Re: [TCML] X-ray cable best practices for feeder cable

["DC Cox" <resonance@xxxxxxxxxxxx>]

If you should elect to use X-Ray cable, I suggest putting a pair of 1" dia.
brass ball sphere gaps directly across the pole or potential transformer.
If resonances should occur, the sparking at the sphere gap will indicate it
and suggest a correction to your circuit.

>From what other members on the list have said, my case may have been an
isolated situation that occured with my 60 nF pri capacitance being
reflected
back into the pole xmfr and establishing some type of unwanted resonance.  I
remember the sparks firing entirely across a 125 kV breakdown rated busing
on the pole xmfr to the case.

Never again for me!  Now I always use 150 kV rated cable with a single 6 AWG
conductor, (never X-ray cable with a grounded outer shield), ground one
bushing, and then run the other bushing thru my protective circuit to the
coil's sparkgap.  For a ground I use some green 10 AWG cable, and, of
course, a separate 2 ought fine stranded welding cable for the high
frequency tank circuit and also to my dedicated external building ground.

I'm also a strong advocate of using a protective filter, even with a robust
pole xmfr.  Some experimenters don't go thru this trouble, but I think this
is poor
engineering practice.  A pole or PT transformer is usually an investment,
and I think a good investment is worth protection.

Jim, with your cable, just strip off the outer grounded shield, use the
cable, and then run a separate 8 AWG ground wire over to the spark gap.
This comes from the case of the xmfr to which the other HV bushing is
grounded.  Then, you don't risk any resonances.

Most power engineers are well aware of nasty unwanted resonances in their
transmission systems, and they rate second only to lightning for damages
inflicted.

Dr. Resonance




On Mon, Jan 5, 2009 at 5:27 PM, Jim Mora <wavetuner@xxxxxxxxx> wrote:

> Hello Fellow List Members,
>
> I am building two cabinets for a pig or my 7500KVA PT @ 18Kv. There are
> lots
> of choices to make in terms of the coil HV design. My 19" control rack will
> be 240V/60 and be at house ground of course. 240-280v will go out to the
> transformer cabinet isolated LV windings that will be configured as
> follows:
> H2 grounded to the cabinet and RF isolated ground. The inner turn of the
> primary and bottom of the 12" coil will be joined to the RF ground
> (something I haven't done before). The primary will be only 1" from the
> secondary; but this setup should allow for no Pri/Sec flashover as one of
> its advantages. K will be kept at around .12 .
>
> Additionally the HV supply cabinet will have  a 50 ohm 3KW resistor bank
> (BIG) resistors - 110 ohm 20"x2.5"x (2) in parallel. This will be tied to
> H1
> and go to a 4" 100 turn air coil inductor and out to the TC per Dr. R's
> experience to reduce the speed of transients to the primo PT via inductive
> reactance/impedance blumline effect.
>
>
> Here we revisit a long,old thread: I have 60 feet of primo, terminated
> 100KV
> x-ray cable. As in turns out the capacitance of the cable is ~ 50pf per
> foot. This by fate this turns out to .003uf and is resonate at very close
> to
> my TC frequency. Ok, so is that maximum power transfer or a formula for PT
> resonance rise failure?
>
> I neglected to say there will be ~19KV set horn gaps at both the PT and the
> TC cap. I would like to use a X-ray screw jack at the HV cabinet to supply
> RF ground and aforementioned HV. I like the engineered safety and balance
> of
> the cable. If the cable was say half as long, (.0015uf) I would use these
> receptacles on both ends.
>
> It is evident I should cut the cable in half and likely strip back the TC
> side for quick connection via threaded brass draw pull knobs at the TC.
>
> This is not meant to begin an old debate but to cause some calculated
> opinions on the design. After I use up my mongo old iron, I will want to go
> to a rat holed VTCC and a SS coil in the future. I guess I like the clearly
> intimidating appearance of big iron which more obviously demands respect
> ;-)
>
>
> I'll post a picture of my 3p Nikkie missile transformer which has 18"
> insulators and is absolutely a Frankenstein-like tube PS if anyone doubts
> that. This baby is DC via (6) 8020 tube rectifiers a 400H filter choke ;-)
>
> Does anyone know the blumline calaculations with a known capacitance? No
> one
> seems to speak to the impedance of X-ray cable in 60hz so that correct
> termination can be had sans all the reflected back garbage.
>
> Happy New to All,
> Jim Mora
>
>
>
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