Time-current curves
Fault Current
10
10
0.01
10000
en05000204.ai
Str
tfunc1
n
n
tfunc2
Trip time
n
IEC05000204 V2 EN
Figure 175: Fault time with maintained selectivity
The operation time can be set individually for each overcurrent protection.
To assure selectivity between different protection functions in the radial network,
there has to be a minimum time difference Dt between the time delays of two
protections. To determine the shortest possible time difference, the operation time of
the protection, the breaker opening time and the protection resetting time must be
known. These time delays can vary significantly between different protective
equipment. The following time delays can be estimated:
Protection operation
time:
15-60 ms
Protection resetting time: 15-60 ms
Breaker opening time: 20-120 ms
Example for time coordination
Assume two substations A and B directly connected to each other via one line, as
shown in the Figure 176. Consider 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
overcurrent protection will start and subsequently trip, and the overcurrent protection
1MRK 502 071-UEN - Section 9
Current protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 333
Application manual
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