6.1.5.3 Possible ASCT connections for REB670It is possible to connect the ASCTs for summated bus differential protection withREB670:• at the end of the main CT circuit (for example, beyond the other protective relays, asshown in figure 75• in series with other secondary equipment when some other relay must be located atthe end of the main CT circuit , as shown in figure 76End connection is the preferred arrangement as it gives greater sensitivity for summationtype bus differential protection (as shown in table 24 for more information).However, it should be noted that these two connection types must not be mixed. Thismeans that within one busbar installation all auxiliary summation CTs have to be eitherend-connected or series-connected.Typical end-connection with ASCT is shown in figure 75.en06000128_ansi.vsdABCNIAIBICINOtherrelaysAuxiliary Summation CTtype SLCE 8;1/1A or 5/1A or 2/1AN4S1S2Main CT2000/1A or 2000/5Aor 2000/2AAI03X40156REB 670 with 1A CTsP1N1P2P3N2P4P5N3P6ISUMMANSI06000128 V1 ENFigure 75: End-connection with ASCT connected to CT3 inputIt is important to notice that even in the case of 5A or 2A main CTs, secondary current ofthe summation CTs shall be connected to the IED with 1A CT inputs (as shown infigure 75). The reason for this is that the rated secondary current of ASCT is always 1Airrespective of the rated secondary current of the main CT.Refer to section "SLCE 8/ASCT characteristics for end-connection" for detailed ASCTcurrent calculations for end-connection.Typical series-connection with ASCT is shown in figure 76.Section 6 1MRK 505 337-UUS -Differential protection154Application manual
