Asynchronous operation of synchronous generators under field failure

A synchronous generator is forced to asynchronous operation under sudden field failure. Under such fault condition, the general practice is to switch off the faulted machine by the action of an off-set type mho-relay after an appropriate time delay to allow for recoverable swings. This practice is good enough for salient pole hydro-alternators for which both the reactive power burden and the pulsating power component are relatively large. But these quantities are not prohibitively high for a cylindrical pole turbo-alternator. Sustained operation under field failure may be continued for limited time for them without any risk of thermal injury, provided the reactive power requirement of the alternator can be met by the system, keeping the voltage profile within acceptable limits. In this paper, the performance variables under sudden field failure for both salient and non-salient pole generators have been computed by specially constructed programs and presented in tabular form. Taking the example of a 210 MW set of BHEL, it has been established that sustained operation of a cylindrical pole generator is not harmful, particularly when the field circuit is disconnected from the source and closed through a properly chosen value of discharge resistance.