Generator Pole Slipping? 3

[majuba]

Hi

Could someone please explain the phenomenon of pole slipping in synchronous generators? What exactly is it and what causes it?

Thank you in advance.

 

[byrdj]

The magnitude of the variable magnitic rotating field pole is controlled by excitition current. This field rotating through the stator generates the output with the sinosoldal peak voltage phased with the field. When syncronized, the phase of the generator output voltage and the system are the same. as the input torque to the generator is increased to produce MWs, the excitation must be increased also. If the torque exceeds the increase in pole stregnth, the generator field will slip slightly out of phase with the system peak voltage. If the pole stregth exceeds the required to be in phase, the system will slightly slip out of phase with the generator.
This small slip is controlled and so noted as power factor.

If the excitation goes to the extreme and the pole and system slip out of phase, you had a serious problem.

 

[ScottyUK]

Pole slipping typically occurs under severe fault conditions which cause a transient torque on the generator which exceeds the ability of the field to hold the generator rotor synchronised to the stator. A generator is most susceptible to this problem when it has a low excitation, as this produces a weak magnetic field. For this reason, capability diagrams show the stability limit for the machine when in an under-excited state. Outside of this line, pole slipping becomes a real possibility in the event of a system fault.

The 'slip' occurs when the rotor experiences a sudden physical and electrical shift in position relative to the stator, after which the field recovers enough strength to lock the rotor back in synch with the stator. When his occurs, the violent acceleration and deceleration associated with pole slip causes enormous stress on the generator and prime mover, and may result in anything from winding movement to shaft fracture or worse. It is a very serious fault.

A correctly set AVR will act to prevent operation at or outside of the stability limit, but certain abrupt changes in the system caused by faults or badly-planned switching operations may exceed the ability of the AVR to respond if the machine is already near the stability limit.



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