ABB REB670 Applications Manual page 159

1MRK 505 337-UUS -
Application manual
1. ASCT type with ratio 1/1A, for balanced 3-Ph current input, shall be used with all
main current transformers with 1A rated secondary current (that is, 2000/1A)
2. ASCT type with ratio 5/1A, for balanced 3-Ph current input, shall be used with all
main current transformers with 5A rated secondary current (that is, 3000/5A)
3. ASCT type with ratio 2/1A, for balanced 3-Ph current input, shall be used with all
main current transformers with 2A rated secondary current (that is, 1000/2A)
Note the following:
• main CT rated primary current is not important for ASCT selection
• possible main CT ratio differences will be compensated by a parameter setting in the
IED
• rated secondary current of ASCT is 1A for all types. That means that secondary
ASCT winding should be always connected to the IED with 1A CT inputs,
irrespective of the rated secondary current of the main CT
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
ASCT SLCE 8; 52
1/1A
ASCT SLCE 8; 12
5/1A
ASCT SLCE 8; 26
2/1A
where:
• N1, N2, N3 & N4 are ASCT windings turn numbers (see figure 74)
• Vkp 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).
N2 N3 N4
52 104 90
12 24 104
26 52 90
Section 6
Differential protection
Vkp [V] Burden [VA]
33 1.0
38 1.0
33 1.0
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