Main Ct Ratio Mismatch Correction; Primary Pick-Up Levels For Summation Type Differential Protection - ABB REB670 Applications Manual

1MRK 505 337-UUS -
6.1.5.4
6.1.5.5
Application manual
Main CT
2000/1A
or 2000/5A
or 2000/2A
A B C
Other
relays
IA IB IC
Other
relays
IN
ANSI06000129 V1 EN
Figure 76: Series-connection with ASCT connected to CT3 input
Refer to section "SLCE 8/ASCT characteristics for
current calculation for series-connection.

Main CT ratio mismatch correction

As stated before, three types of ASCTs for REB670 are available. The first type shall be
used for main CTs with 1A rated secondary current. The second type shall be used for
main CTs with 5A rated secondary current. The third type shall be used with 2A main CTs.
However REB670 with 1A CT inputs is always used. Therefore main CT ratio shall
always be set in such a way that the primary current is entered as for the main CT, but
secondary current is always entered as 1A (that is, 3000/5 main CT will be entered as
3000/1 CT in REB670).

Primary pick-up levels for summation type differential protection

The minimal differential operating current level is entered directly in primary amperes.
However, as stated previously, in case of the summated differential protection the primary
fault sensitivity varies depending on the type of fault and involved phase(s). The entered
value, for the minimal differential operating current level, will exactly correspond to the
REB670 pickup value in the event of a 3-phase internal fault. For all other fault types this
N
Auxiliary Summation CT
type SLCE 8; 1/1 or 5/1A or 2/1A
IA
P1
N1
P2
P3
N2
IC
P4
P5
N3
IN
P6
Differential protection
I
SUMM
REB 670 with 1A CTs
S1
N4
S
2
en06000129_ansi.vsd
series-connection"for detailed ASCT
Section 6
155