GE b30, Instruction Manual

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CHAPTER 9: THEORY OF OPERATION DYNAMIC BUS REPLICA
B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 9-3
9
9.2 Dynamic bus replica
9.2.1 Dynamic bus replica mechanism
The B30 provides protection for one bus differential zone. The bus differential zone of the B30 allows for protecting bus
sections that include circuits that are switchable between different bus 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 ‘source-status’ pairs. The Sources feature of the B30 is a convenient
and flexible mechanism for associating input currents and voltages with protection and control elements.
The Source mechanism permits summing physical input currents and assigning the resulting sum to a Source. It is not
recommended to use this aspect of the Source mechanism for the bus differential protection. If two or more physical
currents are summed using the Source mechanism, and then used as an input to the differential protection element, the
restraining current calculated by the relay does not always reflect external fault currents properly. Consequently, the relay
lacks sufficient bias during certain external faults. Also, the directional principle and saturation detector does not always
work properly. This is not a limitation of the B30, but misapplication of Sources in conjunction with the biased differential
principle.
Normally, each Source defining the input to the bus differential zone of the B30 is associated with a single physical current
transformer bank. The only situation when two or more currents can be summed into a single Source before entering into
the bus zone is when the currents are purely load currents and cannot produce any fault current in any circumstances.
The status signal of a given ‘source-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 switches and/or breakers.
9.2.1.1 Example 1
The following figure shows an example of a circuit that can be connected to two separate bus sections. It is assumed that
each section is protected individually by two B30s. Consider the B30 as protecting the Bus Section 1. The current signals
are connected to the relay using a CT bank, say F1, and assigned to a Source, say SRC 1. The status signal of the switch is
brought into the relay as an input contact, say U7a. The input contact can be used directly (say, Cont Ip 1 On), or further
processed using the FlexLogic for contact discrepancy filtering or extra security. The pair “SRC 1 - Cont Ip 1 On” defines the
input to the Bus Zone 1.