ABB 615 Series ANSI, Техническое руководство

Страница: 124 / 713

1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 117
Technical Manual
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 multiple-stage
overcurrent units. Figure 48 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 51P operates in IDMT-mode and the two higher stages 50P-1/2
and 50P-3 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 pickup value multiplying input of the instantaneous stage. In this way, the
pickup value is multiplied with a predefined setting during the inrush situation and
nuisance tripping can be avoided.