ABB REC670 Applications Manual page 149

1MRK 511 401-UEN A
Bay control REC670 2.2 IEC
Application manual
should be done using the minimum source impedance values for Z
maximum source impedance values for Z
fault current from A to B.
A
Z
~ A
IED
IEC09000022 V1 EN-US
Figure 55: Through fault current from A to B: I
Then a fault in A has to be applied and the through fault current I
calculated, Figure 56. In order to get the maximum through fault current, the
minimum value for Z and the maximum value for Z
B
Z
~
A
Fault
IEC09000023 V1 EN-US
Figure 56: Through fault current from B to A: I
The IED must not trip for any of the two through-fault currents. Hence the
minimum theoretical current setting (Imin) will be:
³ ,
Imin MAX I I
( )
fA fB
EQUATION78 V1 EN-US
A safety margin of 5% for the maximum protection static inaccuracy and a safety
margin of 5% for the maximum possible transient overreach have to be introduced.
An additional 20% is suggested due to the inaccuracy of the instrument
transformers under transient conditions and inaccuracy in the system data.
in order to get the maximum through
B
I
fB
Z
L
I
fA
A
Z
L
IED
Section 8
Current protection
and the
A
B
Z
B
Fault
IEC09000022-1-en.vsd
fB
has to be
fA
have to be considered.
A
B
Z
B
IEC09000023-1-en.vsd
fA
~
~
(Equation 21)
143