Figure 3.52-2 Asynchronous boundary impedance circle for loss of excitation protection
The asynchronous boundary impedance value is:
, Where: Xd’ and Xs are settings.
To prevent misoperation of the stator impedance criterion during normal phase operation of
synchronous motors, two diagonal lines passing through the origin and forming a 15 º angle are
specially designed as the blocking criterion. The opening conditions are: 15°<ψ<165°.
The loss of excitation protection automatically exits during the motor startup process and is
automatically put into operation after startup. In addition, the blocking criteria for loss of excitation
protection also include: PT disconnection blocking, circuit breaker tripping blocking, no current
blocking (Ia1<0.06In and Ic1<0.06In), no voltage blocking (Uca<10V), negative sequence current
blocking (can be on/off, and shares the same negative sequence current blocking element with out
of step protection, which needs to automatically return after a 10 second delay).
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