Asynchronous operation and resynchronisation of synchronous machines

A synchronous machine connected to a power system can pull out of synchronism under a variety of abnormal operating conditions. The conventional practice, under such conditions, is to disconnect the machine from the system. But, under certain conditions, it may be preferable to keep the machine connected to the system with or without the excitation, and to permit a limited period of pole slipping or asynchronous operation. In the case of asynchronous operation, resynchronisation is achieved by closing the excitation voltage at the correct moment in the slip cycle. In the paper the phenomenon of asynchronous operation under different modes of field connection is investigated in detail, with the effect of the prime-mover governor control considered. For each mode, a suitable criterion for the best instant for the application of the excitation voltage for resynchronisation is arrived at.