All of these features simplify the ordering of the ASCTs. Practically, in order to purchase
ASCTs, the only required information is the main CT rated secondary current that is, (1A, 2A or
5A).
Table 23 summarizes the ASCT data:
Table 23: Auxiliary summation CT data
Type of fault N1 N2 N3 N4 Ukp [V] Burden [VA]
ASCT SLCE 8;
1/1A
52 52 104 90 33 1.0
ASCT SLCE 8;
5/1A
12 12 24 104 38 1.0
ASCT SLCE 8;
2/1A
26 26 52 90 33 1.0
where:
• N1, N2, N3 & N4 are ASCT windings turn numbers (see Figure
83)
• Ukp is knee point voltage, at 1.6T, of the secondary winding with N4 turns
• Burden is the total 3Ph load of ASCT imposed to the main CT
Due to ASCT design, the ASCTs for summated bus differential protection, must
always be mounted as close as possible to the IED (that is, in the same
protection cubicle).
6.1.6.3 Possible ASCT connections for REB670
M12137-3 v4
It is possible to connect the ASCTs for summated bus differential protection with REB670:
• at the end of the main CT circuit (for example, beyond the other protective relays, as
shown in Figure
84
• in series with other secondary equipment when some other relay must be located at the
end of the main CT circuit , as shown in Figure
85
End connection is the preferred arrangement as it gives greater sensitivity for summation type
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. This means
that within one busbar installation all auxiliary summation CTs have to be either end-
connected or series-connected.
Typical end-connection with ASCT is shown in Figure
84.
1MRK 505 370-UEN D Section 6
Differential protection
Busbar protection REB670 137
Application manual
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