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Re: Glass cap with polyethylene




From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Sunday, September 14, 1997 2:37 PM
To: 	Tesla List
Subject: 	Re: Glass cap with polyethylene

Alfred and all,

Actually, it's when materials with a significant difference in
dielectric constants are combined that an unequal share of stress occurs
on the dielectric having the lower k. In the case of mineral oil and
polyethylene, the k's are quite close (k of about 2 - 2.3), so there's
no large discontinuity in the electrostatic fields. However, if you add
glass (k of about 5 - 7), it will significantly increase the voltage
stress on the LDPE and mineral oil. This is even worse for if you have
any entrapped air (k=1). 

Assume you have two different dielectric materials of thickness d1 and
d2 and dielectric constants of k1 and k2 respectively. It can be shown
that the ratio of voltage stress across each dielectric will be:
 
    V2/V1 = k1*d2/k2*d1   where V1+V2 = the applied voltage. 

The higher voltage stress is always seen across the material having the
lower dielectric constant. For example, let's look at equal thicknesses
(say 10 mils) of LDPE and glass made into a composite dielectric, and
asume we apply 10 KV of stress.

 Example: 10 mils of LDPE sandwiched with 60 mils of glass:
 Let: k1 = 2.2  (LDPE or Mineral Oil)
      d1 = d2 = 10 mils
      k2 = 7    (Glass)
         
 Then:   V2/V1 = 2.2/7 = 0.31
            V1 = 3.18*V2
 Since V1 + V2 = 10 KV = V1 +3.18*V1
            V1 = 10/4.18 = 2.39 KV  (or 239 volts/mil)
            V2 = 7.61 kV  (or 761 volts/mil)

If we only have mineral oil and LDPE as dielectrics, the voltage stress
will be evenly distributed (in terms of volts/mil...) between the two
materials. However, if we have an entrapped air bubble, we'll see about
2.2X higher stress (V/mil) in the air than in either the oil or the
LDPE. If the stress is too great (say, over 380 volts/mil), the air may
begin break down, creating corona and partial discharges, eventually
leading to capacitor failure. Entrapped air is potentially the kiss of
death for TC capacitors. 

Hope this helps...

-- Bert H --
 

    



Tesla List wrote:
> 
> From:   Alfred A. Skrocki[SMTP:alfred.skrocki-at-cybernetworking-dot-com]
> Sent:   Saturday, September 13, 1997 9:02 PM
> To:     Tesla List
> Subject:        Re: Glass cap with polyethylene
> 
> On  Saturday, September 13, 1997 10:21 AM DR.RESONANCE
> [SMTP:DR.RESONANCE-at-next-wave-dot-net] wrote;
> 
> > To: Rich
> >
> > Using two different dielectrics in a cap is very bad.  The electrostatic
> > fields will not be uniform which could lead to local corona and hence a
> > breakdown.  Suggest using single sheets of plastic and then series
> > connecting each subsection.
> 
> I find one major flaw in your logic, namely the fact that almost all
> polyethylene caps used for TCs are in fact dual dialectric caps. The
> polyethylene being one dielectric and the oil being the other. If
> your logic was true then, all oil immersedimmersed polyethylene caps would
> breakdown . In fact they rarely break down when built to tolerate the
> voltages used.
> 
>                                Sincerely
> 
>                                 \\\|///
>                               \\  ~ ~  //
>                                (  -at- -at-  )
>                         -----o00o-(_)-o00o-----
>                            Alfred A. Skrocki
>                   Alfred.Skrocki-at-CyberNetworking-dot-com
>                              .ooo0   0ooo.
>                         -----(   )---(   )-----
>                               \ (     ) /
>                                \_)   (_/