Radial outgoing feeder overcurrent protection
The basic requirements for feeder overcurrent protection are adequate sensitivity and
operation speed taking into account the minimum and maximum fault current levels
along the protected line, selectivity requirements, inrush currents and the thermal and
mechanical withstand of the lines to be protected.
In many cases the above requirements can be best fulfilled by using a multiple-stage
over current units.
Figure 64 shows an example of this. A brief coordination study
has been carried out between the incoming and outgoing feeders.
The protection scheme is implemented with three-stage numerical over current
protection, where the low-set stage PHLPTOC operates in IDMT-mode and the two
higher stages PHHPTOC and PHIPTOC in DT-mode. Also the thermal withstand of
the line types along the feeder and maximum expected inrush currents of the feeders
are shown. Faults occurring near the station, where the fault current levels are the
highest, are cleared rapidly by the instantaneous stage in order to minimize the effects
of severe short circuit faults. The influence of the inrush current is taken into
consideration by connecting the inrush current detector to the start value multiplying
input of the instantaneous stage. By this way the start value is multiplied with a
predefined setting during the inrush situation and nuisance tripping can be avoided.
Section 5 1MRS756378 D
Protection functions
138 REF615
Application Manual
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