The function shall not operate for any of the calculated currents to the protection. The
minimum theoretical current setting (Imin) will be:
EQUATION284 V2 EN-US (Equation 427)
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:
EQUATION285 V3 EN-US (Equation 428)
In case of parallel lines with zero sequence mutual coupling a fault on the parallel line,
as shown in Figure 267, should be 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-US
Figure 267: Two parallel lines. Influence from parallel line to the through fault
current: I
M
The minimum theoretical current setting (Imin) will in this case be:
I m in M A X I
fA
I
fB
I
M
, ,( )³
EQUATION287 V1 EN-US (Equation 429)
Where:
I
fA
and I
fB
have been described for the single line case.
1MRK 504 163-UUS A Section 9
Current protection
Transformer protection RET670 2.2 ANSI 533
Application manual
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