Analysis of synchronous generator sequential short circuits

A method of analysing sequential short circuits on a synchronous machine is presented, and is applied to the detailed analytical solution for the armature currents following a line-to-line/3-line and a line-to-neutral/3-line-toneutral short circuit. The analysis is carried out in the d, q, o reference frame by a Laplace transform method using the voltages during the unbalanced s.c., which are known from previous analyses. The results show that, following the 3-phase s.c., the currents decay with the same time constants as during a simultaneous 3-phase s.c. together with an additional time constant for the `o¿ component in the s.c. involving the neutral. The magnitudes of all these components, not just the asymmetrical component as indicated by previous workers, are influenced by the duration and time constants of the unbalanced s.c. As pointed out by previous authors, it is shown that a sequential s.c. can be more severe than a simultaneous s.c., if the duration of the initial s.c. is short compared with the subtransient time constants. The effect of subtransient saliency, which is included in the analysis, is also discussed.