1.0 2017-01, Instrument transformers – Guidance for application of currenttransformers in power system protection, is the first official document thathighlighted this development. So far remanence factors of maximum 80% havebeen considered when CT requirements have been decided for ABB IEDs. Even inthe future this level of remanent flux probably will be the maximum level that willbe considered when decided the CT requirements. If higher remanence levelsshould be considered, it should often lead to unrealistic CT sizes.Thus, now there is a need to limit the acceptable level of remanent flux. To be ableto guarantee the performance of protection IEDs, we need to introduce thefollowing classification of CTs.There are many different standards and a lot of classes but fundamentally there arefour different types of CTs:• Very High Remanence type CT• High Remanence type CT• Low Remanence type CT• Non Remanence type CTThe Very High Remanence (VHR) type is a CT with closed iron core (forexample. protection classes TPX, P, PX according to IEC, class C, K according toANSI/IEEE) and with an iron core material (new material, typically new alloybased magnetic materials) that gives a remanent flux higher than 80 % of thesaturation flux.The High Remanence (HR) type is a CT with closed iron core (for example,protection classes TPX, P, PX according to IEC, class C, K according to ANSI/IEEE) but with an iron core material (traditional material) that gives a remanentflux that is limited to maximum 80 % of the saturation flux.The Low Remanence (LR) type is a CT with small airgaps in the iron core (forexample, TPY, PR, PXR according to IEC) and the remanent flux limit is specifiedto be maximum 10% of the saturation flux.The Non Remanence (NR) type is a CT with big airgaps in the core (for example,TPZ according to IEC) and the remanent flux can be neglected.It is also possible that different CT classes of HR and LR type may be mixed.CT type VHR (using new material) should not be used for protection CT cores.This means that it is important to specify that the remanence factor must not exceed80 % when ordering for example, class P, PX or TPX CTs. If CT manufacturers areusing new core material and are not able to fulfill this requirement, the CTs shall bespecified with small airgaps and therefore will be CTs of LR type (for example,class PR, TPY or PXR). Very high remanence level in a protection core CT cancause the following problems for protection IEDs:Section 22 1MRK 505 393-UEN BRequirements424 Line differential protection RED650 2.2 IECApplication manual
