Re: Unexplained arcing current

To: tesla-at-pupman-dot-com
Subject: Re: Unexplained arcing current
From: "B**2" <bensonbd-at-erols-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Mon, 31 May 1999 18:39:04 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme
Hi Gary, All,
    1.  Placing a resistor between the bypass capacitor and the
transformer secondary winding decreases the Q of the RLC circuit
formed by the secondary winding inductance, resistance, and the
capacitance added by the bypass capacitor.  It also absorbs the energy
from the parasitic capacitance of the HV winding as it attempts to
discharge through the spark gap

  "Through simulation, with reasonable component and parasitic values,
I see no evidence of NST secondary oscillations at the gap closures.
Can you suggest a circuit that may show this?"

    2.  Use a current probe and an analog scope.

"the amount of energy dissipated by the NST
secondary capacitance (small compared to the bypass cap values) is
insignificant, not enough to carbonize the tar."

3.  My previous measurements indicated around 2 nF for a 15Kv, 60 mA
transformer from one bushing to ground.  I just now measured 700 pF
from a 12 kV, 30 mA France transformer from one bushing to ground
(This transformer has been unpotted, ultrasonically cleaned, coated
with red glyptal, and reassembled for 12 kV, 50 mA service).
That is a lot of energy wasted and a good argument for a DCTC!
    I have dissected several neons.  In one case (Jefferson 15 kV, 60
mA) the short was between the fine wire connecting the secondary to
the bushing and the metal
transformer casing.  In the other (France 12 kV, 30 mA) the short was
between the same wire and the transformer laminations.  Neither
transformer had a field grading bushing at its output.  In time, with
any kind of abuse,  it will streamer through the tar and short due to
the high field enhancement from the wire.

"There is no doubt that a resonant voltage rise occurs when caps are
added
in parallel to the NST secondary.  This is only evidant however if the
circuit is allowed to ring over multiple half-cycles, something that
cannot happen with a reasonable static gap, and has no bearing on
parasitic oscillations or power dissipated in the resistors."

4.  A static gap fires several times during each half of the sine
wave.  When it does you get the energy stored in the bypass and
secondary parasitic capacitances dumped into the gap through the
resistor.  The two capacitances are charged and discharged many times
during the 4 ms period of the 60 Hz half wave, depending upon how wide
your gap setting is.   I measured ringing current spikes almost twice
as great as the 60 Hz current waveform off the 12 kV, 50 mA France
this morning.

"I'm not sure I see the difference, as far as what current pulses the
NST secondary sees,  between an R-C-R network, and an 2*R-C network.
With
the RCR network, the parasitic capacitance of the secondary is
discharged
by a resistance of R+R through the spark gap.  With an RC network,
assuming the total resistance is the same, the discharge current is
the
same as wellWith reasonable break rates,

Regards, Gary Lau Waltham, MA USA"

5.  The R between the NST and bypass capacitor slows down its
charging.  This helps to reduce the high frequency ringing. See (5)
above.