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Tank protection: common mode vs differential mode transients
Hello.
Within the THOR project, we are at the moment also struggling to get some order
with the current paths. As we are actually measuring the HV tank output, we
need
to ground the negative pole of it. This adds a third way for the current
supplied to the primary capacitor to circulate:
1 - the positive pole cable
2 - the negative pole cable
3 - the common ground
But I'll get to the point now. In series with the cables charging the capacitor
we had two 3 mH inductors, to stop the HF coming back. As we were measuring
(with a HF current probe) the above three currents, we noticed that:
- the two inductors generated an unwanted high current ringing during the
capacitor charging
- there was an enormous difference between differential current (the one
actually charging the capacitor) and common-mode current (the one flowing
in the
same direction in both cables). Differential current was the expected 0.5 A,
while common-mode current had large ringings and spikes up to 20 A!
Removing the two 3 mH inductors the unwanted ringing was gone, but the
common-mode spikes were still there. This led us to think that a better
solution
would be a COMMON-MODE inductor inserted between the cables and the capacitor.
We are now building that with a big double ferrite U core and two anti-parallel
windings.
Terry and others:
have you ever measured common-mode transients on the cables from the tank
to the
capacitor? You can do that easily by passing both the cables through a current
sensor. In theory you should always read zero, but we didn't.
About the 3th current path, the ground:
You may think that your primary winding + capacitor are floating, but you can
still have that path through parasitics. For instance, we too should have it
floating, but the capacitor cans are grounded and we probably have some
hundreds
of picofarads parasitic capacitance from the capacitor pole to its can. On the
tank side, you'll have your NST case grounded with another winding-to-case
capacitance: here is your third current path.
Any comments or help on this?
Regards