No information is available about how frequent the use of the new iron core materialis for protection CT cores, but it is known that some CT manufacturers are using thenew material while other manufacturers continue to use the old traditional corematerial for protection CT cores. In a case where VHR type CTs have been alreadyinstalled, the calculated values of Eal for HR type CTs, for which the formulas aregiven in this document, must be multiplied by factor two-and-a-half in order for VHRtype CTs (i.e. with new material) to be used together with ABB protection IEDs.However, this may result in unacceptably big CT cores, which can be difficult tomanufacture and fit in available space.Different standards and classes specify the saturation e.m.f. in different ways but it ispossible to approximately compare values from different classes. The rated equivalentlimiting secondary e.m.f. Eal according to the IEC 61869–2 standard is used to specifythe CT requirements for the IED. The requirements are also specified according toother standards.24.1.2 ConditionsThe 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-phasefaults have been tested for different relevant fault positions for example, close inforward and reverse faults, zone 1 reach faults, internal and external faults. Thedependability and security of the protection was verified by checking for example,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 unwanted operationsare not acceptable at all maximum remanence has been considered for fault casescritical for the security, for example, faults in reverse direction and external faults.Because of the almost negligible risk of additional time delays and the non-existentrisk of failure to operate the remanence have not been considered for the dependabilitycases. The requirements below are therefore fully valid for all normal applications.It is difficult to give general recommendations for additional margins for remanenceto avoid the minor risk of an additional time delay. They depend on the performanceand economy requirements. When current transformers of low remanence type (forexample, TPY, PR) are used, normally no additional margin is needed. For currenttransformers of high remanence type (for example, P, PX, TPX) the small probabilityof fully asymmetrical faults, together with high remanence in the same direction as theflux generated by the fault, has to be kept in mind at the decision of an additional1MRK 502 071-UEN - Section 24RequirementsGenerator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 745Application manual
