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Re: remote location of NSTs - away from coil base?

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi David,
RG8U has a capacitance of 29.5pf per foot. a 40ft piece would be about 1.2nf and this seems way too small to resonate with the PIGs ballast inductance at line frequency. So I'm still not sure what is going on. Any tranmission line (RG8 coax included) will allow an incremental waveform propagating from the TC end to the PIG end to double in voltage if the termination impedance at the PIG end is high. 

Vload = Vincident + Vrefected (at the PIG)

Vreflected = Vincident * Kr (reflection coefficient)
Kr = (Zload - Zo) / (Zload + Zo) so if the termination impedance Zload = high, Kr = 1 and the incremental voltage doubles.
If Cp were fully charged to say Vmax (Vmax/2 on one bushing and -Vmax/2 on the other bushing) when the SG fires, the +Vmax/2 coax would start an incremental wave propagating toward the PIG of -Vmax/2. When this voltage doubles to -Vmax, the voltage at the bushing would go from +Vmax/2 to -Vmax/2 and would still be OK from the bushings breakdown point of view. So, I'm still not understanding the mechanism. If one could recreate the breakdown effect, one could experiment with termination to see if reflections were involved. If a resonance was really occuring with the 1.2 nf coax, it would be very helpful to know what it was resonating with and at what frequency. 

Gerry R.


Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Hi Gary,




Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
Do you have a pointer to a document explaining this affect?? I did a google on it and what I found seems to indicate a transmission line reflection affect. 

Wouldn't  "transmission line reflection effect" be capacitance based?
I don't know myself but the coax cable is abviously going to add a
capacitive effect and at a certain capacitance, it would have to achieve
resonance depending upon the circuit component parameters, wouldn't
it?

.

BTW, since Blumlein is a result of capacitive resonance on the line,


Could you describe what the coax capacitance is resonating with?? topology???


I would assume the transformer and the circuit components. I'm not
sufficiently educated in this subject to discuss theory of operation.
All I can report is that I do NOT have any huge voltage rise at the out-
put bushings of my pig by using a ~40 ft length of 75 kVDC x-ray cable to
run the high voltage output from my pig to the base of my coil. The trans-
former is a standard 15 kVA rated 14,400/240 volt double bushing,
single phase, pole mount distribution transformer and the primary ca-
pacitor is a .1 uFd (measured cap .0985 uFd), 50 kVDC Hipotronics
pulse capacitor obtained from Jeff Parisse of KVA Effects. I used the
inner conductor core of the cable for the "hot" and the outer braid as
the "ground", so the transmission line as a single cable and the 120V
for the ARSG in Romex shield as the other line -  just 2 cables from
the control panel to the base of the coil ;^).

David Rieben