Current Transformer - 3
Protection Core Current Transformer As per IEC 61889-2:
1)?????Current Transformer – Composite Error Requirement
·????????Class P Protective Current Transformer - These are Protective current transformers without remnant flux limit for which the saturation behavior in the case of asymmetrical short circuit is specified.
The standard accuracy limit factors (ALF) are: 5-10-15-20-30.
The accuracy class is designated using the highest permissible percentage of the composite error followed by the letter (standing for protection) and the ALF value. The standard accuracy classes are 5P and 10P.
·????????Class PR Protective Current Transformer - These are Protective current transformers with remnant flux limit for which the saturation behavior in the case of a symmetrical short circuit is specified.
2)?????Current Transformer – Magnetizing characteristics
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·????????Class PX Protective Current Transformer - These are Protective current transformers of low leakage reactance without remnant 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 PXR Current Transformer - These are Protective current transformers with remnant 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. These are particularly meant for situations where low DC currents are continuously flowing through current transformer. Therefore in order to stop the CT from saturating, current transformers with gaps but with the same performance as class PX is used. For more details of class PX CTs the IEC standard may be referred.
3)?????Current Transformer – Transient Error Requirement
·????????Class TPX Protective Current Transformer - These are Protective current transformers with remnant flux limit for which the saturation behavior in case of a transient short circuit current is specified by the peak value of the instantaneous error.
·????????Class TPY Protective Current Transformer - These are Protective current transformers with remnant flux limit, for which the saturation behavior in case of a transient short circuit current is specified by the peak value of the instantaneous error.
·????????Class TPZ Protective Current Transformer - These are Protective current transformers with a specified secondary time constant for which the saturation behavior in case of a transient short circuit current is specified by the peak value of the alternating error component.
Reference : CBIP Manual of Protection System