ABB Relion 670 Series Applications Manual page 170

Section 7
Current protection
shortest fault time with maintained selectivity. Selectivity is assured if the time difference between the
curves is larger than a critical time difference.
tfunc1
tfunc2 n
0.01
IEC05000204 V2 EN-US
Figure 93:
The operation time can be set individually for each overcurrent protection. To assure selectivity between
different protective protections, in the radial network, there have to be a minimum time difference Dt
between the time delays of two protections. The minimum time difference can be determined for different
cases. To determine the shortest possible time difference between we must have knowledge about
operation time of protections, breaker opening time and protection resetting time. These time delays can
vary significantly between different pieces of equipment. The following time delays can be estimated:
protection operation
time:
protection resetting time:
Breaker opening time:
Example
Assume two substations A and B directly connected to each other via one line, as shown in the figure
below. We study a fault located at another line from the station B. The fault current to the overcurrent
protection of IED B1 has a magnitude so that the protection will have instantaneous function. The
overcurrent protection of IED A1 must have a delayed function. The sequence of events during the fault
can be described using a time axis, see figure 94.
164
10
n
10
Fault time with maintained selectivity
15-60 ms
15-60 ms
20-120 ms
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Time-current curves
Str
n
Fault Current
1MRK 505 370-UUS Rev. K
10000
en05000204.ai
Busbar protection REB670
Application manual