General Fault Admittance Method Solution for a Line-to-Line-to-Line-to-Ground Unsymmetrical Fault

Abstract

Line-to-line-to-line-to-ground unsymmetrical faults involving ground are difficult to analyse using the classical fault analysis approach. This is because the classical solution approach requires knowledge of the connection of symmetrical component sequence networks in order to find the symmetrical fault currents. In the classical solution approach, the phase fault constraints are converted into symmetrical sequence constraints. The sequence networks are connected in a way that satisfies the constraints and the sequence currents determined. A consideration of the symmetrical component constraints for an unsymmetrical three-phase fault involving ground does not lead to an easy connection of the sequence networks. The connection of the positive, negative and zero sequence networks is difficult to deduce when the fault is unsymmetrical and involves ground. 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 to ground unsymmetrical fault, for various fault admittances in each phase. Using the general fault admittance method, a computer program is developed to analyse an unsymmetrical three-phase earth fault on a power system with a delta-earthed-star connected transformer.