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Hi Doug,
Tesla Coil caps see a very nasty oscillatory, high-current, high
rep-rate, high-voltage environment that is extremely challenging to a
capacitor's dielectric and metalization system. Fortunately, the
self-healing feature of these particular caps allows you to overstress
them without suffering immediate catastrophic failure. It allows you to
trade off capacitor run-time life versus initial cost for the capacitor
bank. Excessive voltage stress on your self-healing caps results in the
eventual death of your tank cap, and the greater the overstress, the
shorter the expected life.
For TC use, moderately conservative design practice suggests that each
MMC string should have a total DC rating no less that 2.5X - 3X your HV
source RMS face plate voltage rating (30 - 36 kVDC for your 12 kV NST).
However, you CAN choose to use a lower factor - with more risk and
reduced lifetime. Some MMC design charts even show a factor as low as
1.33X (i.e., only 8 caps in series for a 12 kV NST!). Using fewer caps
in each MMC string significantly increases the voltage stress on each
cap. In TC caps, overvolting is most often caused by voltage reversals
in the ringing tank circuit. The dielectric system of a cap that rapidly
reverses polarity "sees" a voltage stress that's 2X as high as the
initial capacitor voltage.
Initial symptoms of overvolting are typically silent, showing up as
small sparks (partial discharges) along the boundary between the
capacitor's metalization and adjacent dielectric. When the dielectric
fails, a short-circuit and self-healing event occurs. Self-healing
events near the outside tape layer can sometimes be seen as flashes of
light. Once this destructive process begins, it progressively chews at
the polypropylene dielectric, causing large numbers of
short-circuit/self-healing clearing events. These progressively damage,
and eventually destroy, your tank caps. Depending on the degree of
overstress, this can take minutes, hours, or days. By using more caps in
each string to reduce voltage stress, the degradation process can be
avoided and the usable lifetime of the caps can be extended indefinitely.
Looking at the physics and of the internal structure of these
self-healing capacitors, I would recommend using a factor no less that
2.5x to 3x Vsupply(RMS), or 15 to 18 caps/string for your 12 kV NST.
If you only need a few hours of run-time life, you can further reduce
the number in each string to perhaps 2X or less. I wouldn't, but you
can... :)
Bert
doug wrote:
I have 3 MMC’s each consisting of 15 .15u X 2 Kv Caps. [10nf X 30Kv] I can connect them P or S. Which would give me the best setup using a 12X30 NST.
Doug
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