ABB Relion REC615, Техническое руководство

Страница: 225 / 869

In many cases the above requirements can be best fulfilled by using multiple-stage
overcurrent units.
Figure 117
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 (F)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. In this way the start value is multiplied
with a predefined setting during the inrush situation and nuisance tripping can be
avoided.
Figure 117: Functionality of numerical multiple-stage overcurrent protection
The coordination plan is an effective tool to study the operation of time selective
operation characteristics. All the points mentioned earlier, required to define
the overcurrent protection parameters, can be expressed simultaneously in a
coordination plan. In
Figure 118
, the coordination plan shows an example of
operation characteristics in the LV-side incoming feeder and radial outgoing feeder.
1MRS758755 C Protection functions
REC615 & RER615
Technical Manual
225