Re: transmission lines from transformer /was: First Light HELP

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Jim Lux" <jimlux@xxxxxxxxxxxxx>

If there's a resonance effect (high frequencies where the transmission line
is close to a big fraction of a wavelength, with something to excite those
resonances) you could get stacked up transients to maybe 3-4 x the RMS
voltage.  This isn't something that's going to build up from bang to bang or
cycle to cycle of the line frequency though.  It would be a much higher
frequency thing.


Also, don't forget that the peak voltage (which is what causes the
breakdown)  is already 1.4 times the rms voltage.  A double height peak
(easy to get) would be pretty close to 3x the RMS voltage.

You could also get a voltage doubling from having two transmission lines and
just having one discharged at the "wrong" time relative to the other (which
could be the result of the rotary gap firing).  This is the classic Blumlein
doubler (you have two equal length transmission lines charged in parallel
and you discharge just one.  The reflected wave comes back at -V, so you've
got double voltage relative to the other one, still at +V.  Sort of like two
capacitors charged in parallel and effectively discharged in series.. like a
Marx scheme, except with traveling waves).

Add to the 2.8 Vrms, that another potential doubling from having parallel
transmission lines from the two leads from the pig, and you're up to 6x the
RMS voltage (90kV off a 14.4kV transformer)

Add to that the flashover potential across a surface being much lower than
the free air flashover voltage, and you're all set up for unexpected
failures.

Yes, this pulsed HV stuff has all sorts of traps for the unwary.

----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, May 22, 2005 2:15 PM
Subject: Re: transmission lines from transformer /was: First Light HELP


> Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>
>
> On 21 May 2005, at 12:17, Tesla list wrote:
>
>  > Original poster: Steve Conner <steve@xxxxxxxxxxxx>
>  >
>  > At 11:51 20/05/05 -0600, you wrote:
>  >
>  > >Using RG-8 cable with a 10 kVA pig it was "safety gapping" over the
>  > >entire HV bushing surface---- definitely way above 14.4 kV.
>  >
>  > I don't see how any ringing or transmission line effect can ever
>  > produce more than twice the transformer's peak output voltage. Must be
>  > something weird going on!
>  >
>  > Steve Conner
>
> Agreed. Speaking for myself I previously stated that I have observed
> voltages far higher than the o/c output voltage of the transformer
> but never mentioned a specific figure and certainly never claimed
> more than 2x o/c or 2x o/c for that matter. As far as the bushing
> observation goes (not mine) there may have been surface tracking at
> work (possibly? perhaps??).
>
> Malcolm
>
>