Modern Design Principles for Numerical Busbar Differential Protection

For busbar protection, it is extremely important to have good security since an unwanted operation might have severe consequences. The unwanted operation of the bus differential relay will have the similar effect as simultaneous faults on all power system elements connected to the bus. On the other hand, the relay has to be dependable as well. Failure to operate or even slow operation of the differential relay, in case of an actual internal fault, can have fatal consequences. These two requirements are contradictory to each other. To design the differential relay to satisfy both requirements at the same time is not an easy task. Busbar protection shall also be able to dynamically include and/or exclude individual bay currents from differential zones. Therefore it must contain so-called dynamic zone selection in order to adapt to changing topology of substation for multi-zone applications. The software based dynamic zone selection ensures: 1.Dynamic linking of measured bay currents to the appropriate differential protection zone(s) as required by substation topology. Efficient merging of differential zones when required by substation topology (that is, zone interconnection or load-transfer). 2.Easy zone merging initiated externally by closing of bus-sectionalizing disconnectors. 3.Selective operation of busbar differential protection to ensure tripping only of circuit breakers connected to the faulty zone. 4.Selective tripping for circuit breaker failure protection. 5.Correct marshaling of backup-trip commands from internally integrated or external circuit breaker failure protections to all surrounding circuit breakers. 6.Easy incorporation of bus-section and/or bus-coupler bays (that is, tie-breakers) with one or two sets of CTs into the protection scheme. 7.Disconnector and/or circuit breaker status supervision. Modern design for a Busbar Differential Protection IED [10] containing six differential protection zones and fulfilling all of the above mentioned requirements will be presented in the paper.