ABB RET670 Applications Manual page 417

1MRK504116-UUS C
Application manual
The operate times of the phase overcurrent protection has to be chosen so that the fault
time is so short that protected equipment will not be destroyed due to thermal overload,
at the same time as selectivity is assured. For overcurrent protection, in a radial fed
network, the time setting can be chosen in a graphical way. This is mostly used in the
case of inverse time overcurrent protection. Figure
current curves are plotted in a diagram. The time 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
Figure 186: 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:
Section 3
IED application
186 shows how the time-versus-
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