Section 12 Requirements for measurementtransformers12.1 Current transformers12.1.1 Current transformer requirements for overcurrent protectionFor reliable and correct operation of the overcurrent protection, the CT has to bechosen carefully. The distortion of the secondary current of a saturated CT mayendanger the operation, selectivity, and co-ordination of protection. However, whenthe CT is correctly selected, a fast and reliable short circuit protection can be enabled.The selection of a CT depends not only on the CT specifications but also on thenetwork fault current magnitude, desired protection objectives, and the actual CTburden. The protection settings of the protection relay should be defined in accordancewith the CT performance as well as other factors.12.1.1.1 Current transformer accuracy class and accuracy limit factorThe rated accuracy limit factor (Fn) is the ratio of the rated accuracy limit primarycurrent to the rated primary current. For example, a protective current transformer oftype 5P10 has the accuracy class 5P and the accuracy limit factor 10. For protectivecurrent transformers, the accuracy class is designed by the highest permissiblepercentage composite error at the rated accuracy limit primary current prescribed forthe accuracy class concerned, followed by the letter "P" (meaning protection).Table 712:Limits of errors according to IEC 60044-1 for protective current transformersAccuracy class Current error atrated primarycurrent (%)Phase displacement at rated primarycurrentComposite error atrated accuracy limitprimary current (%)minutes centiradians5P ±1 ±60 ±1.8 510P ±3 - - 10The accuracy classes 5P and 10P are both suitable for non-directional overcurrentprotection. The 5P class provides a better accuracy. This should be noted also if thereare accuracy requirements for the metering functions (current metering, powermetering, and so on) of the protection relay.The CT accuracy primary limit current describes the highest fault current magnitudeat which the CT fulfils the specified accuracy. Beyond this level, the secondary current1MRS758755 A Section 12Requirements for measurement transformersREC615 and RER615 761Technical Manual
