Section 25 Requirements25.1 Current transformer requirements IP15171-1 v2M11609-3 v2The 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 ofsome functions. Consequently CT saturation can have an influence on both thedependability and the security of the protection. This protection IED has been designedto permit heavy CT saturation with maintained correct operation.25.1.1 Current transformer basic classification and requirements M11611-3 v2M11611-4 v5To 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. Tofulfill the requirement on a specified time to saturation the CTs must fulfill therequirements of a minimum secondary e.m.f. that is specified below.CTs are specified according to many different classes and standards. In principle, thereare three different types of protection CTs. These types are related to the design of theiron core and the presence of airgaps. Airgaps affects the properties of the remanentflux.The following three different types of protection CTs have been specified:• The High Remanence type with closed iron core and no specified limit of theremanent flux• The Low Remanence type with small airgaps in the iron core and the remanentflux limit is specified to be maximum 10% of the saturation flux• The Non Remanence type with big airgaps in the iron core and the remanent fluxcan be neglectedEven though no limit of the remanent flux is specified in the IEC standard for closedcore CTs, it is a common opinion that the remanent flux is normally limited tomaximum 75 - 80 % of the saturation flux.Since approximately year 2000 some CT manufactures have introduced new corematerials that gradually have increased the possible maximum levels of remanent fluxeven up to 95 % related to the hysteresis curve. Corresponding level of actual remanent1MRK 504 163-UUS A Section 25RequirementsTransformer protection RET670 2.2 ANSI 983Application manual
