Micom P721, Technical Manual

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Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 37/52
The trip command should be configured as Trip [87N] tDiff, so that relay output 1 is asserted
when the differential element operates:
4.2 Busbar applications
FIGURE 31 shows an application of the P72x for high impedance protection of a typical
132 kV double bus generating station, consisting of two 100 MVA generators and step-up
transformers, two bus-couplers and four overhead transmission lines. Each busbar is
sectionalized and therefore gives a requirement for four discriminating zones and one overall
check zone. Therefore, for this application five P72x relays are required. One P72x is
configured as the discriminating zone for each bus bar section and one P72x is configured
as the overall check zone.
Any circuit in FIGURE 31 may be transferred from one bar to the other by isolator switches.
Auxiliary contacts from the isolator must transfer the current transformers between the
zones. The relay trip output must also be switched to the appropriate zone. These auxiliary
switches should close before the main isolator closes and should open after the main isolator
opens to ensure stability during switching operation.
100MVA 15kV
100MVA 132/15kV
Main
reserve
132kV
P1520ENa
FIGURE 31: THREE PHASE AND EARTH FAULT PROTECTION OF A DOUBLE
BUSBAR GENERATING STATION
FIGURE 32 shows the switching configuration for any of the feeders or transformers given in
FIGURE 31: Only normally open auxiliary contacts a and b are needed in the CT secondary
circuit for switching purposes.