The residual current out on the line is calculated at a fault on the remote busbar (one-
or two-phase-to-ground 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.
step1 0I 1.2 3I (remote busbar)³ ×
EQUATION1199 V3 EN-US (Equation 39)
As a consequence of the distribution of zero sequence current in the power system, the
current to the protection might be lar
ger if one line out from the remote busbar is taken
out of service, see Figure
72.
en05000151_ansi.vsd
67N
One- or two-phase-ground-fault
ANSI05000151 V2 EN-US
Figure 72: Step 1, second calculation. Remote busbar with, one line taken out of
service
The requirement is now according to Equation
40.
step1 0I 1.2 3I (remote busbar with one line out)³ ×
EQUATION1200 V3 EN-US (Equation 40)
A higher value of step 1 might be necessary if a big power transformer (Y0/D) at
remote bus bar is disconnected.
A special case occurs at double circuit lines, with mutual zero-sequence impedance
between the parallel lines, see Figure 73.
Section 8 1MRK 511 401-UUS A
Current protection
180 Bay control REC670 2.2 ANSI
Application manual
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