ABB REG670 Applications Manual page 327

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
impedance values for Z
B
B.
A
Z
~
A
IED
IEC09000022 V1 EN
Figure 168: Through fault current from A to B: I
Then a fault in A has to be applied and the through fault current I
Figure 169. In order to get the maximum through fault current, the minimum value for
Z and the maximum value for Z
B
Z
~ A
Fault
IEC09000023 V1 EN
Figure 169: 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 I
( )
fA fB
EQUATION78 V1 EN
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.
in order to get the maximum through fault current from A to
I
fB
Z
L
have to be considered.
A
I
fA
A
Z
L
IED
Section 9
Current protection
B
Z
B
Fault
IEC09000022-1-en.vsd
fB
has to be calculated,
fA
B
Z
B
IEC09000023-1-en.vsd
fA
~
~
(Equation 175)
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