ABB RET650 Applications Manual page 125

1MRK 504 169-UEN A
Transformer protection RET650 2.2 IEC
Application manual
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 = 1.3 × I
s min
EQUATION285 V3 EN-US
In case of parallel lines with zero sequence mutual coupling a fault on the parallel
line, as shown in Figure 50, should be calculated.
A
Z
~
A
Fault
IED
IEC09000025 V1 EN-US
Figure 50: Two parallel lines. Influence from parallel line to the through fault
current: I
The minimum theoretical current setting (Imin) will in this case be:
³ , ,
I m in M A X I ( I I )
fA fB M
EQUATION287 V1 EN-US
Where:
I and I have been described for the single line case.
fA fB
Considering the safety margins mentioned previously, the minimum setting (Is) is:
I = 1.3 × I
s min
EQUATION288 V3 EN-US
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:
Line 1
C B
Z
L1
M
Z
L2
I
M
Line 2
M
Section 7
Current protection
(Equation 27)
Z
~
B
IEC09000025-1-en.vsd
(Equation 28)
(Equation 29)
119