ABB Relion 670 Series Applications Manual page 127

1MRK 511 358-UUS A
~
ANSI09000022 V1 EN-US
Figure 51:
Then a fault in A has to be applied and the through fault current I
In order to get the maximum through fault current, the minimum value for Z
value for Z
~
ANSI09000023 V1 EN-US
Figure 52: 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
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 3-phase
output is then:
³
I 1.3
s
EQUATION79 V3 EN-US
Application manual
A
Z
A
IED
Through fault current from A to B: I
have to be considered.
A
A
Z
A
IED
Fault
,
( I )
fA fB
×
I
min
I
fB
B
Z
Z
L B
Fault
fB
I
fA
B
Z
L
fA
~
ANSI09000022-1-en.vsd
has to be calculated, figure 52.
fA
and the maximum
B
Z
~
B
ANSI09000023-1-en.vsd
Section 7
Current protection
(Equation 22)
(Equation 23)
121