Comments for Chris Reeland, Ladd Illinois USA, on VTTC power measurements:
Caution using clamp-on current meters as they are highly inductive. Please
watch out for loose connections and inductive kickback. Do you know how much
phase shift you are getting through the internal circuit? This may be
modifying your screen waveform. If you are very careful, I would recommend a
series current shunt at the lowest potential point with respect to ground.
And float your meter if necessary.
Some Tektronix probes are prone to overheat when measuring continuous high
voltage - I have toasted two 35 kV probes with my little 811A VTTC. Have you
considered building a bi-filar (non-inductive) probe of Nichrome wire (high
side) and koolohm (N.I.) low side resistor.
Back a few years when I was impulse testing UHV insulators, we used the
original Tektronix 507 Dual Beam Oscilloscope. We experienced oscillations
in the input amplifier circuits which appeared as oscillating time (negative
time) during the rise of the voltage rise for each impulse. The fix was to
remove the input amplifiers completely and directly connect the incoming
signal to the vertical plates of the 507 scope. The images were captured
single-trace on high speed Polaroid flatpack film. A pair of 1000 mm
(40-inch) spheres were used to calibrate the total measurement for a
uni-potential pulse from a 5000 kV Marx generator. {
https://www.google.com/maps/@41.0378424,-81.7583024,52m/data=!3m1!1e3 }
--------- note: be sure that your scope is reading the true risetime of the
voltage waveform!
Rule of thumb - you need at least 1 ft of clearance for every 100 kV (peak)
of arc potential with a nominal 1 kv/uSec rise time. Faster risetime has a
higher breakdown voltage, where slower risetime has a lower breakdown
voltage. Ref-IEEE-4.
Dick
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