ABB RELION 670 Series Technical Manual page 1388

Section 26
Inverse time characteristics
Time
IEC05000131 V1 EN-US
Figure 702: Inverse time overcurrent characteristics with inst. function
The inverse time characteristic makes it possible to minimize the fault clearance time and still
assure the selectivity between protections.
To assure selectivity between protections there must be a time margin between the operation
time of the protections. This required time margin is dependent of following factors, in a simple
case with two protections in series:
• Difference between pickup time of the protections to be co-ordinated
• Opening time of the breaker closest to the studied fault
• Reset times of the protections
• Margin dependent of the time delay inaccuracy of the protections
Assume we have the following network case.
A1
51
t=0
ANSI05000132 V1 EN-US
Figure 703: Selectivity steps for a fault on feeder B1
where:
t=0
t=t
1
t=t
2
t=t
3
1382
t=t t=t
1 2
is The fault occurs
is Protection B1 trips
is Breaker at B1 opens
is Protection A1 resets
B1
51
t=t
3
en05000132_ansi.vsd
1MRK 502 066-UUS B
Fault point
position
en05000131.vsd
Feeder
Time axis
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