Scottf may be along to add something since he does this for a living.
But it's not like people are forming opinions. The sources of error are pretty well known and understood. A few minutes of Googling will turn up more references.
I think the errors can be calculated fairly closely from an equivalent circuit which includes an ideal CT and a magnetizing reactance branch. The current through the magnetizing branch is the main source of error.
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and of course when you get into saturation the linear circuit model no longer holds. But the magnetizing currents go up dramatically and accuracy goes to the pits.
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I guess my real question is if the turns ratio can be measured and corrected, and the phase angle error can be measured and compensated for, why are CT's still given 1% accuracy? Where does the remaining error originate?
Hi.
Please pay attention on the Electricpete's "Ideal CT".
You havent it., becouse CT is trafo include iron with losses.
BTW, why 1%, for the meas you have core 0.2 class too.
I don't know , maybe with new tecnologies possible build today ideal CT, but what will price.
Regards.
Slava.
Intresting Q.
I don't remeber exactly this number, but possible with high level of test eq. check/write down meas point of CT.
New meters are include up to 8( maybe more) correction points for the CT ratio and angle errors.
If possible say, that now it's 0.5 class CT, I don't know. not sure. And take in account , you need provide this procedure for each phase.
If you are going to go the electronic / software route you might want to consider a Rogowski coil. They avoid most of the non-ideal CT properties because they do not have an iron core and can be made to high accuracy standards. The big downside is that they can not drive anything directly: the integrating amplifier is required to produce an output proportional to current. They have not been adopted to any significant degree by the rather conservative protection engineers, or at least not in the UK so far. A CT output is capable of driving a relay directly: simple, rugged, reliable. Now CTs are routinely used with numerical relays the argument about avoiding electronics becomes a bit weak so given time perhaps the Rogowski coil will gain favour.
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