Page 372 - Practical Power System and Protective Relays Commissioning

368 Practical Power System and Protective Relays Commissioning

polarity on one side of the power transformer CTs. The secondary relay
will pickup as the differential current is increased, with the relay before
the test isolated from the tripping circuit, then the current polarity of the
CTs is restored.
2. Busbar differential protection
With the trip contacts of busbar protection are isolated we can measure
the differential current on the relay by shorting the CT input of each feeder
connected to the specified zone of protection. We should have a rise in dif-
ferential current and the relay should issue a trip signal to the bus zone
which this feeder is connected to.