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Source of CT Error

Source of CT Error

Source of CT Error

(OP)
What is the consensus on where CT error originates? Is it in the turns ratio not being exactly as given, or would you say it is the phase lag?

RE: Source of CT Error

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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RE: Source of CT Error

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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RE: Source of CT Error

(OP)
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?

RE: Source of CT Error

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.

RE: Source of CT Error

(OP)
Is it possible to calibrate a 1% CT (by use of metering software correction) with the use of a CT test box to act as a higher say 0.5% precision CT?

RE: Source of CT Error

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.

RE: Source of CT Error

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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If we learn from our mistakes I'm getting a great education!
 

RE: Source of CT Error

Cooper Power Systems has come up with a very creative use of Rogo CT for EAF (Arc Furnace) transformers.

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