ABB Relion 670 Series Applications Manual page 148

Section 8
Current protection
tfunc1
tfunc2 n
0.01
IEC05000204 V2 EN-US
Figure 63: 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:
Protection resetting
time:
Breaker opening time:
Example for time coordination
Assume two substations A and B directly connected to each other via one line, as shown in the
Figure 64. 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 of IED A1 must have a delayed operation in order
to avoid maloperation. The sequence of events during the fault can be described using a time axis
shown in Figure 64.
142
10
n
10
15-60 ms
15-60 ms
20-120 ms
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Time-current curves
Str
n
Fault Current
1MRK 511 401-UUS F
10000
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Bay control REC670
Application manual