The different steps can be described as follows.
Step 1
M15282-123 v6
This step has directional instantaneous function. The requirement is that
overreaching of the protected line is not allowed.
IEC05000150-3-en.vsd
IN
3I
0
One- or two-phase earth-fault
or unsymmetric short circuit
without earth connection
IEC05000150 V4 EN-US
Figure 85: 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
113.
step1 0
I 1.2 3I (remote busbar)³ ×
EQUATION1199 V3 EN-US (Equation 113)
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 86.
Section 8 1MRK 505 393-UEN B
Current protection
176 Line differential protection RED650 2.2 IEC
Application manual
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