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Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
I like small DRSSTCs - my current one runs at about 220kHz.
Now, I was wondering if anyone had done any work on the 3dB points of various CT design (with the exception of Pearson/Rogowski coils).
Most CTs of standard design, even HE/MR designs, roll off at about 100kHz, though some exotics seem to go to maybe twice that, and you pay the consequential price...
I do have a few Pearsons (150/410/411) , so I could calibrate a standard CT maybe, but it seems a bit excessive... I'm really looking for a deterministic rather than a qualitative solution...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hi Nicko
They have a self-resonant frequency determined by the winding capacitance and leakage inductance, which together form a 2nd order low-pass filter. The output peaks at the resonant frequency and falls off at 12dB/octave thereafter, with an accompanying 180' phase lag.
I believe that the lower the ratio and the smaller the burden, the higher the self-resonant frequency and the better damped the peak will be.
Pearsons and some other designs have "distributed termination" that turns the whole winding into a 50 ohm transmission line. We have CTs in our lab using a similar construction, that are usable above 50MHz.
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
I measured the total leakage inductance in a home-made CT (25mm ferrite core, Al=5.4uH, 1:30 ratio) and I found a value of about 100nH (@100kHz)...
Given this low value, I suspect that the self resonant freq. should be much higher than 100kHz... Assuming something around 30pF as self capacitance (1pF per turn) the res. freq. would be 91MHz!
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Frequency response of current transformers...
