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Current Transformer Turns Ratio and current ratio

Current Transformer Turns Ratio and current ratio

Current Transformer Turns Ratio and current ratio

Dear Friends,

Could someone tell me the reason for following:

In PX class Current transformer ,as per IEC the acceptance criteria for error is turns ratio error shall be less than .25%. But in class x CT it is current ratio error as the deciding criteria. why in PX class it is Turns ratio and in Class X current ratio.

Please clarify.


RE: Current Transformer Turns Ratio and current ratio

With no knowledge of IEC standards I'll go out on a limb and say that turns ratio error is measured at nominal secondary rating, where the CT is fairly linear but that current ratio is measured down in the weeds at the low end where the response is less than perfectly linear. Do you want broad range fault protection or are you looking for good low-end performance?

Hopefully somebody who has a better handle on IEC will drop by soon.

RE: Current Transformer Turns Ratio and current ratio

Are we comparing protection CTs with revenue metering CTs?
If so then that may be your answer.

"Why not the best?"
Jimmy Carter

RE: Current Transformer Turns Ratio and current ratio

class PX protective current transformer:
protective current transformer of low-leakage reactance without remanent flux limit for which knowledge of the excitation characteristic and of the secondary winding resistance, secondary burden resistance and turns ratio, is sufficient to assess its performance in relation to the protective relay system with which it is to be used.

class P protective current transformer
protective current transformer without remanent flux limit, for which the saturation behavior in the case of a symmetrical short-circuit is specified

RE: Current Transformer Turns Ratio and current ratio

Hi friends,
Thank you for the reply .
But I know that protection class CT(PX class) error is measured in terms of Turns ratio not current ratio , unlike 5P20 and class x CT.
My doubt is why we use Turns ratio for PX whereas Current ratio for others?

Any significant reason to do so?


RE: Current Transformer Turns Ratio and current ratio

you are comparing CT's made and measured according to two different standards. one BS and the other IEC. X is the old standard. I think you better stick with the new one PX.

RE: Current Transformer Turns Ratio and current ratio

Class PX is defined by the excitation characteristics (Vkp, Iexc, Rct), therefore, a turns ratio check is done and not an "accuracy" test...in addition to verification of Vkp, Iexc, and Rct.

Class P is defined as an accuracy limit up to a over-current factor, therefore and accuracy test is done measuring current ratio error and phase angle error. For example, 5P20 means <5% error up to 20 times over-current.

The error is defined as composite/total error and the limits at rated current at +/-1% ratio error / +/-50 min phase error at rated current and <5% composite error at accuracy limit primary current (20 x Inom in this case).

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