ABB RELION 670 Series Applications Manual page 142

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
6.1.5.2 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 74.
Auxiliary Summation CT
type SLCE 8; X/1A
P1
P2
P3
P4
P5
P6
ANSI03000118 V1 EN-US
Figure 74: Principle ASCT drawing
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
information).
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:
136
24
N1
S1
N4
N2
N3
S2
en03000118_ansi.vsd
1MRK 505 337-UUS A
M12136-4 v3
74 for more
Application manual