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Utility Capacitors



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From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Tuesday, June 16, 1998 7:06 AM
To:  Tesla List
Subject:  Re: Utility Capacitors

Tesla List wrote:
> 
> ----------
> From:  ESchulz531-at-aol-dot-com [SMTP:ESchulz531-at-aol-dot-com]
> Sent:  Monday, June 15, 1998 6:27 PM
> To:  tesla-at-pupman-dot-com
> Subject:  Utility Capacitors
> 
> Is it possible to use a Utility Capacitor.
> I see them everywhere.  What type of
> ratings do these have and would they
> work for RF loads like in a tesla coil?
> 
> Erik -> Gradient

Erik,

It depends! Utility Power factor Correction (PFC) caps are not
constructed for efficient pulse duty. However, other than needing low RF
losses and capability to withstand high RF voltages, Tesla Coil service
doesn't really demand low-inductance, high pulse current construction,
with peak primary currents typlically being under 1000 amps for
small-medium coils with a low duty cycle. The more common PFC's seen on
local feeders tend to have relatively low operating voltages and
relatively high capacitance (for TC use at least). Unfortunately, they
also tend to use dielectrics which are not optimal for TC use
(paper-oil, paper-PCB's, or mylar). The previously discussed GE
Dielektrol caps with polypropylene dielectric were the exception, not
the rule. PFC caps also tend to be quite large for their rated
capacitance and voltage (when compared to true pulse caps). 

Utility PFC's are typically not rated in uF, but in KVAR (Kilovolt
Amperes Reactive) at a given working voltage. However you can convert
between KVAR and uF through the following formula:

     C = 1000*KVAR/(2*Pi*F*(V^2))
      where KVAR = faceplate KVAR rating
              Pi = 3.14159... 
               F = operating frequency (Hertz)
               V = AC operating voltage (RMS)

For example, a 20 KVAR capacitor designed for 4200 volt operation will
have a nominal capacitance of 3.61 uF. 

It IS possible [but not necessarily recommended] to take a capacitor
similar to the one above and "rewire it" internally to get higher
operating voltage and lower capacitance. This is definately not
recommended for PCB-filled caps. Internally the caps typically consist
of identical capacitor rolls connected in parallel. By disconnecting the
individual rolls from the common bus, and then reconnecting them in
series, it's possible to build a HV cap with the low capacitance and
high operating voltage. One of Richard Hull's videotapes describes this
process (tape #19?). However, unless you start out with a cap having
polypropylene dielectric, you end up with a bulletproof lossy cap -
hardly worth the effort and mess.  

Safe cappin' to you!

-- Bert --