Re: High voltage side PFC caps

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Terry,

On 28 Nov 2001, at 11:13, Tesla list wrote:

> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> 
> Hi Marc,
> 
> Normally, there is nothing wrong with using PFC caps on the secondary side
> of HV transformers.  This is often done by utility companies since high
> voltage, but far lower value, caps are easier to make and they can easily
> install them on poles as needed.
> 
> However, if you have a big cap on the output of and NST, when the main gap
> fires it would tend to short the cap directly and perhaps waste a lot of
> power and mess with the gap firing.  A high side PFC cap is not isolated
> from the main gap by the NST current limiting.  A filter network "may" fix
> that problem so perhaps there is some merit to the idea.  I don't know
> right off how such a PFC cap would affect LTR charging and other charging
> circuit operations.  The arc may stay lit until the PFC cap discharges
> since there is nothing to stop the current flow once the arc starts (rotary
> gap?).
> 
> A 100uF cap on the input of a 15/60 NST is equivalent to a:
> 
> 100uF / (15000/120)^2 = 6.4nF    (someone check me on this ;-))

Sounds ballpark right to me (without doing the arithmetic).
 
> cap on the output side.  This cap would not have to be a high current pulse
> cap or anything other than a very typical high voltage cap.  Although MMC
> type caps are probably the easiest to use and you would only need one
> string since there should not be any high currents like a main cap would see.
> 
> So it is an interesting idea but it is not obvious what the "answer" is.  I
> have never heard of this idea before so it would be new as far as I know.

In fact it happens by proxy on resonant charging TC systems. But PFC 
caps are designed to pull line current into phase with voltage for no 
or light load conditions. So if the transformer is significantly 
loaded with a resistive load, a PFC cap won't achieve much. If the 
transformer is unloaded, the downside of the scheme is that it places 
a severe voltage burden on the windings since PFC is a resonant 
condition. 

Regards,
malcolm