Section 19 Requirements19.1 Current transformer requirementsThe performance of a protection function will depend on the quality of the measuredcurrent signal. Saturation of the current transformers (CTs) will cause distortion of thecurrent signals and can result in a failure to operate or cause unwanted operations of somefunctions. Consequently CT saturation can have an influence on both the dependabilityand the security of the protection. This protection IED has been designed to permit heavyCT saturation with maintained correct operation.19.1.1 Current transformer classificationTo guarantee correct operation, the current transformers (CTs) must be able to correctlyreproduce the current for a minimum time before the CT will begin to saturate. To fulfillthe requirement on a specified time to saturation the CTs must fulfill the requirements ofa minimum secondary e.m.f. that is specified below.There are several different ways to specify CTs. Conventional magnetic core CTs areusually specified and manufactured according to some international or national standards,which specify different protection classes as well. There are many different standards anda lot of classes but fundamentally there are three different types of CTs:• High remanence type CT• Low remanence type CT• Non remanence type CTThe high remanence type has no limit for the remanent flux. This CT has a magnetic corewithout any airgaps and a remanent flux might remain almost infinite time. In this type oftransformers the remanence can be up to around 80% of the saturation flux. Typicalexamples of high remanence type CT are class P, PX, TPX according to IEC, class P, Xaccording to BS (old British Standard) and non gapped class C, K according to ANSI/IEEE.The low remanence type has a specified limit for the remanent flux. This CT is made witha small air gap to reduce the remanence to a level that does not exceed 10% of thesaturation flux. The small air gap has only very limited influences on the other propertiesof the CT. Class PXR, TPY according to IEC are low remanence type CTs.1MRK 505 337-UUS - Section 19Requirements483Application manual
