ABB Relion 670 Series Applications Manual page 144

Section 6
Differential protection
• Only one measuring circuit is utilized for all fault types (that is, no redundancy for multi-phase faults)
• Primary fault sensitivity varies depending on the type of fault and involved phase(s), see Table
• The load currents in the healthy phases might produce the stabilizing current when an internal single
phase to ground fault occurs. However, there is no problem for solidly gounded systems with high
ground-fault currents
• No indication of faulty phase(s) in case of an internal fault
• Not possible to fully utilize Open CT detection feature
6.1.6.2 Auxiliary summation CTs
Auxiliary Summation Current Transformer (that is, ASCT in further text) of the type SLCE 8 is used with
the summation principle of the IED. The principle drawing of one such ASCT is shown in Figure 84.
Auxiliary Summation CT
type SLCE 8; X/1A
P1
P2
P3
P4
P5
P6
ANSI03000118 V1 EN-US
Figure 84:
The ASCT has three primary windings and one secondary winding. In further text, turn numbers of these
windings will be marked with N1, N2, N3 & N4, respectively (see Figure
There are three types of ASCT for REB670:
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:
138
N1
S1
N4
N2
N3
S2
en03000118_ansi.vsd
Principle ASCT drawing
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1MRK 505 370-UUS Rev. K
84 for more information).
Busbar protection REB670
Application manual
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M12136-4 v4