CHAPTER 9: THEORY OF OPERATION DYNAMIC BUS REPLICA
B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 9-3
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9.2 Dynamic bus replica
9.2.1 Dynamic bus replica mechanism
The bus differential zones of the B90 allow for protecting bus sections that include circuits that are switchable between
different sections. Proper relay operation is achieved by associating a status signal with each input current. This
mechanism is referred to as a dynamic bus replica.
The dynamic bus zone is programmed as a number of CT input–status–direction entries.
The status signal of a given CT input–status pair of the dynamic bus replica is a FlexLogic operand created to indicate
whether or not the associated circuit (current) is connected to the protected bus zone. Normally, the status signals are to
be created from input contacts wired to appropriate auxiliary contacts of isolator switches or breakers.
9.2.2 CT ratio matching
The B90 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 B90 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 1
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
• 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.
9.3 Differential principle
9.3.1 Biased differential characteristic
The B90 uses a dual-slope dual-breakpoint operating characteristic as shown in the following figure.
The
PICKUP setting is provided to cope with spurious differential signals when the bus carries a light load and there is no
effective restraining signal.
The first breakpoint (
LOW BPNT) is provided to specify the limit of guaranteed linear operation of the CTs in the most
unfavorable conditions, such as high residual magnetism left in the magnetic cores or multiple autoreclosure shots. This
point defines the upper limit for the application of the first slope (
LOW SLOPE).
The second breakpoint (HIGH BPNT) is provided to specify the limits of operation of the CTs without any substantial
saturation. This point defines the lower limit for the application of the second slope (HIGH SLOPE).
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