General Fault Admittance Method Line-to-Ground Faults in Reference and Odd Phases
J.D. Sakala, J.S.J. Daka

Abstract
Line-to-ground faults are usually analysed using symmetrical components. As a first step, a reference phase is chosen which results in the simplest connection of the symmetrical component sequence networks for the fault. The simplest connection of symmetrical component sequence networks is a series one when the line-to-ground fault is in the reference phase, say phase a of an a b c phase system. Putting the fault on an odd phase results in series connections of sequence networks that involve phase shifts, and the solution is more demanding. In practice, the results for the fault in the reference phase may be translated to the odd phase by appropriate substitution of phases. In this approach, the solution proceeds by assuming that the fault is in the reference phase and that the symmetrical sequence networks are connected in series. The series connection of the sequence networks at the fault point is solved for the symmetrical component currents and voltages. These are then used to determine the symmetrical component voltages at the other busbars and hence the symmetrical component currents in the rest of the system. The connection of the sequence networks must be known for the common fault types. In contrast, a solution by the general method of fault admittance matrix does not require prior knowledge of how the sequence networks are connected. A line-to-ground fault may be on any phase, reference or odd, and a solution is obtained for the particular fault. It is therefore more versatile than the classical methods in that it does not depend on prior knowledge of how the sequence networks are connected. The paper presents solutions for line-to-ground faults on the reference and odd phases of a simple power system containing a delta-earthed star connected transformer. The results, which include the effects of the delta-earthed star connected transformer, show that the general fault admittance method can be used to solve line-to-ground faults on odd phases.

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