Re: Paper/Oil Capacitors

["Bert Hickman" <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Tesla list wrote:
> 
> Original poster: "Gregory R. Hunter" <ghunter-at-accucomm-dot-net>
> 
> Thanks for all the info guys, but I'm still intrigued by paper/foil/oil
> caps.  Where can I find performance data on oiled paper, such as dielectric
> constant, puncture voltage, lossiness, etc.  There's tons of data available
> on plastics & ceramics, but very little on paper.  I know paper has it's
> characteristics, and so does oil, but when the two are combined, we have a
> different animal.
> 
> Anybody have a link I can look at on oiled paper?
> 
> Cheers,
> 
> Gregory R. Hunter

Gregory,

There are no web links that I'm aware of, but there are a few books that
have information on paper dielectric capacitors. Unfortunately, most of
these tend to be "older" books, reflecting the age of the technology,
and the information tends to be a bit scattered. I believe all are out
of print, but some may be available in the used book marketplce (ABE or
Bibliofind), or in the technical libraries of some universities. Some of
the better references include:

- Bloomquist, W.C., "Capacitors for Industry - Selection, Application,
and Economics", John Wiley & Sons, 1950

- Brotherton, M., "Capacitors; Their Use in Electronic Circuits". Van
Nostrand, 1967, 107pp

- Marbury, R. E., "Power Capacitors", McGraw-Hill, 1949,  205pp

- Longland, T., "Power Capacitor Handbook", Butterworth-Heinemann, 1984,
308pp 

The information below was gleaned from some of the above. 

Paper/Oil caps are still used extensively by the power industry for PFC
use, although more modern kraft paper/polypropylene and hazy-film
polypropylene caps are rapidly displacing them. Paper-oil caps also
found use in low repetition rate pulse discharge capacitors, where they
can provide a robust (but bulky) energy source for Marx, pulsed-power,
or resonant-charging applications.

The kraft paper used in these caps is made from high-grade wood pulp
that's been specially processed to have few conducting particles or
pinholes, low porosity, and uniform density. Because it readily absorbs
atmospheric water, it should be carefully dried before use (capacitor
makers typically used vacuum drying ovens). One pound of capacitor paper
can easily contain a couple of teaspoons of water at equillibrium prior
to drying!

Typical paper thickness is 0.3 - 0.5 mils (0.00030 - 0.0005 inches).
Multiple layers are always used to prevent single-point defects from
causing failures and to increase operating voltage. For utility PFC use,
voltage stress was limited to about 300 volts/mil on the paper
dielectric, and no more than 1000 volts/mil in a controlled DC
environment (Brotherton). Longland indicates that the dielectric
constant of mineral oil-impregnated capacitor paper ranges from about
3.0 (low-density) to 4.3 (high-density). 

Because of the comparatively higher losses, paper-oil caps are usually
used in DC, low rep-rate pulse, or 60 Hz applications. Even at 60 Hz,
paper-oil caps typically have 10-20X the loss of plastic film caps. At
higher frequencies, Marbury shows dissipation factors of betwen 0.01 and
0.02 at 100 kHz, and 0.025 - 0.03 at 1 MHz for paper-mineral oil caps,
putting this dielectric system in the same league as mylar caps... quite
LOSSY. 

Are you sure you don't want to build an MMC or polypropylene-oil cap
instead?  :^)

Hope this helps...

-- Bert --
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-com
Web Site: http://www.teslamania-dot-com