ABB REB670 Applications Manual page 162

Section 6
Differential protection
156
value must be multiplied by a coefficient shown in the table
actual primary pickup value.
Table 24: Pickup coefficients for Summated Differential Protection
Type of fault A-Gnd
ASCT end 0.434
connected
ASCT series 1.732
connected
The coefficients in table
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 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 – = 0.578 1250
Pickup
EQUATION1106 V1 EN
- =
I ( C Gnd ) 0.578 1250
Pickup
EQUATION1784 V1 EN
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).
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:
B-Gnd C-Gnd A-B
0.578 0.867 1.732
0.867 0.578 1.732
24 are only relevant for ideal internal faults (that
×
= 722.5A
× =
722.5 A
1MRK 505 337-UUS -
24 in order to calculate the
BC C-A ABC
1.732 0.867 1.0
1.732 0.867 1.0
(Equation 14)
(Equation 14)
Application manual