ABB Relion 670 Series Applications Manual page 147

1MRK 505 370-UUS Rev. K
Refer to Section
calculation for series-connection.
6.1.6.4 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).
6.1.6.5 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 value must be multiplied by a coefficient shown in the Table
calculate the actual primary pickup value.
Table 24:
Type of fault
ASCT end
connected
ASCT series
connected
Example 1:
The minimal differential operating current level in the IED is set to 1250A. All ASCTs are series
connected. What is the theoretical primary pickup value in case of C-Gnd fault?
Answer 1:
According to Table 24, pickup coefficient for this type of ASCT connection and this type of fault is 0.578.
Therefore:
(
I L3 Gnd
Pickup
EQUATION1106 V1 EN-US
I ( C Gnd
Pickup
EQUATION1784 V1 EN-US
This means that if 722.5 primary amperes is injected only in phase C of any of the connected main CTs,
the IED shall display the differential current of 1250A (primary) and should be on the point of the pickup
(that is, trip).
Busbar protection REB670
Application manual
"SLCE 8/ASCT characteristics for series-connection"
Pickup coefficients for Summated Differential Protection
A-Gnd B-Gnd
0.434 0.578
1.732 0.867
The coefficients in Table
currents do not exist in the healthy phases).
) ×
– = 0.578 1250 =
- = × =
) 0.578 1250
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C-Gnd A-B B C
0.867 1.732 1.732
0.578 1.732 1.732
24 are only relevant for ideal internal faults (that is, load
722.5A
722.5 A
Differential protection
for detailed ASCT current
24 in order to
C-A ABC
0.867 1.0
0.867 1.0
(Equation 14)
(Equation 14)
Section 6
M12138-3 v2
M12139-3 v4
141