Machine Riddle No.42 - Subtransient negative sequence reactance
While the positive sequence reactance  has different values during sub transient, transient and steady state periods, why these are not applicable for negative sequence reactance. Why there  is no sub transient or transient negative sequence reactance?

#1
Tue, April 10th, 2012 - 11:41
Thank you very much for your wonderful conceptual question.
As you know, positive sequence reactance of generator is including two parts, one part due to winding leakage flux (transformer effects) and other due to armoutor voltage reaction which related to flux linkage between all involved conductive paths of generator. After switching/changing of armoutor current, the flux linkage cannot change in the short-circuited stator winding. Therefore the currents in the excitation winding and in the damper winding have to magnetize oppositely a counteracting. Following equations define the generator transient and subtransient inductance.

The reaction effects of armoutor winding can be observed in all phenomenons which involved to rotation field and the mutual inductance between two or more winding just can be exist in synchronizing situation. Therefore only positive sequence of reactance can be affected from armoutor reaction.
The initial effective values of currents for single-phase and two – phase short circuits are calculated by formulas similar to those for sustained currents of asymmetrical short circuits, in which only X1=Xd is replaced by X'd (or corresponding by X''d), while X2 and X0 remain practically the same both for the steady-state and transient conditions. Indeed the negative and zero sequence stator fluxes, as distinct from the positive sequence flux, enter into transformer linkage with the rotor windings already in the steady state condition and therefore as concerns the manifestation and action of these fluxes, there is no significant difference between steady state and transient conditions.

#2
Thu, April 12th, 2012 - 06:25
with reference to your answer,since the negative sequence currents  also produce a rotating magnetic field which rotates in a direction opposite to that of positive sequence current ,how is it,it is immune from significant changes?

#3
Thu, April 12th, 2012 - 09:25

#4
Thu, April 12th, 2012 - 13:15
However since the negative sequence currents also produce a rotating magnetic field
in a direction opposite to that of positive sequence currents,how is negative sequence
reactance is immune from changes in value unlike positive sequence reactance.

#5
Thu, April 12th, 2012 - 13:38
I have tried to explain it. Please note that an important portion of synchronizing reactance is related to armoutor reaction or winding induction voltage (emf). This reaction effect just may be for positive sequence which is synchronous with main rotation field and cannot exist for any asynchronous rotation field such as zero and negative sequence. Also the difference value between transient and stady state condition is related to armoutor reaction or induction voltage produced by stator windings.