ABB REQ650 - Page 120

~ ~
Z
A
Z
B
Z
L
A B
IED
I
fB
Fault
IEC09000022-1-en.vsd
IEC09000022 V1 EN
Figure 56: Through fault current from A to B: I
fB
IEC09000023-1-en.vsd
~ ~
Z
A
Z
B
Z
L
A B
IED
I
fA
Fault
IEC09000023 V1 EN
Figure 57: Through fault current from B to A: I
fA
The function shall not operate for any of the calculated currents to the protection. The
minimum theoretical current setting (Imin) will be:
Imin MAX I
fA
I
fA
,( )³
EQUATION284 V1 EN (Equation 25)
A safety margin of 5% for the maximum static inaccuracy and a safety margin of 5%
for maximum possible transient overreach have to be introduced. An additional 20%
is suggested due to inaccuracy of instrument transformers under transient conditions
and inaccuracy in the system data.
The minimum primary current setting (Is) is:
m
1.3.Is inI ³
EQUATION285 V2 EN (Equation 26)
In case of parallel lines with zero sequence mutual coupling a fault on the parallel line,
as shown in figure 58, should be calculated.
Section 6 1MRK 505 291-UEN A
Current protection
114
Application manual