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# Machine Riddle No.42 – Subtransient negative sequence reactance

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• #944
D.Suryanarayana

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?
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?

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• #3023
Hamid

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

#3026
D.Suryanarayana

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?

#3027
Hamid