ABB Relion REC670, Applications Manual

Page: 189 / 658

en05000156_ansi.vsd
67N
One phase
ground fault
3I0
67N
3I0
x
ANSI05000156 V1 EN-US
Figure 76: Step 3, Selectivity calculation
1
0
step3 step2
02
3I
I .2 I
3I
³ × ×
EQUATION1204 V4 EN-US
(Equation 44)
where:
I
step2
is the chosen current setting for step 2 on the faulted line.
Step 4
M15282-177 v4
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.
8.5 Four step directional negative phase sequence
overcurrent protection NS4PTOC (46I2)
GUID-E8CF8AA2-AF54-4FD1-A379-3E55DCA2FA3A v1
1MRK 511 401-UUS A
Section 8
Current protection
Bay control REC670 2.2 ANSI 183
Application manual