TSSP: List Archives

From: boris petkovic
Date: Sun, 5 May 2002 10:03:25 -0700 (PDT)
Subject: Re: [TSSP] Secondary voltage stress factor

After rereading I noticed you refer to local strenght
of E field,while from mine post(sort of retorical
question in the end) could be missunderstood that 12
KV/cm was measured max  field in these exps.To avoid
possible misinterpretations I must elaborete more on
this.
12kv/cm was only average field (voltage between plates
/diastance).And when I said quasi-uniform I meant
partly uniform.This considers that shape of E-field
"lines" (field orientation vector and its scalar
density) highly uniform and paralel tru space before
inserting the insultors,changes after inserting such
dielectric body in Field.In this way dielectric
objects (insulators here) act like sort of lens and
the effect of coverging E-field lines is most
expressed in surface of the dielectric while voltage
between big plate electrodes remained same as before.
How much of bending and where exactly and to what
degree on the permitivity of dielectric and system
geometry.
Thus,originally uniform E=12kv/cm distribution between
mentioned electrodes changes in described fashion
points of Emax>12 kv/cm emerged somewhere at
insulator's surface (even mathematical smoothness
taken).Would it be enough to increase Emax to 30 kv/cm
somewhere?I dunno.Hardly to theoretically happen in
case of our coil form if perfect geometry of
dielectric taken I think.
I hope I made my point clearer now.

regards,
Boris 

> Bert wrote:
> 
>  There are a number of studies that imply
> > surface fields of
> > 5-10kV/cm are sufficient to cause further leader
> > growth once a leader
> > has been initiated. But I believe Boris is also
> > correct - the point of
> > initiation will still require a local E-field in
> the
> > 26-30 kV/cm range to
> > trigger initial ionization. E-fields of this
> > magnitude might occur at
> > localized defects in the winding insulation,
> > imbedded particulate
> > matter, or defects in the surface coating. 
> ---
I wrote:

> I seem to remember reading somewhere the study 
> where
> authors put very clean excelent insulators  having
> smooth cylindrical walls, between two parallel big
> plate electrodes.Species were tested by 50 Hz and DC
> voltage.No matter how they tried hard no insulator
> shown more than 12kV/cm strenght before flashover
> occured.I can't be sure but I think they reported
> somewhat less strenght in DC voltage tests.
> Humidity and some other influences (appart from
> possible microscopic increase of local field
> magnitude
> due to never perfectly smooth insulator surface)may
> be
> reason for only 12Kv/cm limit?
> 
> regards,
> Boris      
> 
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