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Registered Member #118
Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72
Hi Harry,
I suspect that most folks in this forum tend to use various dry polymer or polymer-oil systems instead of kraft paper/oil for most TC and non-commercial HV applications... hence the lack of a response.
As you undoubtedly know, high quality, high density kraft paper, baked to remove any moisture, impregnated with oil, and maintained within oil, makes for a robust dielectric system in PFC and high energy pulse capacitors, older style power cables, and utility transformers. It remains as a valuable dielectric system even today. Kraft paper and castor oil (or other high dielectric constant synthetic oils) have been used in demanding high energy discharge capacitors for over 50 years - especially with low duty cycle oscillatory discharges. Its capability to withstand extended operation at high temperatures make it very useful for many power utility applications. The key to reliable performance is preventing any moisture from being absorbed into the kraft paper. Moisture not only reduces insulation resistance, it also promotes a variety of degradation processes that can dramatically shorten the useful life of the dielectric system.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Hi Bert,
as you point out, kraft paper and oil are a widely-used dielectric in industrial high voltage applications, and I wondered why this practice had not carried over to enthusiasts.
Not only are kraft paper and vegetable oil dielectric systems non-toxic, bio-degradable, and environmentally friendly, but have working lives measured in decades, and a very long research history as to their failure modes.
While kraft paper used in most modern designs is specially treated to improve its lifespan and dielectric properties, older iterations simply call for pure kraft paper, which is readily available in lengths and grades more friendly to the enthusiast's pocket.
Kraft paper oil capacitors are cost effective as large energy storage devices - lower energy density and lack of self-healing - and are widely used in industrial pulsed power applications, including pulse welding, Marx generators, and pulse power lasers.
Millimetre for millimetre KPO cannot compete with polymers in terms of dielectric strength and dissipation, and are not suited to use much above ELF, but who can complain when the building blocks are paper, foil, scissors, and supermarket sunflower oil?
Soggy oil HT capacitors go for a small fortune on ebay, so why not make them ourselves?
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
80 degrees! Nothing I make is meant to run for more than for a few minutes at a time, and to hit 80 degrees while in an oil bath would surely take a few tens of minutes. But for continuous operation I see your point.
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Kraft Paper and Oil
I don't want my transformer to melt @80C