From: Bert Hickman
Date: Sun, 24 Jun 2001 17:15:20 -0500
Subject: Re: [TSSP] Direct Voltage Measurement of Topload Voltage for TSSP
Terry and all, I concur with Paul's assessment regarding field concentration around the central conductor. The end of the central conductor should also be made hemispherical to prevent E-field concentration. Since oil and LDPE have similar dielectric constants, you may be able to increase the standoff voltage by carefully adding loosely spiraled lengths of LDPE or PP in the space between the inner and outer electrode, and letting the oil displace entrapped air. Thin layers of LDPE should have a much greater standoff voltage than the oil, and their presence should also disrupt the formation of continuous particle or bubble chains that may otherwise form under high E-fields in the oil alone. The advantage of pure mineral oil is that it would be self healing. However, its dielectric breakdown strength is only about 1/4 that of thin sheets of LDPE or PP. Mylar or PVC should not be used because the higher k of these materials may overstress the oil between layers. Also, I assume that the outer container is made from a nonconducting tank - I've found (the hard way) that polypropylene does NOT maintain satisfactory mechanical strength when used as a container for mineral oil - does Marc have more of those fiberglass tanks? -- Bert -- -- Bert Hickman Stoneridge Engineering Email: bert.hickman@aquila.net Web Site: http://www.teslamania.com Terrell W. Fritz wrote: > > Hi All, > > I sketched up some ideas for a 500kV capacitive voltage divider at: > > http://hot-streamer.com/temp/HighVoltageProbe.gif > > The top diagram shows the circuit. The 5pF and 250,000pF (oops, says > 50,000pF :-)) capacitors form a 50,000:1 divider. This takes a 500kV input > and divides it to 10Volts. That signal is buffered and gain adjusted to a > nice 100000:1 output signal that can be feed to a fiber optic coupling > system. The voltage could actually be taken off the lower cap and feed > directly to a scope with two 50ohm matching resistor and 50 ohm coax if one > was "feeling lucky". That is how most commercial dividers do it. > > The 5pF cap can be a coaxial capacitor made from a 12 inch sphere with an > ~12 inch long 1/4 inch rod protruding from the bottom. The rod leads > inside an oil bath. There is a 5 inch diameter "can" inside the bath into > which the rod goes in about 5 inches. This basically forms a 5pF cap that > can stand off 500kV (hope hope!). There is another grounded can around the > first which basically acts as a shield for the inside can and adds some to > the divider capacitance. All the parts are oil insulated and it would > stand about 30-36 inches high and ~12 inches in diameter. It would need > 6-10 gallons of oil. The top sphere maybe could be smaller for less > capacitive load. There would also need to be a "cable" from the coil to > this probe that would add capacitance and maybe add other problems? There > really aren't probes like this around. The commercial ones are > high-frequency but to catch "spikes" rather than to continually run. The > "real" high frequency probes don't go this high in voltage... I bet there > are reasons for that :-)) > > One problem may be that the amplifiers can't take a 20V p-p signal given > their unity gain bandwidth and all. However, that is not hard to fix. I > could easily add the direct 50 ohm stuff for safer lower power tests. > > This is just my quick sketch of the thing. If anyone has suggestions or > sees any flaws in my thinking here, I am all ears. The commercial dividers > use gas (probably SF6) but that is hard to get. The fiber optics add a lot > of complexity but until one is sure it works or one can develop really good > proven protection circuits, the fiber optics provide absolute protection no > matter what. I expect this stuff as full of "problems" :-)) > > Cheers, > > Terry
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.