ABB REL650 Applications Manual page 218

Section 7
Current protection
212
3I
0
50/51N
67N
ANSI05000154 V2 EN
Figure 105: 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 123.
³ ×
I 0.7 3I (at remote busbar)
step2 0
EQUATION1202 V4 EN
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 106.
3I
0
50/51N
67N
ANSI05000155 V2 EN
Figure 106: Step 2, selectivity calculation
A second criterion for step 2 is according to equation 124.
3I
0
³ × ×
I 1 .2 I
step2 step1
3I
0 1
EQUATION1203 V4 EN
where:
Istep1 is the current setting for step 1 on the faulted line.
One- or two-phase ground-fault
ANSI05000154_2_en.vsd
3I
0x
One phase-ground-fault
50/51N
67N
ANSI05000155_2_en.vsd
1MRK 506 334-UUS A
(Equation 123)
(Equation 124)
Application manual