Hi David and Steve,
There are some actual measurements of capacitor voltages at my website, and
the overshoot to the reversed state is very slight, certainly not over 5 %.
But the peak voltage over the cap is not much lower than two times the
transformer peak voltage, which is to be expected for a resonant charging
condition, or for my coil about 32 kV for a 12,5 kV rms transformer. So far
my MMC designed for 36 kV DC has stood up.
But 32 kV is a lot, and I had some serious corona and arcing problems
initially before I made some necessary changes to the MMC layout.
Jan
Stockholm, Sweden
Message: 12
Date: Tue, 15 Sep 2020 06:31:27 -0700
From: jimlux <jimlux@xxxxxxxxxxxxx <mailto:jimlux@xxxxxxxxxxxxx> >
To: tesla@xxxxxxxxxxxxxxxxxx <mailto:tesla@xxxxxxxxxxxxxxxxxx>
Subject: Re: [TCML] Asynchronous versus synchronous gaps, was Old Post
Message-ID: <5359d93f-b686-68c2-cb74-012ab0fdb2c7@xxxxxxxxxxxxx
<mailto:5359d93f-b686-68c2-cb74-012ab0fdb2c7@xxxxxxxxxxxxx> >
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On 9/14/20 5:03 AM, David Rieben wrote:
> Hi Steve,
>
> To be perfectly honest, since I have no formal training in electrical
engineering, I?m afraid your inquiry is beyond my collective expertise. ? I
will say that I?ve heard that the voltage reversal vs shot life issues can
be reasonably dealt with by choosing a cap whose maximum voltage rating is a
minimum of 3x the peak input AC voltage (rms x 1.41), assuming that it is
properly designed with a low loss dielectric system. My transformer can
produce up to 16.8 kvAC so 16800 x 1.41=23688, and 23688 x 3 = 71,064. My
primary tank capacitor is rated at 75 kV, so I should be ?good?.
>
> David
>
> Sent from my iPhone
>
>> On Sep 13, 2020, at 12:38 PM, Steve White <steve.white1@xxxxxxxxx
<mailto:steve.white1@xxxxxxxxx> > wrote:
>>
>> ?Hi David,
>>
>> I've always wondered if a ARSG has some negative effect on capacitor
life. With a ARSG essentially firing at slow-rolling random times it seems
as if many of those firing events would be during capacitor voltage
reversals during the charging process. Of course the frequency of the 60 Hz
charging voltage reversals is much lower than the voltage reversals
experienced in the primary coil tank circuit. Since percent voltage reversal
is a key capacitor lifetime parameter, it seems like something to consider.
>>
>> That potential problem can't happen with a SRSG because the firing points
are nominally at 0 and 180 degrees for a 120 PPS system or at 0, 90, 180,
and 225 degrees for a 240 PPS system.
>>
>> Maybe the effect is negligible.
>>
>> Thoughts?
>>
>> Steve White
>> Cedar Rapids, Iowa
>>
>> --
Pulse discharge operation, particularly with underdamped (ringing)
discharges, places large internal stresses on components. Empirical
expressions have been developed to predict the life of a pulse discharge
capacitor under conditions other than the nominal design. For instance, in
an article by K. Salisbury, S. Lloyd, and Y.G. Chen at Maxwell Laboratories,
"A transportable 50 KA Dual Mode Lightning Simulator", the following
equation is given.
Lx = Lref * (Qref/Qx)^1.6 * (Vref/Vx)^7.5
where
x subscript refers to the application
ref subscript refers to the reference data L is the expected life (in
shots), Q is the discharge waveform Q V is the capacitor charge voltage.
Note that capacitor voltage is the most important life determinant, with a
7.5 exponent. A little reduction in voltage leads to a huge increase in
life.
For large-ish Q
Q = sqrt( 1 + 1 /( 2 /pi*ln(VR))^2)
VR = exp( - pi/2 * sqrt(1/(q^2-1)))
------------------------------
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