The evaluation of simultaneous faults on three-phase systems

Abstract

Equivalent circuits representing simultaneous dissymmetries on 3-phase systems have been treated previously, 1, 2, 3 but the analysis has been purely mathematical and the ultimate evaluations have been formal. In this paper the more direct method of superposition is employed, as this retains the physical aspect and in general reduces the labour of computation; it also has the indirect advantage that the faults are developed in proper sequence.All possible combinations of two dissymmetries are analysed, with the exception of simultaneous open-circuits. The results are tabulated suitably for direct computation.The primary operation is to reduce the symmetrical-component sequence networks of a given system to the simplest arrangement of impedances between the source and the dissymmetry locations. The networks are then interconnected so that the pertinent constraints between the sequence currents and voltages are complied with at both locations. In general the interconnections at one location can be made direct, whereas at the other magnetic coupling will be required. The constraints and the direct- and magnetically-coupled interconnections for all types of faults and open-circuits are tabulated.The interconnections for any particular dissymmetry can be introduced for faults by the closure, and for open-circuits by the opening, of a switch. The procedure employed is, with the switch (S1) controlling the magnetic interconnections open, to close or open the switch (S2) controlling the direct interconnections, and to determine the current and voltage distributions throughout the network, in particular the voltage across S1. The current or voltage distribution which would be due to the closure of S1 is then determined by an operation equivalent to closing it, i.e. by applying across its terminals a voltage equal and opposite to that determined from the operation of S2. The actual current or voltage distribution for the simultaneous dissymmetries is then the vector sum of the distributions due to the closure (or opening) of S2 and the equivalent closure of S1.