ABB Relion REC670 Applications Manual page 109

1MRK 511 358-UEN A
~
IEC09000023 V1 EN-US
Figure 40: 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
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.
The minimum primary setting (Is) for the instantaneous phase overcurrent protection 3-phase
output is then:
³
I 1.3
s
EQUATION79 V3 EN-US
The protection function can be used for the specific application only if this setting value is
equal to or less than the maximum fault current that the IED has to clear, I
~
IEC09000024 V1 EN-US
Figure 41: Fault current: I
>>=
IP
EQUATION1147 V3 EN-US
Application manual
A
Z
A
IED
Fault
,
( I )
fA fB
×
I
min
I
F
A
Z
A
IED
F
Is
×
100
IBase
I
fA
B
Z
L
fA
B
Z Z
L B
Fault
Current protection
Z
~
B
IEC09000023-1-en.vsd
(Equation 22)
(Equation 23)
in figure 41.
F
~
IEC09000024-1-en.vsd
(Equation 24)
Section 7
103