ABB REC650 - Page 97

A fault in C has to be applied, and then the maximum current seen from the IED (I
M
) on
the healthy line (this applies for single-phase-to-ground and two-phase-to-ground faults)
is calculated.
~ ~
Z
A
Z
B
Z
L1
A B
I
M
Fault
IED
Z
L2
M
C
Line 1
Line 2
ANSI09000025_2_en.vsd
ANSI09000025 V2 EN
Figure 34: Two parallel lines. Influence from parallel line to the through fault current:
I
M
The minimum theoretical current setting for the overcurrent protection function (Imin)
will be:
Imin MAX I
fA
I
fB
I
M
, ,( )³
EQUATION82 V1 EN
(Equation 26)
Where I
fA
and I
fB
have been described in the previous paragraph. Considering the safety
margins mentioned previously, the minimum setting (Is) for the instantaneous phase
overcurrent protection 3-phase output is then:
Is
³1.3·Imin
EQUATION83 V2 EN (Equation 27)
The protection function can be used for the specific application only if this setting value
is equal or less than the maximum phase fault current that the IED has to clear.
The IED setting value Pickup is given in percentage of the primary base current value,
IBase. The value for Pickup is given from this formula:
100
Is
Pickup
IBase
= ×
ANSIEQUATION1147 V1 EN (Equation 28)
1MRK 511 286-UUS A Section 6
Current protection
91
Application manual