9-4 B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUALDYNAMIC BUS REPLICA CHAPTER 9: THEORY OF OPERATION9Figure 9-2: Dynamic bus replica mechanismIf a given circuit cannot be connected to any power system element other than the protected bus, then fix its status signalusing the FlexLogic “On” constant.9.2.2 CT ratio matchingThe B30 allows for using CTs with various rated secondary currents and transformation ratios. Scaling to a common baseis performed internally by the relay. The maximum allowable ratio mismatch is 32:1. For proper setting of the differentialcharacteristic, 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 ratedprimary current is not connected to a given bus zone at a given time, the scaling base does not change.9.2.2.1 Example 2Assume that 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 selected as the base upon configuration of the BUSZONE 1. 1 per unit (pu) represents 1000 A primary.
