ABB REC650 - Page 95

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