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HV coupling caps: Good for anything?

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J. Aaron Holmes

Tue Aug 22 2006, 03:23AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

My local utility recently declared their willingness to part with a number of coupling caps. They're oil-filled porcelain-body caps of the sort usually used to couple transmission lines to small PTs. They're 16" in diameter and about 4' tall, weighing about 400lbs, and are rated at .22uF at 170kV! A friend of mine said he took one of these apart one time and that it was filled with hundreds of little green doorknob caps in strings--basically a gigantic MMC

Of course, they're not meant for pulse discharge. Not, at least, at their rated voltage. Any thoughts as to their fitness for a particular HV sport? I don't have a part number, unfortunately (trying to get that now).

Regards,
Aaron, N7OE

Avalanche

Tue Aug 22 2006, 03:42AM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

IIRC, there are two grades of ceramic caps, 1 type being more suited to pulse discharge use than the other. I tried some ceramic caps in a mini SGTC recently, and they died a few minutes later, they're too lossy :p

I guess you could use them in a (huge!) marx gen, or for smoothing the supply from a flyback for a large lifter or something... but i'm sure someone will come up with a better idea

Hazmatt_(The Underdog)

Tue Aug 22 2006, 08:18AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I would probably use them as dividers for Tesla Coil measurements. Now I say that knowing that their intended frequency is 40-400 Hz. What I would do is run a frequency sweep in simulation to give me a derating characteristic curve, and possibly build a 30kv inverter and do a live sweep of the capacitor to find out its characteristics. Then determine the correction factor, and use it as the divider.

Sulaiman

Tue Aug 22 2006, 08:54AM

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

Ceramic capacitors come in grades,
e.g.
CLASS/Tempco/D.F.PERCENT@1MHz
1/NP0/ 0.1
1/N750/0.1
1/N2200/0.2
1/N3200/0.2
1/N5250/ 0.3
2/X5T/1.5
2/X5V/1.5

N750 MEANS -750 PPM/DEGREE CELCIUS etc.
d.c. smoothing, and disk capacitors are usually classII
which is what I'd expect a hv line frequency capacitor to be.
I've used classII ceramic capacitors for a sgtc
they aren't very good,
but suitably (?) derated will survive ok.
(monitor for overheating)

Steve Conner

Tue Aug 22 2006, 09:45AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think those might be used as capacitive dividers, in which case the caps have to be precision. So they would have to be one of the better grades of ceramic (as the poor grades all have lousy tolerances and tempco's) and hence I think they might do well in RF and pulse operation. Even poor capacitors seem to work OK in Marx banks, anyway.

If it were me, I think I'd try and build it into a Marx bank inside the original body. Or make the porcelain body into a big oil filled Tesla resonator and use the guts I removed as the tank cap for it.

Or if you can get four, you could make a colossal 1600lb 4 stage Marx out of them

J. Aaron Holmes

Tue Aug 22 2006, 03:41PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

I'll get more info and maybe post a pic. Something tells me that I won't be able to afford *four* of them!

Maybe I should see if they'll keep the porcelain and let me take the guts? The porcelain is awfully cool looking, though!

BTW, I said "coupling", but really meant what Steve pointed out, which is that these are for voltage dividing and measurement. I suspect just about everybody here has seen what I'm talking about; they're in almost every substation.

I like the idea about building a Marx or something else *inside* the porcelain tank. Hmmm...

Regards,
Aaron, N7OE

Hazmatt_(The Underdog)

Tue Aug 22 2006, 11:31PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I still think using them as dividers and contributing data from a carefully controlled experiment would be worth much more to the forum then another Marx generator.

Sulaiman

Wed Aug 23 2006, 08:21AM

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

IF the internals are many small ceramic capacitors in a series/parallel mmc array
I would be tempted to put as many in series as neccessary
inside a Tesla Coil resonator from ground to topload.
This would;
A)
Allow the 'bottom' capacitor to be tapped off for an accurate measurement of topload voltage/waveform, and/or provide a voltage feedback signal
B)
Significantly lower the resonant frequency for easier electronic drive.
C)
You could drive the resonator from the bottom of the capacitor string.
Unlike driving from the bottom of the resonator,
when an arc forms, the input impedance would drop
allowing very 'hot' arcs.
AFAIK an untried/new topology.
A CFPR/Royer drive would be ideal for this.

Since there's over 3kJ worth of capacitors in there the rest could be used as an MMC
for SGTC or SSTC primary capacitor.

With over 3kJ of capacitance just about any hv project becomes viable;
Can Crusher, Railgun, nitrogen/CO2 Laser, CW/Villard multiplier, Marx etc.......

Steve Conner

Wed Aug 23 2006, 10:06AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That would be a great experiment that would give us some new data on streamer behaviour. IOW: If you add a great big toroid to your coil, how does it change the spark length, as opposed to keeping a small toroid (or even taking the toroid off altogether) and using a capacitor string to bring the capacitance up to the same as the big toroid would have.

I always wanted to try it, but never found a cheap capacitor that could take the output voltages of my coils. The mini OLTC gave about 120kV, the DRSSTC 300kV and the OLTC2 600kV.

J. Aaron Holmes

Wed Aug 23 2006, 03:24PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Well shucks, you all have me interested in the voltage measurement thing now

Trouble is, I've got neither the cash nor the coiling experience to do much of that (yet). I suppose what I really need to do is collect some of this cool junk Seattle City Light loves to throw away, buy a beat-up old F150, and drive out to Cheesehead or some other Teslathon and hand the stuff out to people who can actually do stuff with it

Regards,
Aaron, N7OE

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