Pole slip, in the art also called out of step, is a specific phenomenon, caused either by low excitation, faulty regulator or abrupt changes of load within the mains, which causes severe mechanical stress on the machine, putting windings at risk, shaft damage or effecting normal operation of control systems. Moreover, the simultaneous very high currents can be thermally very devastating and lead to complete generator destruction and massive instability to the grid.
Conventional protection, such as over-current, vector shift or ROCOF, is not sufficient because it is not fast and reliable enough especially if oscillations caused by large load jumps develop.
caused either by low excitation, faulty regulator or abrupt changes of load within the mains