ABB Relion REC670 Applications Manual page 132

Section 7
Current protection
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 57.
3I
IN
One phase-to-earth fault
IEC05000152 V2 EN-US
Figure 57: 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.
³
I 1.2 3I
step1
EQUATION1201 V3 EN-US
The current setting for step 1 is chosen as the largest of the above calculated residual
currents, measured by the protection.
Step 2
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.
3I
0
IN
>
IEC05000154 V2 EN-US
Figure 58: Step 2, check of reach calculation
126
0
>
×
0
IEC05000152-en-2.vsd
One- or two-phase earth-fault
IEC05000154-en-2.vsd
1MRK 511 358-UEN A
(Equation 41)
M15282-144 v6
Application manual