ABB RELION RET670 Applications Manual page 522

Section 9
Current protection
516
A
Z
~
A
Fault
ANSI09000023 V1 EN-US
Figure 257: 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.
The minimum primary setting (Is) for the instantaneous phase overcurrent protection is
then:
³ ×
I 1.3 I
s min
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
Figure 258.
I
fA
Z
L
IED
1MRK 504 163-UUS A
B
Z
B
ANSI09000023-1-en.vsd
fA
Transformer protection RET670 2.2 ANSI
Application manual
~
(Equation 415)
(Equation 416)
in
F