I am doing a power system study for a client. Our software indicates that the HGC3 breaker that they have is over dutied at 92kA. I have read articles that indicate that we should separated the utility fault current from the generator fault current and size them for the largest of the two. Why shouldn't we consider the total available fault current?
Generator circuit-breakers have two short-circuit current ratings, namely generator-source short-circuit current (when the fault is between the GCB and the step-up transformer) and system-source short-circuit current (when the fault is between the generator and the GCB).
In order to size the GCB for a given project, plant-specific studies for both fault cases should be performed. The results of these studies should then be compared to the corresponding ratings of the GCB. A GCB can be considered as suitable if both its rated generator-source short-circuit current and its rated system-source short-circuit current are not less the corresponding values calculated for the project.
Sizing a GCB (installed between generator and step-up transformer) based on the sum of generator-source short-circuit current and of system-source short-circuit current is not correct because the GCB might have to interrupt either one or the other current, but never both at the same time.
More details about selection and sizing of GCBs can be found in IEEE C37.013 which also contains a practical case study.
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