ABB RELION RET670 Applications Manual page 535

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
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tfunc1
n
tfunc2 n
0.01
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IEC05000204 V2 EN-US
Figure 263: 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 15-60 ms
time:
Protection resetting time: 15-60 ms
Breaker opening time: 20-120 ms
Time-current curves
Str
n
Fault Current
Section 9
Current protection
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