| Summary: | Line-to-line-to-line unsymmetrical faults either involving or not involving ground are in the classical fault analysis approach difficult to analyse. This is because the classical solution requires use of the knowledge of connection of symmetrical component sequence networks for various common faults. In this approach, the phase fault constraints are converted into symmetrical sequence constraints and the sequence networks connected in a way that satisfies the constraints. The symmetrical component constraints for an unsymmetrical three-phase fault not involving ground do not lend themselves easy to the connection of the sequence networks. The exception is that, because the phase currents at the fault summate to zero, the zero sequence current is zero and therefore the zero sequence network is not connected. The connection of the positive and negative sequence networks is
difficult to deduce when the fault is unsymmetrical. A classical solution is therefore difficult to find. In contrast, a solution by the general method of fault admittance matrix does not require prior knowledge of how the sequence networks are connected. It is therefore more versatile than the classical methods. The paper presents a procedure for solving a three-phase unsymmetrical fault, with different fault impedances, hence fault admittances in each phase. A computer program based on the general fault admittance method is developed and used to analyse an unsymmetrical three-phase fault on a simple power system
with a delta-earthed-star connected transformer.
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