ABB REL650 Applications Manual page 182

Section 7
Current protection
176
Then a fault in A has to be applied and the through fault current I
figure 83. In order to get the maximum through fault current, the minimum value for Z
and the maximum value for Z
A
Z
~
A
IED
Fault
ANSI10000276 V1 EN
Figure 83: 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
Im in MAX(I , I )
fA fB
EQ1 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. The minimum primary setting (Is) for
SPTPIOC (50) is then:
³ ×
I 1.3 Im in
s
EQ2 V1 EN
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 ) in figure 84.
F
have to be considered.
A
I
fA
B
Z
L
fA
1MRK 506 334-UUS A
has to be calculated,
fA
Z
~
B
ANSI10000276-1-en.vsd
(Equation 100)
(Equation 101)
Application manual
B