To assure selectivity between different 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, the operation time of protections, breaker opening
time and 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
50. 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
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 50.
Section 6 1MRK 505 291-UEN A
Current protection
102
Application manual
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