ABB REL650 series, Applications Manual

Page: 219 / 516

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
107 .
en05000156_ansi.vsd
67N
One phase
ground fault
3I0
67N
3I0
x
ANSI05000156 V1 EN
Figure 107: Step 3, Selectivity calculation
1
0
step3 step2
02
3I
I .2 I
3I
³ × ×
EQUATION1204 V4 EN
(Equation 125)
where:
Istep2 is the chosen current setting for step 2 on the faulted line.
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
0
). In many
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.
1MRK 506 334-UUS A
Section 7
Current protection
213
Application manual