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Lets use simple English terms, Pole Slipping is aka, Out of Step, meaning it will result to Islanding or island non-synch operations. Common culprits are faulty regulator, low excitation, abrupt changes of load as in a power grid. 78 can be used here.
- Pole slipping causes a torque on the rotor the generator which exceeds the ability of the field to hold the rotor synchronised with the magnetic field caused by the stator .
- The generator can be affected when it has a low excitation , as this problem causes a weak magnetic field
- The slip is caused when a difference happens in the angular velocity between the rotor and stator , caused by a sudden mechanical or electrical shift , after which the field recovers enough strength to bring the rotor back in sync with the stator , and when this occurs the violent acceleration or deceleration with the slippage causes a high torsion stress on the shaft that may lead to its fracture
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.
Pole slipping occurs under severe fault conditions and these cause a transient torque on the generator which produces a weak magnetic field. 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 synchronizing 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. A correctly set AVR will act to prevent operation at or outside of the stability limit.
when transient torque on generator exceeds the ability of rotor which hold the stator. its sever fault, slip mainly occurs when rotor experience sudden physical or electrical shift of the stator.
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.
Pole slip, also known as ‘out of step’ is a phenomenon caused either by low excitation, a faulty regulator or abrupt changes of load within the mains. It can cause severe mechanical stresses within the generator, putting winding at risk, causing shaft damage or adversely affecting the control operating system. Very high currents can cause thermal instability and ultimately lead to complete destruction of the generator and massive instability on the grid. It is a serious fault and should repaired asap.
pole slips due to out of step phenomenon i.e. due to failure of regulator, sudden increase or decrease of large amount of load, by low excitation. effects are likely to be damaging shaft, control operating unit will be deeply effected, winding losses.
When there is loss in synchronism between rotor speed and magnetic field, pole slips.
