140 uF @ 10 kV? Why so much? Do you really need all that?

Yes.  Dynamic regulation is a good thing, when you can get it.

Hence the reason most don't use a doubler in AM service.


--Shane

[John K5PRO]

The caps failed about a year after their full warranty ran out. (10 years of operation, ran half time, so about 20 years old). It is a complicated situation that caused it. They are charged by a three phase rectitifier, and run 88 kV total, or 44 kV per cap, series parallel arrangement with grading resistors. The discharging current is not to zero, we have a few kV of sawtooth ripple from it. They are discharged at anywhere from 20 to 120 pulses per second. To save money, it was decided to discharge them with less pulses, but not evenly spaced out. (don't ask, too complicated to explain further). So here is a train of pulses, spaced in a stuttering rate of discharge. I measured the ripple, it was higher, and the FFT of the ripple current was much higher frequency. GE rates their caps with a certain amount of harmonics in the ripple current. We probably exceeded that with old caps. They begin to heat from dielectric loss. Eventually one pack shorted. They kept going, until the second one shorted inside, then it was boom. And all the other capacitors nearby discharged their energy into the short, as the5 ohm wirewound resistors in series with each just turned into plasma and shorted out for a moment.

 

[KA1ZGC]

140 uF @ 10 kV? Why so much? Do you really need all that?

The capacity was needed cuz my power transformer doesn't have the greatest regulation. The core size is probably too small for the job. If I throw a full carrier, the HV sags too much when I had only 35 ufd.  But now it sags a lot less with 140uf -  but the real important thing is now I see NO fluctuation on big AM voice peaks. The dynamic regulation has improved markedly with the X4 capacitance increase.  Same thing on ssb, so less distortion.

Gotcha.

I need to pull the HV supply on the Junkyard Dawg and shove in this 15 uF cap I've got laying around for the same reason. I blew (well, flashed over, anyway) a 10 uF cap ages ago and just pulled it. I think another 10 has opened up and left me with 4 uF grand total, because I'm starting to get noticable sags on modulation peaks that weren't there when I pulled the shorted 10.

FWIW, the power supply in my avatar is 22.5 uF @ 10 kV, about 7 or 8 H, and a 25 kVA pole pig to deliver 2.5 to 3 amps at about 6400 volts. You might want to try that. Sounds about right for your needs. 

BTW, Chuck has about 200 ufd in his HV supply, using a quality Peter Dahl transformer, so I'm in good company, caw mawn.

Yeah, but that's Chuck. Chuck doesn't do anything small. I delivered the tube he's currently running (or would be if he'd ever get on the air, at least on 160). I used to swing by a few times a year when I was working down there. That is quite a few pounds per square inch of rig.

I miss the days of you, him, and several other now-scarce tall ships getting on and peckermatcing. 

Eventually one pack shorted. They kept going, until the second one shorted inside, then it was boom. And all the other capacitors nearby discharged their energy into the short, as the5 ohm wirewound resistors in series with each just turned into plasma and shorted out for a moment.

Yikes! I wouldn't want to be standing within a hundred feet of that. Your ears would probably ring for a month!

Were these PCB caps, or did you have giant rooster-tails of mineral oil simultaneously boiling and burning?

--Thom
Keep Away One Zorched Grid Capacitor

[WA1GFZ]

Any of you guys get to see IEEE Spectrum  3/09 check out the 2.4 million volt Marx generator at LTI in Pittsfield. The have set it off 4 times for me.
Strap a 10 spark. There is a big cap about every 18 inches of height on the charge bank. The charge supply is 100,000 volts. The last two they took pictures for me.

[John K5PRO]

No one was in the building when our caps ruptured. So don't know if they spouted like an oil well or not. But oil was all over every inch of the room, it takes about 12 hours with 3 people or more to clean it, a very disgusting job. Ruins clothing. Not PCB oil, these are vintage 1980s GE capacitors.

[Opcom]

Photos attached of what happens when a 5 kJ capacitor ruptures...Note that the photo of the bank of caps, is only half of them. Identical wall full on the right of the camera. You can see the series wirewound resistor beside one cap. The resistor has no wire (the emperor has no clothes!). The mechanical force of the magnetic field caused the wire to bunch up and rip off the core, despite being epoxied there.

I want to work where you work.

[k4kyv]

The only time I ever blew up an oil cap was once when I dead shorted a 10 mfd 4 kv oil capacitor I was using in a 2500 volt supply, to discharge it.  Nothing big happened; it just sparked as expected.  But next time, the power supply output voltage was way low, and I finally figured out that the capacitor was completely open-circuited.  I pried the top off the case, and saw that the wire leading from the guts of the capacitor up to the terminals was small, only about #20 solid copper, spot welded to the bottom side of the terminal.  The wire had melted in two right at one of the welds.  I probably could have repaired it, but it wasn't worth the trouble , with oil all over the place and no easy way to re-seal the can.  After that, I always use a hefty resistor in series with a shorting stick.

Perhaps a good design would be to make a shorting stick with two tips, one a dead short and the other through the resistor.  You would first touch the resistor to discharge the capacitor, and then do the dead short just to make sure the cap is fully discharged.

I'd bet I could get quite a bang out of those 400 mfd 2500 volt caps I picked up.  The case is made of 1/8" steel.  The fellow who gave them to me said they were used for "energy storage" for some research project at Vanderbilt.