ABB Relion REC670 - Page 109

IEC09000023-1-en.vsd
~ ~
Z
A
Z
B
Z
L
A B
IED
I
fA
Fault
IEC09000023 V1 EN-US
Figure 40: Through fault current from B to A: I
fA
The IED must not trip for any of the two through-fault currents. Hence the minimum
theoretical current setting (Imin) will be:
Imin MAX I
fA
I
fB
,( )³
EQUATION78 V1 EN-US (Equation 22)
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:
min
1.3
s
I I³ ×
EQUATION79 V3 EN-US (Equation 23)
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
F
in figure 41.
IEC09000024-1-en.vsd
~ ~
Z
A
Z
B
Z
L
A B
IED
I
F
Fault
IEC09000024 V1 EN-US
Figure 41: Fault current: I
F
100
Is
IP
IBase
>>= ×
EQUATION1147 V3 EN-US
(Equation 24)
1MRK 511 358-UEN A Section 7
Current protection
103
Application manual