GE Multilin B30 Bus Differential System 8-38 THEORY OF OPERATION 8.2 DYNAMIC BUS REPLICA8If a given circuit cannot be connected to any power system element other than the protected bus, then its status signalshould be fixed using the FlexLogic “On” constant.8.2.2 CT RATIO MATCHINGThe B30 allows for using CTs with various rated secondary currents and transformation ratios. Scaling to a common base isperformed internally by the relay. The maximum allowable ratio mismatch is 32:1. For proper setting of the differential char-acteristic, it is imperative to understand the common base used by the relay.The B30 scales the secondary currents to the maximum primary current among the CTs defining a given bus differentialzone: 1 per unit corresponds to the highest rated primary current.The scaling base is selected automatically by the relay during the configuration phase and is not affected by the dynamicaspect of the bus differential zone. This means that even though the circuit containing the CT with the maximum rated pri-mary current is not connected to a given bus zone at a given time, the scaling base does not change.EXAMPLE 2:Assume the CTs installed in the circuit defining the BUS ZONE 1 have the following ratings:• 1A CT: 600:5• 1B CT: 500:1• 1C CT: 600:5• 1D CT: 1000:5• 1E CT: 500:1• 1F CT: 600:5The maximum of 600, 500, 600, 1000, 500, and 600 is 1000 A which is therefore selected as the base upon configurationof the BUS ZONE 1. 1 per unit (pu) represents 1000A primary.Note that independently from the ratios and rated secondary currents, the per unit values of the differential current retaintheir original meaning regardless of the distribution of the differential current between individual circuits. Assume, for exam-ple, that the differential current is fed by the inputs 1A and 1B exclusively, and consider two situations:• The 1A input supplies 1kA primary, and the 1B input supplies 2kA primary. The currents are in phase. The pu current ofthe 1A source is 1000 A : (600:5) : 5A/pu = 1.67 pu. The pu current of the 1B source is 2000 A : (500:1) : 1A/pu = 4.00pu. The pu differential current is (1000A + 2000A) : 1000A = 3.00 pu.• The 1A input supplies 2kA primary, and the 1B input supplies 1kA primary. The currents are in phase. The pu current ofthe 1A source is 2000 A : (600:5) : 5A/pu = 3.33 pu. The pu current of the 1B source is 1000 A : (500:1) : 1A/pu = 2.00pu. The pu differential current is (1000A + 2000A) : 1000A = 3.00 pu.
