A higher value of step 1 might be necessary if a big power transformer (Y0/D) at remote bus
bar is disconnected.
A special case occurs at double circuit lines, with mutual zero-sequence impedance between
the parallel lines, see figure
IN
>
One phase-to-earth fault
3I
0
IEC05000152-en-2.vsd
IEC05000152 V2 EN-US
Figure 279: Step 1, third calculation
In this case the residual current out on the line can be larger than in the case of earth fault on
the remote busbar.
step1
0
I
1.2 3I
³
×
EQUATION1201 V3 EN-US
(Equation 486)
The current setting for step 1 is chosen as the largest of the above calculated residual
currents, measured by the protection.
Step 2
M15282-144 v6
This step has directional function and a short time delay, often about 0.4 s. Step 2 shall
securely detect all earth faults on the line, not detected by step 1.
IN
>
One- or two-phase earth-fault
3I
0
IEC05000154-en-2.vsd
IEC05000154 V2 EN-US
Figure 280: Step 2, check of reach calculation
Section 8
1MRK 505 343-UEN B
Current protection
426
Application manual
Содержание Relion 670 series
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