ABB Relion 670 Series Applications Manual page 153

1MRK 511 358-UUS A
In this case the residual current out on the line can be larger than in the case of ground 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 ground faults on the line, not detected by step 1.
3I
0
50/51N
67N
ANSI05000154 V2 EN-US
Figure 70: Step 2, check of reach calculation
The residual current, out on the line, is calculated at an operational case with minimal ground-fault
current. The requirement that the whole line shall be covered by step 2 can be formulated
according to equation 42.
³
I 0.7 3I (at remote busbar)
step2
EQUATION1202 V4 EN-US
To assure selectivity the current setting must be chosen so that step 2 does not operate at step 2
for faults on the next line from the remote substation. Consider a fault as shown in Figure 71.
3I
0
50/51N
67N
ANSI05000155 V2 EN-US
Figure 71:
A second criterion for step 2 is according to equation 43.
Application manual
×
0
×
0
Step 2, selectivity calculation
One- or two-phase ground-fault
ANSI05000154_2_en.vsd
3I
0x
One phase-ground-fault
50/51N
67N
ANSI05000155_2_en.vsd
Section 7
Current protection
(Equation 41)
M15282-144 v6
(Equation 42)
147