Re: Weird safety gap behavior

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Ed,

At 12:57 PM 9/5/2004, you wrote:
>"Hi Ed,
>
>I'm thinking my case may be a different phenomenum.  My capacitance is
>26.5
>nf and I'm at 2.5* Cres.  The thevenin inductance of the tranny is 663H
>and
>the thevenin resistance is 15.9Kohms.  The calculated the steady state
>voltage on the capacitance is about 14500 Vpeak (assuming 21200 Vpeak
>for
>the Vs_oc).  The safety gaps are now adjusted to not fire so no
>transients
>present.
>
>Today, Terry and I measured the PF during this event.  Driving a mostly
>pure
>capacitive load and before runaway, the PF was 0.17 (small as one would
>expect).  After runaway, the PF was 0.97 and the line current droped
>slightly.   The power seems to be mostly dissapated in the NST.  The NST
>was
>buzzing but not too violently.   I did note that the sine wave output
>was
>relatively clean before the event and became very distorted after
>runaway.
>The distortion apears to be odd harmonic in nature (symetry seemed to be
>preserved).  A scope capture of the event shows a peak to peak growth
>until
>30KVpeak  is reached.
>
>My current thinking is that what ever the nonlinear effect is,  it is
>causing the effective inductance to decrease to cause the resonant
>rise.  I
>am studying ferroresonance to see what I can learn.
>
>Gerry R."
>
>Gerry:
>
>         Certainly you CAN achieve resonance at high flux density when the
>permeability is decreased after going through peak.  In my case it was
>at low flux densities that I observed the phenomenon.  What puzzles me
>is how your transformer insulation has withstood that high a secondary
>voltage!!!!!!!  I assume you haven't run it that way for long......
>
>         I would think the buzzing indicates core saturation, which should be
>very evident in the input current waveform, which should show big
>spikes.
>
>Ed


That is what I found so odd.  As Gerry was very slowly turning up the 
variac, we watched the output voltage on the scope too.  The output looked 
pretty distorted with higher order harmonics, the current was about 11 amps 
and the power factor was about 0.17.  Then the voltage suddenly (~1/2 sec 
ramp) jumped up to a fairly good 30kV peak sine wave, the current actually 
"decreased" to about 9 amps, the power factor went to 0.97 and the trannys 
hummed like mad.

I always thought that fuses would protect against this condition.  They 
apparently do not!!

It seems like the system is going from a nonresonant low power factor, low 
real power situation (about 224W real)... to a practically unity power 
factor situation (about 1080 watts real).

It all has me confused %:-p

Cheers,

         Terry