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Capacative measurement on high voltage powerline.

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7kasper:
Hello fellow engineers!

I am trying to do a capacitive measurement on a conductor.
In simple form I basically apply force a small voltage on the conductor and measure the same output voltage. Based on the change in rise time I can detect a (significant) change in capacitance loading the conductor. This seems to work just fine.

Now comes the problem: the conductor I do my measurements on will sometimes have around 6kV on it (with respect to ground).
The measurement device doesn't have to measure when the high voltage is on, but the key point here is that it just musn't break.


I have thought of multiple solutions. These are the main ones I came up with:
1. Completely seperate the measurement circuit.
Basically put one or two relais after the measurment circuit. Only make electricty to be able to flow back if the HV line is off.
Problem with this is that you need to have a relais that seperates >5kV and I haven't found something that isn't super expensive.

2. Operate measurement at relative or floating point.
So the voltage doesn't want to go through the measurement circuit. This is quite impossible though. You need your capacitive measurement with respect to something right? Also there will not always be 5kV on the line. It can also be 0v. So I cannot have the plus of the measurement circuit be at 6kV + some test voltage.

3. Diode seperation.
I believe there are some diodes with capable reverse blocking voltages. I basically put them in line with the capacity measurement circuit.
If the HV is applied the diode should stop the reverse current into the measurement circuit.

Questions:
Method 3 seems most promising to me. But can it work? I guess the capacity will change somewhat because of the added diode that may or may not have wierd effects. But perhaps the relative measurment should still work, do you aggree?

Second of all, is this a sensible approach for this problem.
And lasly: Can this method be adapted if the 6kV signal was AC instead of DC?

Of course I am open for all (other) ideas to perform such measurement.

Thanks a lot for your effort and creativity!

- Kasper

bob91343:
I think the most sensible approach is to disconnect the measuring apparatus when not actually measuring.  The diode approach might work if you provide a path for its leakage current and can tolerate the transient that occurs when the 6 kV is applied.  If application of the high voltage is sudden, capacitive coupling around the diode may destroy your measurement device.

EPAIII:
What about capacitive coupling the pulse into and out of the conductor. Use some form of over-Voltage protection on the test circuit side of the capacitors to protect that circuit when the HV is switched on or off. Keep your test pulse below the level where that protection becomes active. Your test circuits would only need to withstand that level where the over-Voltage kicks in.

You did not mention any frequency so I do not know how this would work when the HV is AC. Power line frequencies (50 or 60 Hz) would probably be OK but higher frequencies may not.

What over-Voltage devices? Zeners, ordinary diodes in series, or any of many other circuits would be candidates.

MasterTech:
I'm curious, is this for a university assignment or a project for a company you are working with?

asdf336:
Yes AC coupling is the most promising method.   The known capacitance in your device can be backed out with calibration/calculation.

A remaining problem is the inrush event if you accidentally connect your device to such a high voltage.  Hard to know the scope of that problem without knowing more about the C you’re trying to measure (which impacts the series C you’d choose). 

There are few other series components rated for such voltages though you could reference high voltage multimeters which have 1kv type protections on their leads. 

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