ABB REC670 Applications Manual page 188

Section 8
Current protection
182
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 0
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 75.
3I
0
50/51N
67N
ANSI05000155 V2 EN-US
Figure 75: Step 2, selectivity calculation
A second criterion for step 2 is according to equation 43.
3I
0
³ × ×
I 1 .2 I
step2 step1
3I
0 1
EQUATION1203 V4 EN-US
where:
I is the current setting for step 1 on the faulted line.
step1
Step 3
This step has directional function and a time delay slightly larger than step 2, often 0.8
s. Step 3 shall enable selective trip of ground faults having higher fault resistance to
ground, compared to step 2. The requirement on step 3 is selectivity to other ground-
fault protections in the network. One criterion for setting is shown in Figure 76.
1MRK 511 401-UUS A
3I
0x
One phase-ground-fault
50/51N
67N
ANSI05000155_2_en.vsd
Bay control REC670 2.2 ANSI
(Equation 42)
(Equation 43)
M15282-164 v6
Application manual