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Section 7
Current protection
tfunc1
tfunc2 n
0.01
IEC05000204 V2 EN-US
Figure 92: Fault time with maintained selectivity
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 93.
158
10
n
10
15-60 ms
15-60 ms
20-120 ms
Time-current curves
Str
n
Fault Current
1MRK 505 370-UEN D
10000
en05000204.ai
Busbar protection REB670
Application manual
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