ABB RELION 670 SERIES Applications Manual page 527

1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual
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 272.
3I0
67N
ANSI05000156 V1 EN
Figure 272: Step 3, Selectivity calculation
3I
0
³ × ×
I 1 .2 I
step3 step2
3I
02
EQUATION1204 V4 EN
where:
I is the chosen current setting for step 2 on the faulted line.
step2
Step 4
This step normally has non-directional function and a relatively long time delay. The task
for step 4 is to detect and initiate trip for ground faults with large fault resistance, for
example tree faults. Step 4 shall also detect series faults where one or two poles, of a
breaker or other switching device, are open while the other poles are closed.
Both high resistance ground faults and series faults give zero-sequence current flow in the
network. Such currents give disturbances on telecommunication systems and current to
ground. It is important to clear such faults both concerning personal security as well as risk
of fire.
The current setting for step 4 is often set down to about 100 A (primary 3I
applications definite time delay in the range 1.2 - 2.0 s is used. In other applications a
current dependent inverse time characteristic is used. This enables a higher degree of
selectivity also for sensitive ground-fault current protection.
Section 9
Current protection
3I0
x
One phase
ground fault
67N
en05000156_ansi.vsd
(Equation 466)
). In many
0
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