GE BUS1000, Instructions Manual

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3. OPERATION PRINCIPLES
3-4 BUS1000 Busbar Protection GEK-98514B
3.2.2. BEHAVIOUR WITH EXTERNAL FAULTS
3.2.2.1. Without saturation
During the time prior to the saturation of any of the main C.T's and assuming ideal conditions for an external
fault, the fault current flows through input circuits of the various positions without any differential current.
In these conditions the value of V
D
= 0 and in our case, the value of V
F
will be proportional to twice the fault
current. See fig. 2.
3.2.2.2. With saturation
In the case of an external fault, saturation may be produced in the current transformers associated to any of
the protected bus positions. In this case, the inputs' currents will not be compensated; thus a differential
current will be produced which must not lead to the operation of the relay. The combination of the percent
restraint operation characteristic together with the R
E
stabilization resistance in the differential circuit ensures
the correct behaviour of the unit in these circumstances.
The worst case from the point of view of the possibility of false operations with external faults is that of a
complete saturation (total absence of signal in the secondary) of only one of the main C.T's while the rest
behave correctly.
In our case, the equivalent circuit is shown in fig. 4. Here, the fault current provided by the rest of the current
transformers is divided between the totally saturated I
X
circuit and the I
D
differential circuit in an inversely
proportional way to the resistance of every circuit.
Thus, when the R
E
resistance value increases in the differential circuit, the differential current flowing
erroneously in case of saturation of a current transformer decreases. In the same way, when the K restraint
percent value increases, a greater differential current is allowed without providing a trip in the unit since V
F
will increase.