For synchronous motor of low excitation
and loss-of-magnetic state, when the end of
measured impedance locus once crossed the
static-stable impedance, motor inevitably enter
the out-of-step operation. before
loss-of-excitition the active power is greater, the
time is shorter by loss-of-excitition to the
out-of-step. In general,before out-of-step , the
unit or the system will not harm themself.After
out-of-step, active power swings, over current
and low voltage is harm to synchronous motor.
Therefore, in order to maintain the safety of
unit and system, we can take the static-stable
impandence as a criterion for a differential low
excitation or loss-of-excitition faults. So we can
monitor fault before the harm of synchronous
motors’ loss-of-excitation.