ABB RER670 - Page 165

~ ~
Z
A
Z
B
Z
L
A B
IED
I
fB
Fault
IEC09000022-1-en.vsd
IEC09000022 V1 EN-US
Figure 47: 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
48. 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.
IEC09000023-1-en.vsd
~ ~
Z
A
Z
B
Z
L
A B
IED
I
fA
Fault
IEC09000023 V1 EN-US
Figure 48: 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-US (Equation 35)
A safety margin of 5% for the maximum protection static inaccuracy and a safety
mar
gin 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:
1MRK 506 375-UEN A Section 8
Current protection
Railway application RER670 2.2 IEC 159
Application manual