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 multiple-stage
overcurrent units.
Figure 74 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 overcurrent
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.
1YHT530004D05 D Section 4
Protection functions
615 series 149
Technical Manual
To Next Page
Loading...
