Re: Capacitor voltage and corona

Tesla List wrote:
> Original Poster: "Howden, Brian FOR:EX" <Brian.Howden-at-gems1.gov.bc.ca>
> At what voltage do you have to start submerging capacitors in oil to reduce
> corona / improve insulation?  It would appear that a lot of the bother with
> rolled polyethylene caps is the juggling of several layers of dielectric and
> the mess / bother / weight of submerging them.  Is there any merit to
> combining some of the MMC approaches with a group of dry, single 6 mil poly
> / Al foil caps in series?  Judging by the prices of MMC caps, this isn't
> going to be a cost saving issue as much as availability for some.  I live in
> a <350,000 pop center with no suppliers of any of the types of caps being
> referenced, and living in Canada makes mail order subject to nasty surprises
> with unpredictable charges from customs and extreme difficulty in returning
> incorrect / warranty orders.
> Brian Howden


There has been previous discussion of making "dry caps", and some
coilers currently make expendable caps for quick testing that will fail
over a given number of hours of operating time. If you connected enough
high capacitance dry caps in series so that the voltage stress across
any single cap was below that necessary to ionize air, you could (in
theory at least) make oil-less caps. However, you'd most likely need to
drop the voltage per cap to 1500 volts or less, and you'd still want to
use two layers of 6-mil poly dielectric between plates tightly rolled.
In an oil-immersed cap, the oil fills in gaps between the plates and the
poly, so a dry cap will have substantially less capacitance per area due
to the large amount entrapped air.

-- Bert --