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Re: Tank protection: common mode vs differential mode transients

Hi Marco,

At 02:02 PM 02/22/2000 +0200, you wrote:
>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

You bet they will ;-))

>- 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!

Are you sure this isn't noise giving a false reading ??  There are some cap
to ground parasitics that by be unbalanced and there is pure RF emission too.

>
>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.

Try big ferrite beads too.  They work wonders.

>
>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.

I have not tried this.

>
>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?
>

When you are dealing with high voltages and fast dV/dT s all kinds of fun
noisy things go on.  Keep lead paths short, avoid ferrous metal, and use
lots of big ferrites...

Cheers,

	Terry

>Regards
>