ABB REB670 Series, Applications Manual

Page: 145 / 476

6.1.6.4 Main CT ratio mismatch correction
M12138-3 v2
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
M12139-3 v4
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 start
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 24 in order to calculate the actual primary start
value.
Table 24: Start coefficients for Summated Differential Protection
Type of fault L1-Gnd L2-Gnd L3-Gnd L1-L2 L2-L3 L3-L1 L1L2L3
ASCT end
connected
0.434 0.578 0.867 1.732 1.732 0.867 1.0
ASCT series
connected
1.732 0.867 0.578 1.732 1.732 0.867 1.0
The coefficients in Table 24 are only relevant for ideal internal faults (that is,
load currents do not exist in the healthy phases).
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 start value in case of L3-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
Pickup
L3 Gnd –
( )
0.578 1250
×
722.5A = =
EQUATION1106 V1 EN-US
(Equation 11)
( ) 0.578 1250 722.5
Pickup
I C Gnd A
- = × =
EQUATION1784 V1 EN-US
(Equation 11)
This means that if 722.5 primary amperes is injected only in phase L3 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 start (that is, trip).
SEMOD127594-37 v4
In addition to busbar protection differential zones, the IED can incorporate other additional
functions and features. If and how they can be used together with summation busbar
protection design is shown in Table 25 :
1MRK 505 370-UEN D Section 6
Differential protection
Busbar protection REB670 139
Application manual