ABB RET670 Applications Manual page 404

Section 3
IED application
398
A
Z
~
A
Fault
ANSI09000023 V1 EN
Figure 180: 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
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:
I ³
1.3.I
s m in
EQUATION79 V2 EN
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 181.
I
fA
Z
L
IED
1MRK504116-UUS C
B
Z
B
ANSI09000023-1-en.vsd
fA
Application manual
~
(Equation 334)
(Equation 335)
in
F