Q4Power System - I

Sub-transient reactance Xd″ is the reactance that determines the initial (highest-magnitude) symmetrical fault current immediately after a sudden short circuit at a synchronous machine's terminals, valid only for the first few cycles (typically the first 0.03-0.05 seconds) following the disturbance. Physically, at the very instant of the fault, both the rotor's damper winding (amortisseur winding) and its field winding act to oppose the sudden change in armature-linked flux (by the principle of constant flux linkage), each inducing currents that momentarily maintain a low-reluctance flux path; the combined opposing effect of both windings acting together gives the smallest of the machine's three characteristic reactances (Xd″ < Xd′ < Xd), producing the largest initial fault current. As the damper-winding currents decay away rapidly (due to their comparatively low time constant), the effective reactance transitions to the larger transient reactance Xd′, governed by the field winding alone, and finally to the still-larger steady-state synchronous reactance Xd once all transient effects have decayed.