1. Unwanted operation of differential (i.e. unit) protections for external faults2. Unacceptably delayed or even missing operation of all types of protections (forexample, distance, differential, overcurrent, etc.) which can result in loosingprotection selectivity in the networkNo information is available about how frequent the use of the new iron corematerial is for protection CT cores, but it is known that some CT manufacturers areusing the new material while other manufacturers continue to use the old traditionalcore material for protection CT cores. In a case where VHR type CTs have beenalready installed, the calculated values of Eal for HR type CTs, for which theformulas are given in this document, must be multiplied by factor two-and-a-half inorder for VHR type CTs (i.e. with new material) to be used together with ABBprotection IEDs. However, this may result in unacceptably big CT cores, which canbe difficult to manufacture and fit in available space.Different standards and classes specify the saturation e.m.f. in different ways but itis possible to approximately compare values from different classes. The ratedequivalent limiting secondary e.m.f. Eal according to the IEC 61869–2 standard isused to specify the CT requirements for the IED. The requirements are alsospecified according to other standards.23.1.2 Conditions M11610-3 v1M11610-4 v4The requirements are a result of investigations performed in our network simulator.The current transformer models are representative for current transformers of highremanence and low remanence type. The results may not always be valid for nonremanence type CTs (TPZ).The performances of the protection functions have been checked in the range fromsymmetrical to fully asymmetrical fault currents. Primary time constants of at least120 ms have been considered at the tests. The current requirements below are thusapplicable both for symmetrical and asymmetrical fault currents.Depending on the protection function phase-to-earth, phase-to-phase and three-phase faults have been tested for different relevant fault positions for example,close in forward and reverse faults, zone 1 reach faults, internal and external faults.The dependability and security of the protection was verified by checking forexample, time delays, unwanted operations, directionality, overreach and stability.The remanence in the current transformer core can cause unwanted operations orminor additional time delays for some protection functions. As unwantedoperations are not acceptable at all maximum remanence has been considered forfault cases critical for the security, for example, faults in reverse direction andexternal faults. Because of the almost negligible risk of additional time delays andthe non-existent risk of failure to operate the remanence have not been consideredfor the dependability cases. The requirements below are therefore fully valid for allnormal applications.1MRK 511 407-UEN C Section 23RequirementsPhasor measurement unit RES670 2.2 IEC 357Application manual
