In symmetrical three phase fault analysis, we represent the equivalent circuits with sub-transient, transient and steady state d-axis reactance. Why don't we use q-axis reactance to model the generators under fault in these cases?
Modern high speed, high power generators are round rotor, where Xd=Xq, For low speed hydral generator, rotor are heavy with large inertia, also direct axis offers low. reluctant path, with increased regiedity, are the reasons to considered Xd in faults calculation.In symmetrical fault X +ve seq, is equal to Xd, while X -ve and zero are almost zero.
If you neglect the resistances and the load current,, there is no current in the q-axix windings. When modelling short circuit current in the time domain (EM transient or dynamic transients) the effects of the current in the q-axis windings are accounted.
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