ABB REC670 Applications Manual page 176

Section 8
Current protection
170
The different steps can be described as follows.
Step 1
This step has directional instantaneous function. The requirement is that
overreaching of the protected line is not allowed.
3I
0
IN
IEC05000150 V4 EN-US
Figure 71: Step 1, first calculation
The residual current out on the line is calculated at a fault on the remote busbar
(one- or two-phase-to-earth fault). To assure selectivity it is required that step 1
shall not give a trip at this fault. The requirement can be formulated according to
Equation 39.
³ ×
I 1.2 3I (remote busbar)
step1 0
EQUATION1199 V3 EN-US
As a consequence of the distribution of zero sequence current in the power system,
the current to the protection might be larger if one line out from the remote busbar
is taken out of service, see Figure 72.
1MRK 511 401-UEN A
One- or two-phase earth-fault
or unsymmetric short circuit
without earth connection
IEC05000150-3-en.vsd
Bay control REC670 2.2 IEC
Application manual
M15282-123 v6
(Equation 39)