Section 18 Requirements18.1 Current transformer requirements IP15171-1 v2M11609-3 v2The performance of a protection function will depend on the quality of the measured currentsignal. Saturation of the current transformers (CTs) will cause distortion of the current signalsand can result in a failure to operate or cause unwanted operations of some functions.Consequently CT saturation can have an influence on both the dependability and the securityof the protection. This protection IED has been designed to permit heavy CT saturation withmaintained correct operation.18.1.1 Current transformer classification M11611-3 v1M11611-4 v4To 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 fulfill therequirement on a specified time to saturation the CTs must fulfill the requirements of aminimum secondary e.m.f. that is specified below.There are several different ways to specify CTs. Conventional magnetic core CTs are usuallyspecified and manufactured according to some international or national standards, whichspecify different protection classes as well. There are many different standards and a lot ofclasses 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. Typical examplesof high remanence type CT are class P, PX, TPX according to IEC, class P, X according to BS (oldBritish 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 with asmall air gap to reduce the remanence to a level that does not exceed 10% of the saturationflux. The small air gap has only very limited influences on the other properties of the CT. ClassPXR, TPY according to IEC are low remanence type CTs.The non remanence type CT has practically negligible level of remanent flux. This type of CThas relatively big air gaps in order to reduce the remanence to practically zero level. In thesame time, these air gaps reduce the influence of the DC-component from the primary faultcurrent. The air gaps will also decrease the measuring accuracy in the non-saturated region ofoperation. Class TPZ according to IEC is a non remanence type CT.Different standards and classes specify the saturation e.m.f. in different ways but it is possibleto approximately compare values from different classes. The rated equivalent limitingsecondary e.m.f. Eal according to the IEC 61869–2 standard is used to specify the CTrequirements for the IED. The requirements are also specified according to other standards.18.1.2 Conditions M11610-3 v1M11610-4 v4The requirements are a result of investigations performed in our network simulator. Thecurrent transformer models are representative for current transformers of high remanence1MRK 504 158-UEN A Section 18Requirements263Application manual
