ABB Relion 670 Series Applications Manual page 138

Section 7
Current protection
IEC05000204 V1 EN-US
Figure 58: Fault time with maintained selectivity
The operation time can be set individually for each overcurrent protection.
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:
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 59. 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 59.
132
15-60 ms
15-60 ms
20-120 ms
1MRK 511 358-UUS A
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Application manual