ABB RELION 670 Series Applications Manual page 159

1MRK 505 337-UUS A
can be chosen in a graphical way. This is mostly used in the case of inverse time overcurrent
protection. Figure
setting is chosen to get the shortest fault time with maintained selectivity. Selectivity is assured if
the time difference between the curves is larger than a critical time difference.
IEC05000204 V1 EN-US
Figure 80: 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 81. Consider a fault located at another line from the station B. The fault current to the
Application manual
80 shows how the time-versus-current curves are plotted in a diagram. The time
15-60 ms
15-60 ms
20-120 ms
Current protection
en05000204.wmf
Section 7
153