ABB Relion 670 Series Applications Manual page 164

Section 8
Current protection
³
I 1
step2
EQUATION1203 V4 EN-US
where:
I
step1
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 77.
3I0
67N
ANSI05000156 V1 EN-US
Figure 77:
³
I 1
step3
EQUATION1204 V4 EN-US
where:
I
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
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.
158
3I
0
× ×
.2 I
step1
3I
0 1
is the current setting for step 1 on the faulted line.
Step 3, Selectivity calculation
3I
0
× ×
.2 I
step2
3I
02
is the chosen current setting for step 2 on the faulted line.
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3I0
x
One phase
ground fault
67N
en05000156_ansi.vsd
). In many applications
0
1MRK 511 401-UUS F
(Equation 46)
M15282-164 v6
(Equation 47)
M15282-177 v4
Bay control REC670
Application manual