ABB Relion REQ650, Applications Manual

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IEC10000276-1-en.vsd
~ ~
Z
A
Z
B
Z
L
A B
IED
I
fA
Fault
IEC10000276 V1 EN
Figure 43: 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:
fA fB
Imin MAX(I , I )
³
EQ1 V1 EN
(Equation 9)
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 SPTPIOC is then:
s
I 1.3 Imin
³ ×
EQ2 V1 EN
(Equation 10)
The protection function can be used for the specific application only if this setting
value is equal to or less than the maximum fault current that the IED has to clear, fault
current (I
F
) in figure 44 .
IEC10000275-1-en.vsd
~ ~
Z
A
Z
B
Z
L
A B
IED
I
F
Fault
IEC10000275 V1 EN
Figure 44: Fault current: I
F
Section 6
1MRK 505 291-UEN A
Current protection
92
Application manual