Ct Ratio Matching - GE B30 Instruction Manual

DYNAMIC BUS REPLICA
If a given circuit cannot be connected to any power system element other than the protected bus, then fix its status signal
using the FlexLogic "On" constant.

9.2.2 CT ratio matching

The B30 allows for using CTs with various rated secondary currents and transformation ratios. Scaling to a common base
is performed internally by the relay. The maximum allowable ratio mismatch is 32:1. For proper setting of the differential
characteristic, 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 differential
zone: 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 dynamic
aspect of the bus differential zone. This means that even though the circuit containing the CT with the maximum rated
primary current is not connected to a given bus zone at a given time, the scaling base does not change.
9.2.2.1 Example 2
Assume 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
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• 1D CT: 1000:5
• 1E CT: 500:1
• 1F CT: 600:5
The maximum of 600, 500, 600, 1000, 500, and 600 is 1000 A, which is selected as the base upon configuration of the BUS
ZONE 1. 1 per unit (pu) represents 1000 A primary.
9-4
Figure 9-2: Dynamic bus replica mechanism
CHAPTER 9: THEORY OF OPERATION
B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL