ABB ret650 - Page 125

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:
I
s
= 1.3 × I
min
EQUATION285 V3 EN-US (Equation 27)
In case of parallel lines with zero sequence mutual coupling a fault on the parallel
line, as shown in Figure
50, should be calculated.
IEC09000025-1-en.vsd
~ ~
Z
A
Z
B
Z
L1
A B
I
M
Fault
IED
Z
L2
M
C
Line 1
Line 2
IEC09000025 V1 EN-US
Figure 50: 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 28)
Where:
I
fA
and I
fB
have been described for the single line case.
Considering the safety margins mentioned previously, the minimum setting (Is) is:
I
s
= 1.3 × I
min
EQUATION288 V3 EN-US (Equation 29)
The IED setting value IN>> is given in percent of the primary base current value,
IBase. The value for IN>> is given by the formula:
1MRK 504 169-UEN A Section 7
Current protection
Transformer protection RET650 2.2 IEC 119
Application manual