ABB REC650 Applications Manual page 95

1MRK 511 286-UUS A
Application manual
A
Z
~
A
IED
ANSI09000022 V1 EN
Figure 31: Through fault current from A to B: I
Then a fault in A has to be applied and the through fault current I
figure 32. In order to get the maximum through fault current, the minimum value for Z
and the maximum value for Z
A
Z
~
A
Fault
ANSI09000023 V1 EN
Figure 32: 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.
I
fB
Z
L
Fault
have to be considered.
A
I
fA
Z
L
IED
Section 6
Current protection
B
Z
~
B
ANSI09000022-1-en.vsd
fB
has to be calculated,
fA
B
Z
B
ANSI09000023-1-en.vsd
fA
B
~
(Equation 23)
89